Search

To constrain models of high-mass star formation, the Herschel/HOBYS KP aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC6334, one of the best-studied HOBYS molecular cloud complexes. We used Herschel PACS and SPIRE 70-500mu images of the NGC6334 complex complemented with (sub)millimeter and mid-infrared data. We built a complete procedure to extract ~0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their SEDs. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75Msun for MDCs in NGC6334. MDCs have temperatures of 9.5-40K, masses of 75-1000Msun, and densities of 10^5-10^8cm-3. Their mid-IR emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70mu emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1-7x10^4yr and at most 3x10^5yr respectively, suggest a dynamical scenario of high-mass star formation. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star formation. High-mass prestellar cores may not exist in NGC6334, favoring a scenario presented here, which simultaneously forms clouds and high-mass protostars., Comment: 36 pages, 14 figures, accepted by A&A. Complete appendix could be requested to F. Motte

Aims. To constrain models of high-mass star formation, the Herschel-HOBYS key program aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC 6334, one of the best-studied HOBYS molecular cloud complexes. Methods. We used Herschel /PACS and SPIRE 70 500 mu m images of the NGC 6334 complex complemented with (sub) millimeter and mid-infrared data. We built a complete procedure to extract 0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their spectral energy distributions (SEDs). We also identified the densest pc-scale cloud structures of NGC 6334, one 2 pc x 1 pc ridge and two 0.8 pc x 0.8 pc hubs, with volume-averaged densities of 10(5) cm(-3). Results. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75 M-circle dot for MDCs in NGC 6334. MDCs have temperatures of 9 : 5 40 K, masses of 75 1000 M-circle dot, and densities of 1 x 10(5) 7 x 10(7) cm(-3). Their mid-infrared emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70 mu m emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1-7 x 10(4) yr and at most 3 105 yr respectively, suggest a dynamical scenario of high-mass star formation. Conclusions. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star

Aims. To constrain models of high-mass star formation, the Herschel-HOBYS key program aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC 6334, one of the best-studied HOBYS molecular cloud complexes. Methods. We used Herschel /PACS and SPIRE 70 500 mu m images of the NGC 6334 complex complemented with (sub) millimeter and mid-infrared data. We built a complete procedure to extract 0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their spectral energy distributions (SEDs). We also identified the densest pc-scale cloud structures of NGC 6334, one 2 pc x 1 pc ridge and two 0.8 pc x 0.8 pc hubs, with volume-averaged densities of 10(5) cm(-3). Results. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75 M-circle dot for MDCs in NGC 6334. MDCs have temperatures of 9 : 5 40 K, masses of 75 1000 M-circle dot, and densities of 1 x 10(5) 7 x 10(7) cm(-3). Their mid-infrared emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70 mu m emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1-7 x 10(4) yr and at most 3 105 yr respectively, suggest a dynamical scenario of high-mass star formation. Conclusions. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star

To constrain models of high-mass star formation, the Herschel/HOBYS KP aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC6334, one of the best-studied HOBYS molecular cloud complexes. We used Herschel PACS and SPIRE 70-500mu images of the NGC6334 complex complemented with (sub)millimeter and mid-infrared data. We built a complete procedure to extract ~0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their SEDs. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75Msun for MDCs in NGC6334. MDCs have temperatures of 9.5-40K, masses of 75-1000Msun, and densities of 10^5-10^8cm-3. Their mid-IR emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70mu emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1-7x10^4yr and at most 3x10^5yr respectively, suggest a dynamical scenario of high-mass star formation. The present study provides good mass estimates for a statistically significant sample, covering the earliest phases of high-mass star formation. High-mass prestellar cores may not exist in NGC6334, favoring a scenario presented here, which simultaneously forms clouds and high-mass protostars., Comment: 36 pages, 14 figures, accepted by A&A. Complete appendix could be requested to F. Motte

The accretion mechanism producing the short flares observed from the Supergiant Fast X-ray Transients (SFXT) is still highly debated and forms a major part in our attempts to place these X-ray binaries in the wider context of the High Mass X-ray Binaries. We report on a 216 ks INTEGRAL observation of the SFXT IGR J16328-4726 (August 24-27, 2014) simultaneous with two fixed-time observations with XMM Newton (33ks and 20ks) performed around the putative periastron passage, in order to investigate the accretion regime and the wind properties during this orbital phase. During these observations, the source has shown luminosity variations, from 4x10^{34} erg/s to 10^{36} erg/s, linked to spectral properties changes. The soft X-ray continuum is well modeled by a power law with a photon index varying from 1.2 up to 1.7 and with high values of the column density in the range 2-4x10^{23}/cm^2. We report on the presence of iron lines at 6.8-7.1 keV suggesting that the X-ray flux is produced by accretion of matter from the companion wind characterized by density and temperature inhomogeneities.

The accretion mechanism producing the short flares observed from the Supergiant Fast X-ray Transients (SFXT) is still highly debated and forms a major part in our attempts to place these X-ray binaries in the wider context of the High Mass X-ray Binaries. We report on a 216 ks INTEGRAL observation of the SFXT IGR J16328-4726 (August 24-27, 2014) simultaneous with two fixed-time observations with XMM Newton (33ks and 20ks) performed around the putative periastron passage, in order to investigate the accretion regime and the wind properties during this orbital phase. During these observations, the source has shown luminosity variations, from 4x10^{34} erg/s to 10^{36} erg/s, linked to spectral properties changes. The soft X-ray continuum is well modeled by a power law with a photon index varying from 1.2 up to 1.7 and with high values of the column density in the range 2-4x10^{23}/cm^2. We report on the presence of iron lines at 6.8-7.1 keV suggesting that the X-ray flux is produced by accretion of matter from the companion wind characterized by density and temperature inhomogeneities.

Aims. Fundamental to any theory of high-mass star formation are gravity and turbulence. Their relative importance, which probably changes during cloud evolution, is not known. By investigating the spatial and density structure of the high-mass star-forming complex NGC 6334 we aim to disentangle the contributions of turbulence and gravity. Methods. We used Herschel PACS and SPIRE imaging observations from the HOBYS key programme at wavelengths of 160, 250, 350, and 500 μm to construct dust temperature and column density maps. Using probability distribution functions (PDFs) of the column density determined for the whole complex and for four distinct sub-regions (distinguished on the basis of differences in the column density, temperature, and radiation field), we characterize the density structure of the complex. We investigate the spatial structure using the Δ-variance, which probes the relative amount of structure on different size scales and traces possible energy injection mechanisms into the molecular cloud. Results. The Δ-variance analysis suggests that the significant scales of a few parsec that were found are caused by energy injection due to expanding H ii regions, which are numerous, and by the lengths of filaments seen everywhere in the complex. The column density PDFs have a lognormal shape at low densities and a clearly defined power law at high densities for all sub-regions whose slope is linked to the exponent α of an equivalent spherical density distribution. In particular with α = 2.37, the central sub-region is largly dominated by gravity, caused by individual collapsing dense cores and global collapse of a larger region. The collapse is faster than free-fall (which would lead only to α = 2) and thus requires a more dynamic scenario (external compression, flows). The column density PDFs suggest that the different sub-regions are at different evolutionary stages, especially the central sub-region, which seems to be in a more evolved stage. © 2013 ESO.

Aims. Fundamental to any theory of high-mass star formation are gravity and turbulence. Their relative importance, which probably changes during cloud evolution, is not known. By investigating the spatial and density structure of the high-mass star-forming complex NGC 6334 we aim to disentangle the contributions of turbulence and gravity. Methods. We used Herschel PACS and SPIRE imaging observations from the HOBYS key programme at wavelengths of 160, 250, 350, and 500 μm to construct dust temperature and column density maps. Using probability distribution functions (PDFs) of the column density determined for the whole complex and for four distinct sub-regions (distinguished on the basis of differences in the column density, temperature, and radiation field), we characterize the density structure of the complex. We investigate the spatial structure using the Δ-variance, which probes the relative amount of structure on different size scales and traces possible energy injection mechanisms into the molecular cloud. Results. The Δ-variance analysis suggests that the significant scales of a few parsec that were found are caused by energy injection due to expanding H ii regions, which are numerous, and by the lengths of filaments seen everywhere in the complex. The column density PDFs have a lognormal shape at low densities and a clearly defined power law at high densities for all sub-regions whose slope is linked to the exponent α of an equivalent spherical density distribution. In particular with α = 2.37, the central sub-region is largly dominated by gravity, caused by individual collapsing dense cores and global collapse of a larger region. The collapse is faster than free-fall (which would lead only to α = 2) and thus requires a more dynamic scenario (external compression, flows). The column density PDFs suggest that the different sub-regions are at different evolutionary stages, especially the central sub-region, which seems to be in a more evolved stage. © 2013 ESO.

We report on temporal and spectral analysis of the INTEGRAL fast transient candidate IGR J16328-4726 observed with Beppo SAX in 1998 and more recently with INTEGRAL. The MECS X-ray data show a frequent micro activity typical of the intermediate state of Supergiant Fast X-ray Transients and a weak flare with duration of ~4.6 ks. The X-ray emission in the 1.5-10 keV energy range is well described through the different time intervals by an absorbed power law model. Comparing spectra from the lower emission level up to the peak of the flare, we note that while the power-law photon index was constant (~2), the absorption column density varied by a factor of up to ~6-7, reaching the value of ~2x10^{23}cm^{-2} at the peak of the flare. Analysis of the long-term INTEGRAL/IBIS light curve confirms and refines the proposed ~10.07 day period, and the derived ephemeris places the Beppo SAX observations away from periastron. Using the near and the mid-IR available observations, we constructed a spectral infrared distribution for the counterpart of IGR J16328-4726, allowing us to identify its counterpart as a High Mass OB type star, and to classify this source as a firm HMXB. Following the standard clumpy wind theory, we estimated the mass and the radius of the clump responsible of the flare. The obtained values of M~4x10^{22}g and R~4.4x10^6 km are in agreement with expected values from theoretical predictions., Comment: arXiv admin note: text overlap with arXiv:1106.5125 by other authors

We report on temporal and spectral analysis of the INTEGRAL fast transient candidate IGR J16328-4726 observed with Beppo SAX in 1998 and more recently with INTEGRAL. The MECS X-ray data show a frequent micro activity typical of the intermediate state of Supergiant Fast X-ray Transients and a weak flare with duration of ~4.6 ks. The X-ray emission in the 1.5-10 keV energy range is well described through the different time intervals by an absorbed power law model. Comparing spectra from the lower emission level up to the peak of the flare, we note that while the power-law photon index was constant (~2), the absorption column density varied by a factor of up to ~6-7, reaching the value of ~2x10^{23}cm^{-2} at the peak of the flare. Analysis of the long-term INTEGRAL/IBIS light curve confirms and refines the proposed ~10.07 day period, and the derived ephemeris places the Beppo SAX observations away from periastron. Using the near and the mid-IR available observations, we constructed a spectral infrared distribution for the counterpart of IGR J16328-4726, allowing us to identify its counterpart as a High Mass OB type star, and to classify this source as a firm HMXB. Following the standard clumpy wind theory, we estimated the mass and the radius of the clump responsible of the flare. The obtained values of M~4x10^{22}g and R~4.4x10^6 km are in agreement with expected values from theoretical predictions., Comment: arXiv admin note: text overlap with arXiv:1106.5125 by other authors

Following the INTEGRAL/JEM-X detection of the unidentified source 1RXS J194211.9+255552 (ATel #3816) on December 18, we asked for a Swift/XRT follow-up observation. Swift observed the source field on December 21, 2011 at 06:10:09.7 (UTC), with a net exposure of 1756 s. Within the ROSAT error circle there is only one pointlike source, at the following position (J2000): RA(hh mm ss.s) = 19h42m11.13s, Dec(dd mm ss.s) = +25:56:07.32 (3.6 arcsec error radius). The source light curve is variable on time scale of about 1000 s (similar to the Jem-X light curve). A fit to the XRT spectrum (1-10 keV) with a power law model resulted in a good fit (reduced chi_sq = 1.101 for 36 degrees of freedom) with an absorption of NH = 1.28E22 cm-2 (+0.57, -0.52) (consistent with the total Galactic value), a photon index of 0.64 (+0.30, -0.28) and an unabsorbed flux (1-10 keV) of 7.88E-11 ergs/cm2/s. A black body model resulted in a much worse fit (reduced chi_sq = 1.367 for 36 dof). The source is a factor of 2.5 fainter than during the Jem-X detection (ATel #3816), two days before. This is the first characterization of the source X-ray spectrum. A search at other wavelengths resulted in a possible counterpart in the 2MASS All-Sky Catalog of Point Sources, 19421116+2556056 (J=13.479, H=12.834, K=12.793 mag; although we note that the 2MASS image indicates a blend of two stars, the brighter more consistent with the XRT position), as well as a single star in the USNO-B1.0 catalog, 1159-0412038, at 1.62 arcsec offset from the XRT coordinates, with B1=20.55, R1=17.34, B2=19.85, R2=17.12, I=15.44 mag. Although the hard power law spectrum could indicate a high mass X-ray binary, other observations are needed to fully identify the nature of this X-ray transient. Further multiwavelength observations are encouraged. We would like to thank the Swift Team for making these observations possible, in particular Neil Gehrels, the duty scientists and the science planners.

We present the initial highlights of the HOBYS key program, which are based on Herschel images of the Rosette molecular complex and maps of the RCW120 H II region. Using both SPIRE at 250/350/500 mu m and PACS at 70/160 mu m or 100/160 mu m, the HOBYS survey provides an unbiased and complete census of intermediate-to high-mass young stellar objects, some of which are not detected by Spitzer. Key core properties, such as bolometric luminosity and mass (as derived from spectral energy distributions), are used to constrain their evolutionary stages. We identify a handful of high-mass prestellar cores and show that their lifetimes could be shorter in the Rosette molecular complex than in nearby low-mass star-forming regions. We also quantify the impact of expanding H II regions on the star formation process acting in both Rosette and RCW 120., authorCount :49

Context. The Rosette molecular cloud is promoted as the archetype of a triggered star-formation site. This is mainly due to its morphology, because the central OB cluster NGC 2244 has blown a circular-shaped cavity into the cloud and the expanding H II-region now interacts with the cloud. Aims. Studying the spatial distribution of the different evolutionary states of all star-forming sites in Rosette and investigating possible gradients of the dust temperature will help to test the "triggered star-formation" scenario in Rosette. Methods. We use continuum data obtained with the PACS (70 and 160 mu m) and SPIRE instruments (250, 350, 500 mu m) of the Herschel telescope during the science demonstration phase of HOBYS. Results. Three-color images of Rosette impressively show how the molecular gas is heated by the radiative impact of the NGC 2244 cluster. A clear negative temperature gradient and a positive density gradient (running from the H II-region/molecular cloud interface into the cloud) are detected. Studying the spatial distribution of the most massive dense cores (size scale 0.05 to 0.3 pc), we find an age-sequence (from more evolved to younger) with increasing distance to the cluster NGC 2244. No clear gradient is found for the clump (size-scale up to 1 pc) distribution. Conclusions. The existence of temperature and density gradients and the observed age-sequence imply that star formation in Rosette may indeed be influenced by the radiative impact of the central NGC 2244 cluster. A more complete overview of the prestellar and protostellar population in Rosette is required to obtain a firmer result., authorCount :46

The Herschel OB young stellar objects survey (HOBYS) has observed the Rosette molecular cloud, providing an unprecedented view of its star formation activity. These new far-infrared data reveal a population of compact young stellar objects whose physical properties we aim to characterise. We compiled a sample of protostars and their spectral energy distributions that covers the near-infrared to submillimetre wavelength range. These were used to constrain key properties in the protostellar evolution, bolometric luminosity, and envelope mass and to build an evolutionary diagram. Several clusters are distinguished including the cloud centre, the embedded clusters in the vicinity of luminous infrared sources, and the interaction region. The analysed protostellar population in Rosette ranges from 0.1 to about 15 M-circle dot with luminosities between 1 and 150 L-circle dot, which extends the evolutionary diagram from low-mass protostars into the high-mass regime. Some sources lack counterparts at near-to mid-infrared wavelengths, indicating extreme youth. The central cluster and the Phelps & Lada 7 cluster appear less evolved than the remainder of the analysed protostellar population. For the central cluster, we find indications that about 25% of the protostars classified as Class I from near-to mid-infrared data are actually candidate Class 0 objects. As a showcase for protostellar evolution, we analysed four protostars of low-to intermediate-mass in a single dense core, and they represent different evolutionary stages from Class 0 to Class I. Their mid-to far-infrared spectral slopes flatten towards the Class I stage, and the 160 to 70 mu m flux ratio is greatest for the presumed Class 0 source. This shows that the Herschel observations characterise the earliest stages of protostellar evolution in detail., authorCount :42

The Rosette molecular cloud is promoted as the archetype of a triggered star-formation site. This is mainly due to its morphology, because the central OB cluster NGC 2244 has blown a circular-shaped cavity into the cloud and the expanding HII-region now interacts with the cloud. Studying the spatial distribution of the different evolutionary states of all star-forming sites in Rosette and investigating possible gradients of the dust temperature will help to test the 'triggered star-formation' scenario in Rosette. We use continuum data obtained with the PACS (70 and 160 micron) and SPIRE instruments (250, 350, 500 micron) of the Herschel telescope during the Science Demonstration Phase of HOBYS. Three-color images of Rosette impressively show how the molecular gas is heated by the radiative impact of the NGC 2244 cluster. A clear negative temperature gradient and a positive density gradient (running from the HII-region/molecular cloud interface into the cloud) are detected. Studying the spatial distribution of the most massive dense cores (size scale 0.05 to 0.3 pc), we find an age-sequence (from more evolved to younger) with increasing distance to the cluster NGC 2244. No clear gradient is found for the clump (size-scale up to 1 pc) distribution. The existence of temperature and density gradients and the observed age-sequence imply that star formation in Rosette may indeed be influenced by the radiative impact of the central NGC 2244 cluster. A more complete overview of the prestellar and protostellar population in Rosette is required to obtain a firmer result., Comment: Letter in press for Astronomy & Astrophysics Herschel Special Issue

The Herschel OB young stellar objects survey (HOBYS) has observed the Rosette molecular cloud, providing an unprecedented view of its star formation activity. These new far-infrared data reveal a population of compact young stellar objects whose physical properties we aim to characterise. We compiled a sample of protostars and their spectral energy distributions that covers the near-infrared to submillimetre wavelength range. These were used to constrain key properties in the protostellar evolution, bolometric luminosity, and envelope mass and to build an evolutionary diagram. Several clusters are distinguished including the cloud centre, the embedded clusters in the vicinity of luminous infrared sources, and the interaction region. The analysed protostellar population in Rosette ranges from 0.1 to about 15 Msun with luminosities between 1 and 150 Lsun, which extends the evolutionary diagram from low-mass protostars into the high-mass regime. Some sources lack counterparts at near- to mid-infrared wavelengths, indicating extreme youth. The central cluster and the Phelps & Lada 7 cluster appear less evolved than the remainder of the analysed protostellar population. For the central cluster, we find indications that about 25% of the protostars classified as Class I from near- to mid-infrared data are actually candidate Class 0 objects. As a showcase for protostellar evolution, we analysed four protostars of low- to intermediate-mass in a single dense core, and they represent different evolutionary stages from Class 0 to Class I. Their mid- to far-infrared spectral slopes flatten towards the Class I stage, and the 160 to 70um flux ratio is greatest for the presumed Class 0 source. This shows that the Herschel observations characterise the earliest stages of protostellar evolution in detail., Comment: Astronomy & Astrophysics letter, 6 pages, 4 figures, accepted for publication in the Special Issue for Herschel first results

We present a preliminary analysis of the small-scale structure found in new 70-520 micron continuum maps of the Rosette molecular cloud (RMC), obtained with the SPIRE and PACS instruments of the Herschel Space Observatory. We find 473 clumps within the RMC using a new structure identification algorithm, with sizes up to ~1.0 pc in diameter. A comparison with recent Spitzer maps reveals that 371 clumps are "starless" (without an associated young stellar object), while 102 are "protostellar." Using the respective values of dust temperature, we determine the clumps have masses (M_C) over the range -0.75 <= log (M_C/M_sun) <= 2.50. Linear fits to the high-mass tails of the resulting clump mass spectra (CMS) have slopes that are consistent with those found for high-mass clumps identified in CO emission by other groups., Comment: 6 pages, to be published in Astronomy & Astrophysics

The Rosette molecular cloud is promoted as the archetype of a triggered star-formation site. This is mainly due to its morphology, because the central OB cluster NGC 2244 has blown a circular-shaped cavity into the cloud and the expanding HII-region now interacts with the cloud. Studying the spatial distribution of the different evolutionary states of all star-forming sites in Rosette and investigating possible gradients of the dust temperature will help to test the 'triggered star-formation' scenario in Rosette. We use continuum data obtained with the PACS (70 and 160 micron) and SPIRE instruments (250, 350, 500 micron) of the Herschel telescope during the Science Demonstration Phase of HOBYS. Three-color images of Rosette impressively show how the molecular gas is heated by the radiative impact of the NGC 2244 cluster. A clear negative temperature gradient and a positive density gradient (running from the HII-region/molecular cloud interface into the cloud) are detected. Studying the spatial distribution of the most massive dense cores (size scale 0.05 to 0.3 pc), we find an age-sequence (from more evolved to younger) with increasing distance to the cluster NGC 2244. No clear gradient is found for the clump (size-scale up to 1 pc) distribution. The existence of temperature and density gradients and the observed age-sequence imply that star formation in Rosette may indeed be influenced by the radiative impact of the central NGC 2244 cluster. A more complete overview of the prestellar and protostellar population in Rosette is required to obtain a firmer result., Comment: Letter in press for Astronomy & Astrophysics Herschel Special Issue

The Herschel OB young stellar objects survey (HOBYS) has observed the Rosette molecular cloud, providing an unprecedented view of its star formation activity. These new far-infrared data reveal a population of compact young stellar objects whose physical properties we aim to characterise. We compiled a sample of protostars and their spectral energy distributions that covers the near-infrared to submillimetre wavelength range. These were used to constrain key properties in the protostellar evolution, bolometric luminosity, and envelope mass and to build an evolutionary diagram. Several clusters are distinguished including the cloud centre, the embedded clusters in the vicinity of luminous infrared sources, and the interaction region. The analysed protostellar population in Rosette ranges from 0.1 to about 15 Msun with luminosities between 1 and 150 Lsun, which extends the evolutionary diagram from low-mass protostars into the high-mass regime. Some sources lack counterparts at near- to mid-infrared wavelengths, indicating extreme youth. The central cluster and the Phelps & Lada 7 cluster appear less evolved than the remainder of the analysed protostellar population. For the central cluster, we find indications that about 25% of the protostars classified as Class I from near- to mid-infrared data are actually candidate Class 0 objects. As a showcase for protostellar evolution, we analysed four protostars of low- to intermediate-mass in a single dense core, and they represent different evolutionary stages from Class 0 to Class I. Their mid- to far-infrared spectral slopes flatten towards the Class I stage, and the 160 to 70um flux ratio is greatest for the presumed Class 0 source. This shows that the Herschel observations characterise the earliest stages of protostellar evolution in detail., Comment: Astronomy & Astrophysics letter, 6 pages, 4 figures, accepted for publication in the Special Issue for Herschel first results

We present a preliminary analysis of the small-scale structure found in new 70-520 micron continuum maps of the Rosette molecular cloud (RMC), obtained with the SPIRE and PACS instruments of the Herschel Space Observatory. We find 473 clumps within the RMC using a new structure identification algorithm, with sizes up to ~1.0 pc in diameter. A comparison with recent Spitzer maps reveals that 371 clumps are "starless" (without an associated young stellar object), while 102 are "protostellar." Using the respective values of dust temperature, we determine the clumps have masses (M_C) over the range -0.75 <= log (M_C/M_sun) <= 2.50. Linear fits to the high-mass tails of the resulting clump mass spectra (CMS) have slopes that are consistent with those found for high-mass clumps identified in CO emission by other groups., Comment: 6 pages, to be published in Astronomy & Astrophysics

PILOT (the Pathfinder for an International Large Optical Telescope) is a proposed 2.5 m optical/infrared telescope to be located at Dome C on the Antarctic plateau. Conditions at Dome C are known to be exceptional for astronomy. The seeing (above ~30 m height), coherence time, and isoplanatic angle are all twice s good as at typical mid-latitude sites, while the water-vapour column, and the atmosphere and telescope thermal emission are all an order of magnitude better. These conditions enable a unique scientific capability for PILOT, which is addressed in this series of papers. The current paper presents an overview of the optical and instrumentation suite for PILO and its expected performance, a summary of the key science goals and observational approach for the facility, a discussion of the synergies between the science goals for PILOT and other telescopes, and a discussion of the future of Antarctic astronomy. Paper II and Paper III present details of the science projects divided, respectively, between the distant Universe (i.e., studies of first light, and the assembly and evolution of structure) and the nearby Universe (i.e., studies of Local Group galaxies, the Milky Way, and the Solar System)., Comment: 19 pages, 8 figures, accepted for publication in PASA

PILOT (the Pathfinder for an International Large Optical Telescope) is a proposed 2.5 m optical/infrared telescope to be located at Dome C on the Antarctic plateau. Conditions at Dome C are known to be exceptional for astronomy. The seeing (above ~30 m height), coherence time, and isoplanatic angle are all twice s good as at typical mid-latitude sites, while the water-vapour column, and the atmosphere and telescope thermal emission are all an order of magnitude better. These conditions enable a unique scientific capability for PILOT, which is addressed in this series of papers. The current paper presents an overview of the optical and instrumentation suite for PILO and its expected performance, a summary of the key science goals and observational approach for the facility, a discussion of the synergies between the science goals for PILOT and other telescopes, and a discussion of the future of Antarctic astronomy. Paper II and Paper III present details of the science projects divided, respectively, between the distant Universe (i.e., studies of first light, and the assembly and evolution of structure) and the nearby Universe (i.e., studies of Local Group galaxies, the Milky Way, and the Solar System)., Comment: 19 pages, 8 figures, accepted for publication in PASA

We present ground-based mid-infrared imaging for 27 M-, S- and C-type Asymptotic Giant Branch (AGB) stars. The data are compared with those of the database available thanks to the IRAS, ISO, MSX and 2MASS catalogues. Our goal is to establish relations between the IR colors, the effective temperature $T_{eff}$, the luminosity $L$ and the mass loss rate $\dot M$, for improving the effectiveness of AGB modelling. Bolometric (absolute) magnitudes are obtained through distance compilations, and by applying previously-derived bolometric corrections; the variability is also studied, using data accumulated since the IRAS epoch. The main results are: i) Values of $L$ and $\dot M$ for C stars fit relations previously established by us, with Miras being on average more evolved and mass losing than Semiregulars. ii) Moderate IR excesses (as compared to evolutionary tracks) are found for S and M stars in our sample: they are confirmed to originate from the dusty circumstellar environment. iii) A larger reddening characterizes C-rich Miras and post-AGBs. In this case, part of the excess is due to AGB models overestimating $T_{eff}$ for C-stars, as a consequence of the lack of suitable molecular opacities. This has a large effect on the colors of C-rich sources and sometimes disentangling the photospheric and circumstellar contributions is difficult; better model atmospheres should be used in stellar evolutionary codes for C stars. iv) The presence of a long-term variability at mid-IR wavelengths seems to be limited to sources with maximum emission in the 8 -- 20 $\mu$m region, usually Mira variables (1/3 of our sample). Most Semiregular and post-AGB stars studied here remained remarkably constant in mid-IR over the last twenty years., Comment: Accepted for publication in the Astronomical Journal - 35 pages (in preprint), 9 figures, 5 tables

We present ground-based mid-infrared imaging for 27 M-, S- and C-type Asymptotic Giant Branch (AGB) stars. The data are compared with those of the database available thanks to the IRAS, ISO, MSX and 2MASS catalogues. Our goal is to establish relations between the IR colors, the effective temperature $T_{eff}$, the luminosity $L$ and the mass loss rate $\dot M$, for improving the effectiveness of AGB modelling. Bolometric (absolute) magnitudes are obtained through distance compilations, and by applying previously-derived bolometric corrections; the variability is also studied, using data accumulated since the IRAS epoch. The main results are: i) Values of $L$ and $\dot M$ for C stars fit relations previously established by us, with Miras being on average more evolved and mass losing than Semiregulars. ii) Moderate IR excesses (as compared to evolutionary tracks) are found for S and M stars in our sample: they are confirmed to originate from the dusty circumstellar environment. iii) A larger reddening characterizes C-rich Miras and post-AGBs. In this case, part of the excess is due to AGB models overestimating $T_{eff}$ for C-stars, as a consequence of the lack of suitable molecular opacities. This has a large effect on the colors of C-rich sources and sometimes disentangling the photospheric and circumstellar contributions is difficult; better model atmospheres should be used in stellar evolutionary codes for C stars. iv) The presence of a long-term variability at mid-IR wavelengths seems to be limited to sources with maximum emission in the 8 -- 20 $\mu$m region, usually Mira variables (1/3 of our sample). Most Semiregular and post-AGB stars studied here remained remarkably constant in mid-IR over the last twenty years., Comment: Accepted for publication in the Astronomical Journal - 35 pages (in preprint), 9 figures, 5 tables

As part of a reanalysis of galactic Asymptotic Giant Branch (AGB) stars at infrared (IR) wavelengths, we discuss a sample (357) of carbon stars for which mass loss rates, near-IR photometry and distance estimates exist. For 252 sources we collected mid-IR fluxes from the MSX (6C) and the ISO-SWS catalogues. Most stars have spectral energy distributions up to 21 microns, and some (1/3) up to 45 microns. This wide wavelength coverage allows us to obtain reliable bolometric magnitudes. The properties of our sample are discussed with emphasis on about 70 stars with astrometric distances. We show that mid-IR fluxes are crucial to estimate the magnitude of stars with dusty envelopes. We construct HR diagrams and show that the luminosities agree fairly well with model predictions based on the Schwarzschild's criterion, contrary to what is widely argued in the literature. A problem with the brightness of C stars does not appear to exist. From the relative number of Mira and Semiregular C-variables, we argue that the switch between these classes is unlikely to be connected to thermal pulses. The relevance of the two populations varies with the evolution, with Miras dominating the final stages. We also analyze mass loss rates, which increase for increasing luminosity, but with a spread that probably results from a dependence on a number of parameters (like e.g. different stellar masses and different mechanisms powering stellar winds). Instead, mass loss rates are well monitored by IR colours, especially if extended to 20 microns and beyond, where AGB envelopes behave like black bodies. From these colours the evolutionary status of various classes of C stars is discussed., Comment: 19 pages, 12 figures, 4 tables. Accepted for publication in "Astronomy & Astrophysics"

As part of a reanalysis of galactic Asymptotic Giant Branch (AGB) stars at infrared (IR) wavelengths, we discuss a sample (357) of carbon stars for which mass loss rates, near-IR photometry and distance estimates exist. For 252 sources we collected mid-IR fluxes from the MSX (6C) and the ISO-SWS catalogues. Most stars have spectral energy distributions up to 21 microns, and some (1/3) up to 45 microns. This wide wavelength coverage allows us to obtain reliable bolometric magnitudes. The properties of our sample are discussed with emphasis on about 70 stars with astrometric distances. We show that mid-IR fluxes are crucial to estimate the magnitude of stars with dusty envelopes. We construct HR diagrams and show that the luminosities agree fairly well with model predictions based on the Schwarzschild's criterion, contrary to what is widely argued in the literature. A problem with the brightness of C stars does not appear to exist. From the relative number of Mira and Semiregular C-variables, we argue that the switch between these classes is unlikely to be connected to thermal pulses. The relevance of the two populations varies with the evolution, with Miras dominating the final stages. We also analyze mass loss rates, which increase for increasing luminosity, but with a spread that probably results from a dependence on a number of parameters (like e.g. different stellar masses and different mechanisms powering stellar winds). Instead, mass loss rates are well monitored by IR colours, especially if extended to 20 microns and beyond, where AGB envelopes behave like black bodies. From these colours the evolutionary status of various classes of C stars is discussed., Comment: 19 pages, 12 figures, 4 tables. Accepted for publication in "Astronomy & Astrophysics"

The results of a deep mid-IR ISOCAM survey of the L1551 dark molecular cloud are presented. The aim of this survey is a search for new YSO (Young Stellar Object) candidates, using two broad-band filters centred at 6.7 and 14.3 micron. Although two regions close to the centre of L1551 had to be avoided due to saturation problems, 96 sources were detected in total (76 sources at 6.7 micron and 44 sources at 14.3 micron). Using the 24 sources detected in both filters, 14 were found to have intrinsic mid-IR excess at 14.3 micron and were therefore classified as YSO candidates. Using additional observations in B, V, I, J, H and K obtained from the ground, most candidates detected at these wavelengths were confirmed to have mid-IR excess at 6.7 micron as well, and three additional YSO candidates were found. Prior to this survey only three YSOs were known in the observed region (avoiding L1551 IRS5/NE and HL/XZ Tau). This survey reveals 15 new YSO candidates, although several of these are uncertain due to their extended nature either in the mid-IR or in the optical/near-IR observations. Two of the sources with mid-IR excess are previously known YSOs, one is a brown dwarf MHO 5 and the other is the well known T Tauri star HH30, consisting of an outflow and an optically thick disk seen edge on., Comment: 14 Pages, 8 Figures

We present results from an ISOCAM survey in the two broad band filters LW2 (5-8.5 mu) and LW3 (12-18 mu) of a 0.13 square degree coverage of the Serpens Main Cloud Core. A total of 392 sources were detected in the 6.7 mu band and 139 in the 14.3 mu band to a limiting sensitivity of ~ 2 mJy. Only about 50% of the mid-IR excess sources show excesses in the near-IR J-H/H-K diagram. In the central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10 times larger than in other young embedded clusters such as rho Ophiuchi or Chamaeleon. The mid-IR fluxes of the Class I and flat-spectrum sources are found to be on the average larger than those of Class II sources. Stellar luminosities are estimated for the Class II sample, and its luminosity function is compatible with a coeval population of about 2 Myr which follows a three segment power-law IMF. For this age about 20% of the Class IIs are found to be young brown dwarf candidates. The YSOs are in general strongly clustered, the Class I sources more than the Class II sources, and there is an indication of sub-clustering. The sub-clustering of the protostar candidates has a spatial scale of 0.12 pc. These sub-clusters are found along the NW-SE oriented ridge and in very good agreement with the location of dense cores traced by millimeter data. The smallest clustering scale for the Class II sources is about 0.25 pc, similar to what was found for rho Ophiuchi. Our data show evidence that star formation in Serpens has proceeded in several phases, and that a ``microburst'' of star formation has taken place very recently, probably within the last 10^5 yrs., Comment: 25 pages, 14 figures, accepted by A&A March 18th, see also http://www.not.iac.es/~amanda

This paper presents a study of the accretion properties of 19 very low mass objects (Mstar .01-0.1 Msun) in the regions Chamaeleon I and rho-Oph For 8 objects we obtained high resolution Halpha profiles and determined mass accretion rate Macc and accretion luminosity Lacc. Pabeta is detected in emission in 7 of the 10 rho-Oph objects, but only in one in Cha I. Using objects for which we have both a determination of Lacc from Halpha and a Pabeta detection, we show that the correlation between the Pabeta luminosity and luminosity Lacc, found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass approx 0.03 Msun; L(Pab) can be used to measure Lacc also in the substellar regime. The results were less conclusive for Brgamma, which was detected only in 2 objects, neither of which had an Halpha estimate of Macc. Using the relation between L(Pab) and Lacc we determined the accretion rate for all the objects in our sample (including those with no Halpha spectrum), more than doubling the number of substellar objects with known Macc. When plotted as a function of the mass of the central object together with data from the literature, our results confirm the trend of lower Macc for lower Mstar, although with a large spread. Some of the spread is probably due to an age effect; our very young objects in rho-Oph have on average an accretion rate at least one order of magnitude higher than objects of similar mass in older regions. As a side product, we found that the width of Halpha measured at 10% peak intensity is not only a qualitative indicator of accretion, but can be used to obtain a quantitative estimate of Macc over a large mass range, from T Tauri stars to brown dwarfs. Finally, we found that some of our objects show evidence of mass-loss., Comment: 11 pages, 8 figures, A&A in press

This paper presents a study of the accretion properties of 19 very low mass objects (Mstar .01-0.1 Msun) in the regions Chamaeleon I and rho-Oph For 8 objects we obtained high resolution Halpha profiles and determined mass accretion rate Macc and accretion luminosity Lacc. Pabeta is detected in emission in 7 of the 10 rho-Oph objects, but only in one in Cha I. Using objects for which we have both a determination of Lacc from Halpha and a Pabeta detection, we show that the correlation between the Pabeta luminosity and luminosity Lacc, found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass approx 0.03 Msun; L(Pab) can be used to measure Lacc also in the substellar regime. The results were less conclusive for Brgamma, which was detected only in 2 objects, neither of which had an Halpha estimate of Macc. Using the relation between L(Pab) and Lacc we determined the accretion rate for all the objects in our sample (including those with no Halpha spectrum), more than doubling the number of substellar objects with known Macc. When plotted as a function of the mass of the central object together with data from the literature, our results confirm the trend of lower Macc for lower Mstar, although with a large spread. Some of the spread is probably due to an age effect; our very young objects in rho-Oph have on average an accretion rate at least one order of magnitude higher than objects of similar mass in older regions. As a side product, we found that the width of Halpha measured at 10% peak intensity is not only a qualitative indicator of accretion, but can be used to obtain a quantitative estimate of Macc over a large mass range, from T Tauri stars to brown dwarfs. Finally, we found that some of our objects show evidence of mass-loss., Comment: 11 pages, 8 figures, A&A in press

The results of a deep mid-IR ISOCAM survey of the L1551 dark molecular cloud are presented. The aim of this survey is a search for new YSO (Young Stellar Object) candidates, using two broad-band filters centred at 6.7 and 14.3 micron. Although two regions close to the centre of L1551 had to be avoided due to saturation problems, 96 sources were detected in total (76 sources at 6.7 micron and 44 sources at 14.3 micron). Using the 24 sources detected in both filters, 14 were found to have intrinsic mid-IR excess at 14.3 micron and were therefore classified as YSO candidates. Using additional observations in B, V, I, J, H and K obtained from the ground, most candidates detected at these wavelengths were confirmed to have mid-IR excess at 6.7 micron as well, and three additional YSO candidates were found. Prior to this survey only three YSOs were known in the observed region (avoiding L1551 IRS5/NE and HL/XZ Tau). This survey reveals 15 new YSO candidates, although several of these are uncertain due to their extended nature either in the mid-IR or in the optical/near-IR observations. Two of the sources with mid-IR excess are previously known YSOs, one is a brown dwarf MHO 5 and the other is the well known T Tauri star HH30, consisting of an outflow and an optically thick disk seen edge on., Comment: 14 Pages, 8 Figures

We present results from an ISOCAM survey in the two broad band filters LW2 (5-8.5 mu) and LW3 (12-18 mu) of a 0.13 square degree coverage of the Serpens Main Cloud Core. A total of 392 sources were detected in the 6.7 mu band and 139 in the 14.3 mu band to a limiting sensitivity of ~ 2 mJy. Only about 50% of the mid-IR excess sources show excesses in the near-IR J-H/H-K diagram. In the central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10 times larger than in other young embedded clusters such as rho Ophiuchi or Chamaeleon. The mid-IR fluxes of the Class I and flat-spectrum sources are found to be on the average larger than those of Class II sources. Stellar luminosities are estimated for the Class II sample, and its luminosity function is compatible with a coeval population of about 2 Myr which follows a three segment power-law IMF. For this age about 20% of the Class IIs are found to be young brown dwarf candidates. The YSOs are in general strongly clustered, the Class I sources more than the Class II sources, and there is an indication of sub-clustering. The sub-clustering of the protostar candidates has a spatial scale of 0.12 pc. These sub-clusters are found along the NW-SE oriented ridge and in very good agreement with the location of dense cores traced by millimeter data. The smallest clustering scale for the Class II sources is about 0.25 pc, similar to what was found for rho Ophiuchi. Our data show evidence that star formation in Serpens has proceeded in several phases, and that a ``microburst'' of star formation has taken place very recently, probably within the last 10^5 yrs., Comment: 25 pages, 14 figures, accepted by A&A March 18th, see also http://www.not.iac.es/~amanda

We present optical spectra of eight candidate brown dwarfs and a previously known T Tauri star (Sz 33) of the Chamaeleon I dark cloud. We derived spectral types based on the strength of the TiO or VO absorption bands present in the spectra of these objects as well as on the PC3 index of Martin et al. (1999). Photometric data from the literature are used to estimate the bolometric luminosities for these sources. We apply D'Antona & Mazzitelli (1997) pre-main sequence evolutionary tracks and isochrones to derive masses and ages. Based on the presence of Halpha in emission, we confirm that most of the candidates are young objects. Our sample however includes two sources for which we can only provide upper limits for the emission in Halpha; whereas these two objects are most likely foreground/background stars, higher resolution spectra are required to confirm their true nature. Among the likely cloud members, we detect one new sub-stellar object and three transition stellar/sub-stellar sources., Comment: 22 pages - manuscript format

We present the results of a mid-infrared imaging spectroscopy survey of Young Stellar Objects (YSOs) and their surrounding environment in four low-mass star formation regions: RCrA, Rho Ophiuchi, Serpens and Chamaeleon I. This survey was performed using ISOCAM and its Circular Variable Filters (CVF) and observed 42 YSO candidates. A number of spectral features were measured, most notably the 9.7um silicate feature, the bending modes of both water and CO2 ices at 6.0 and 15.2um respectively and the well-known unidentified feature at 6.8um. The strength of the unidentified feature was observed to correlate very well with that of the water ice bending mode and far less strongly with the CO2 ice bending mode. This suggests, in a manner consistent with previous observations, that the carrier of the unidentified feature is a strongly polar ice. Absorption profiles of the bending mode of CO2 ice are observed to show a significant long wavelength wing, which suggests that a significant fraction of the CO2 ice observed exists in a polar (H2O-rich) phase. The sources observed in RCrA, Rho Oph and Serpens show similar spectral characteristics, whilst the sources observed in Cha I are somewhat anomalous. However this is consistent with previous studies of this region of the Cha I cloud, which contains an unusual cluster of YSOs. We detect an apparent under-abundance of water ice towards the sources in Rho Oph. This may be indicative of differences in chemical composition between the different clouds, or may be due to evaporation. Finally the CO2:H2O ice ratios observed towards the sources in Rho Oph show significantly greater scatter than in the other regions, possibly due to varying local conditions around the YSOs in Rho Oph. (Abridged), Comment: 15 pages, 9 figures. Accepted for publication in Astronomy & Astrophysics. Version including appendix containing atlas of spectra available at http://www.ast.cam.ac.uk/~rda/cvfpaper.ps.gz

We present optical spectra of eight candidate brown dwarfs and a previously known T Tauri star (Sz 33) of the Chamaeleon I dark cloud. We derived spectral types based on the strength of the TiO or VO absorption bands present in the spectra of these objects as well as on the PC3 index of Martin et al. (1999). Photometric data from the literature are used to estimate the bolometric luminosities for these sources. We apply D'Antona & Mazzitelli (1997) pre-main sequence evolutionary tracks and isochrones to derive masses and ages. Based on the presence of Halpha in emission, we confirm that most of the candidates are young objects. Our sample however includes two sources for which we can only provide upper limits for the emission in Halpha; whereas these two objects are most likely foreground/background stars, higher resolution spectra are required to confirm their true nature. Among the likely cloud members, we detect one new sub-stellar object and three transition stellar/sub-stellar sources., Comment: 22 pages - manuscript format

We present the results of a mid-infrared imaging spectroscopy survey of Young Stellar Objects (YSOs) and their surrounding environment in four low-mass star formation regions: RCrA, Rho Ophiuchi, Serpens and Chamaeleon I. This survey was performed using ISOCAM and its Circular Variable Filters (CVF) and observed 42 YSO candidates. A number of spectral features were measured, most notably the 9.7um silicate feature, the bending modes of both water and CO2 ices at 6.0 and 15.2um respectively and the well-known unidentified feature at 6.8um. The strength of the unidentified feature was observed to correlate very well with that of the water ice bending mode and far less strongly with the CO2 ice bending mode. This suggests, in a manner consistent with previous observations, that the carrier of the unidentified feature is a strongly polar ice. Absorption profiles of the bending mode of CO2 ice are observed to show a significant long wavelength wing, which suggests that a significant fraction of the CO2 ice observed exists in a polar (H2O-rich) phase. The sources observed in RCrA, Rho Oph and Serpens show similar spectral characteristics, whilst the sources observed in Cha I are somewhat anomalous. However this is consistent with previous studies of this region of the Cha I cloud, which contains an unusual cluster of YSOs. We detect an apparent under-abundance of water ice towards the sources in Rho Oph. This may be indicative of differences in chemical composition between the different clouds, or may be due to evaporation. Finally the CO2:H2O ice ratios observed towards the sources in Rho Oph show significantly greater scatter than in the other regions, possibly due to varying local conditions around the YSOs in Rho Oph. (Abridged), Comment: 15 pages, 9 figures. Accepted for publication in Astronomy & Astrophysics. Version including appendix containing atlas of spectra available at http://www.ast.cam.ac.uk/~rda/cvfpaper.ps.gz

We present the results of the first extensive mid-infrared (IR) imaging survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main molecular cloud L1688, as well as L1689N and L1689S, have been completely surveyed for point sources at 6.7 and 14.3 micron. A total of 425 sources are detected including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the rho Ophiuchi central region. The stellar luminosity function of the complete sample of Class II YSOs is derived with a good accuracy down to L= 0.03 Lsun. A modeling of this lumino- sity function, using available pre-main sequence tracks and plausible star for- mation histories, allows us to derive the mass distribution of the Class II YSOs which arguably reflects the IMF of the embedded cluster. We estimate that the IMF in rho Ophiuchi is well described by a two-component power law with a low- mass index of -0.35+/-0.25, a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35), and a break occurring at M = 0.55+/-0.25 Msun. This IMF is flat with no evidence for a low-mass cutoff down to at least 0.06 Msun., Comment: A&A Document Class -- version 5.01, 27 pages, 10 figures v2: typos added including few changes in source numbering

We present the results of the first extensive mid-infrared (IR) imaging survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera on board the ISO satellite. The main molecular cloud L1688, as well as L1689N and L1689S, have been completely surveyed for point sources at 6.7 and 14.3 micron. A total of 425 sources are detected including 16 Class I, 123 Class II, and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR sources coincide with near-IR sources, but a large proportion of them are recognized for the first time as YSOs. Our dual-wavelength survey allows us to identify essentially all the YSOs with IR excess in the embedded cluster down to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II YSOs and represents the most complete census to date of newly formed stars in the rho Ophiuchi central region. The stellar luminosity function of the complete sample of Class II YSOs is derived with a good accuracy down to L= 0.03 Lsun. A modeling of this lumino- sity function, using available pre-main sequence tracks and plausible star for- mation histories, allows us to derive the mass distribution of the Class II YSOs which arguably reflects the IMF of the embedded cluster. We estimate that the IMF in rho Ophiuchi is well described by a two-component power law with a low- mass index of -0.35+/-0.25, a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35), and a break occurring at M = 0.55+/-0.25 Msun. This IMF is flat with no evidence for a low-mass cutoff down to at least 0.06 Msun., Comment: A&A Document Class -- version 5.01, 27 pages, 10 figures v2: typos added including few changes in source numbering

We have compiled the near infrared Point Source Catalogue (PSC) towards the Magellanic Clouds (MCs) extracted from the data obtained with the Deep Near Infrared Survey of the Southern Sky - DENIS (Epchtein et al. 1997). The catalogue covers an area of of 19.87* 16 square degrees centered on (RA, DEC)=(5h27m20s, -69o00'00'') for the Large Magellanic Cloud (LMC) and 14.7* 10 square degrees centered on (RA, DEC)=(h02m40s, -73o00'00'' for the Small Magellanic Cloud (SMC) at the epoch J2000. It contains about 1300000 sources towards the LMC and 300000 sources towards the SMC each detected in at least 2 of the 3 photometric bands involved in the survey (I, J, Ks). 70% of the detected sources are true members of the Magellanic Clouds, respectively and consist mainly of red giants, asymptotic giant branch stars and super-giants. The observations have all been made with the same instrument and the data have been calibrated and reduced uniformly. The catalogue provides a homogeneous set of photometric data., Comment: 1 figure changed, published

We have compiled the near infrared Point Source Catalogue (PSC) towards the Magellanic Clouds (MCs) extracted from the data obtained with the Deep Near Infrared Survey of the Southern Sky - DENIS (Epchtein et al. 1997). The catalogue covers an area of of 19.87* 16 square degrees centered on (RA, DEC)=(5h27m20s, -69o00'00'') for the Large Magellanic Cloud (LMC) and 14.7* 10 square degrees centered on (RA, DEC)=(h02m40s, -73o00'00'' for the Small Magellanic Cloud (SMC) at the epoch J2000. It contains about 1300000 sources towards the LMC and 300000 sources towards the SMC each detected in at least 2 of the 3 photometric bands involved in the survey (I, J, Ks). 70% of the detected sources are true members of the Magellanic Clouds, respectively and consist mainly of red giants, asymptotic giant branch stars and super-giants. The observations have all been made with the same instrument and the data have been calibrated and reduced uniformly. The catalogue provides a homogeneous set of photometric data., Comment: 1 figure changed, published

The Two Micron All Sky Survey (2MASS) will begin operations in the spring of 1997 (Skrutskie et al., this volume). The Infrared Processing and Analysis Center (IPAC) is tasked with processing ~30 GB of data that will be obtained each night by the northern and southern telescopes during the survey. The goal of this processing is to produce three primary data products: 1) the 2MASS Image Atlas that will contain ~ 10⁶ 512 x 1024 (1″/pix) images in J,H and K_s, 2) the 2MASS Point Source Catalog containing accurate astrometry and photometry for ~ 10⁸ point sources, and 3) the 2MASS Extended Source Catalog that will contain accurate positions, photometry and morphological information for ~ 10⁶ small extended sources.

Radiative transfer modelling of AGB circumstellar envelopes is applied to a sample of AGB stars previously observed with the mid-IR imaging camera TIRCAM (Busso et al. 1996: Paper I). We present the results of our simulations, aimed at deriving the physical parameters of the envelope, such as the optical depth and the radial thermal structure, the mass loss and the dust-to-gas mass ratio. The chemical composition of the dust in the observed envelopes is discussed. The ability of different sets of dust opacities to fit the mid-infrared spectra is evaluated. The hypothesis of dust grain aging and annealing in O-rich envelopes is considered in order to explain an apparent inadequacy of the availabe opacities to describe the variety of observed spectra, as previously noted by other authors. Various possible origins of the discrepancies are discussed, together with their consequences on the dust grain formation processes., Comment: 19 pages, LaTex, 5 Postscript figures, uses psfig.sty Submitted to Astronomy and Astrophysics. Gzipped Compressed Postscript also available at http://www.sissa.it/~marengo/sci/tirpap2/tirpap2.ps.gz

Radiative transfer modelling of AGB circumstellar envelopes is applied to a sample of AGB stars previously observed with the mid-IR imaging camera TIRCAM (Busso et al. 1996: Paper I). We present the results of our simulations, aimed at deriving the physical parameters of the envelope, such as the optical depth and the radial thermal structure, the mass loss and the dust-to-gas mass ratio. The chemical composition of the dust in the observed envelopes is discussed. The ability of different sets of dust opacities to fit the mid-infrared spectra is evaluated. The hypothesis of dust grain aging and annealing in O-rich envelopes is considered in order to explain an apparent inadequacy of the availabe opacities to describe the variety of observed spectra, as previously noted by other authors. Various possible origins of the discrepancies are discussed, together with their consequences on the dust grain formation processes., Comment: 19 pages, LaTex, 5 Postscript figures, uses psfig.sty Submitted to Astronomy and Astrophysics. Gzipped Compressed Postscript also available at http://www.sissa.it/~marengo/sci/tirpap2/tirpap2.ps.gz

We study the long-term behaviour of the equatorial disc of the Be/X-ray binary X Persei (X Per), combining new low-resolution IUE spectra and IR photometry with UV, optical and IR observations collected from the literature. We find that the near-UV continuum level of X Per varies along with the optical brightness. From the UV observations, we also find that during optical high states the flux excess due to the intrinsic stellar variability and/or electron scattering in the disc is at most 15–20 per cent of the photospheric flux. From the data taken in discless and near-discless states (optical low states) we show that the stellar photosphere can be modelled with Teff=31000 K and log g=4. With this model we derive E(B?V)=0.39, and estimate the distance to X Per as 950±200 pc (assuming R*=9 R?). We fit the (quasi-)simultaneous optical and IR photometry with a simple disc model including free–bound and free–free radiation. We find that the density of the disc at the photosphere of the star varies along with the brightness of X Per, and that in optical high states the disc in X Per is among the densest of all Be stars: ?0=(1.5±0.3)×10?10 g cm?3. The disc density at the photosphere varies by a factor of at least 20 from optical high to low states. During disc build-up and break-down phases, and also in phases when the disc is relatively stable, we find a very steep radial density gradient of the disc of X Per. This may reflect the limitations of some of the assumptions in our model. We find that in a disc-loss event the disc loses mass at a rate of about 5×10?9 M? yr?1. For a disc build-up phase we find a disc-mass growth rate of about 4×10?9 M? yr?1. This growth rate is consistent with a model that feeds the disc from the 'ordinary' mass-loss of the star, but we cannot exclude the possibility that other phenomena contribute to the disc growth as well.

We present the initial highlights of the HOBYS key program, which are based on Herschel images of the Rosette molecular complex and maps of the RCW120 H II region. Using both SPIRE at 250/350/500 μm and PACS at 70/160 μm or 100/160 μm, the HOBYS survey provides an unbiased and complete census of intermediate- to high-mass young stellar objects, some of which are not detected by Spitzer. Key core properties, such as bolometric luminosity and mass (as derived from spectral energy distributions), are used to constrain their evolutionary stages. We identify a handful of high-mass prestellar cores and show that their lifetimes could be shorter in the Rosette molecular complex than in nearby low-mass star-forming regions. We also quantify the impact of expanding H II regions on the star formation process acting in both Rosette and RCW 120.

We present a preliminary analysis of the small-scale structure found in new 70-520 μm continuum maps of the Rosette molecular cloud (RMC), obtained with the SPIRE and PACS instruments of the Herschel Space Observatory. We find 473 clumps within the RMC using a new structure identification algorithm, with sizes up to ~1.0 pc in diameter. A comparison with recent Spitzer maps reveals that 371 clumps are "starless" (without an associated young stellar object), while 102 are "protostellar". Using the respective values of dust temperature, we determine the clumps have masses (Mc) over the range -0.75 ≤ log (Mc/M⌂) ≤ 2.50. Linear fits to the high-mass tails of the resulting clump mass spectra (CMS) have slopes that are consistent with those found for high-mass clumps identified in CO emission by other groups.

Aims: Fundamental to any theory of high-mass star formation are gravity and turbulence. Their relative importance, which probably changes during cloud evolution, is not known. By investigating the spatial and density structure of the high-mass star-forming complex NGC 6334 we aim to disentangle the contributions of turbulence and gravity. Methods: We used Herschel PACS and SPIRE imaging observations from the HOBYS key programme at wavelengths of 160, 250, 350, and 500 μm to construct dust temperature and column density maps. Using probability distribution functions (PDFs) of the column density determined for the whole complex and for four distinct sub-regions (distinguished on the basis of differences in the column density, temperature, and radiation field), we characterize the density structure of the complex. We investigate the spatial structure using the Δ-variance, which probes the relative amount of structure on different size scales and traces possible energy injection mechanisms into the molecular cloud. Results: The Δ-variance analysis suggests that the significant scales of a few parsec that were found are caused by energy injection due to expanding HII regions, which are numerous, and by the lengths of filaments seen everywhere in the complex. The column density PDFs have a lognormal shape at low densities and a clearly defined power law at high densities for all sub-regions whose slope is linked to the exponent α of an equivalent spherical density distribution. In particular with α = 2.37, the central sub-region is largly dominated by gravity, caused by individual collapsing dense cores and global collapse of a larger region. The collapse is faster than free-fall (which would lead only to α = 2) and thus requires a more dynamic scenario (external compression, flows). The column density PDFs suggest that the different sub-regions are at different evolutionary stages, especially the

Context. The Rosette molecular cloud is promoted as the archetype of a triggered star-formation site. This is mainly due to its morphology, because the central OB cluster NGC 2244 has blown a circular-shaped cavity into the cloud and the expanding H II-region now interacts with the cloud. Aims. Studying the spatial distribution of the different evolutionary states of all star-forming sites in Rosette and investigating possible gradients of the dust temperature will help to test the "triggered star-formation" scenario in Rosette. Methods. We use continuum data obtained with the PACS (70 and 160 μm) and SPIRE instruments (250, 350, 500 μm) of the Herschel telescope during the science demonstration phase of HOBYS. Results. Three-color images of Rosette impressively show how the molecular gas is heated by the radiative impact of the NGC 2244 cluster. A clear negative temperature gradient and a positive density gradient (running from the H II-region/molecular cloud interface into the cloud) are detected. Studying the spatial distribution of the most massive dense cores (size scale 0.05 to 0.3 pc), we find an age-sequence (from more evolved to younger) with increasing distance to the cluster NGC 2244. No clear gradient is found for the clump (size-scale up to 1 pc) distribution. Conclusions. The existence of temperature and density gradients and the observed age-sequence imply that star formation in Rosette may indeed be influenced by the radiative impact of the central NGC 2244 cluster. A more complete overview of the prestellar and protostellar population in Rosette is required to obtain a firmer result

To constrain models of high-mass star formation, the Herschel/HOBYS KP aims at discovering massive dense cores (MDCs) able to host the high-mass analogs of low-mass prestellar cores, which have been searched for over the past decade. We here focus on NGC 6334, one of the best-studied HOBYS molecular cloud complexes. We used Herschel PACS and SPIRE 70-500 µm images of the NGC 6334 complex complemented with (sub)millimeter and mid-infrared data. We built a complete procedure to extract ~0.1 pc dense cores with the getsources software, which simultaneously measures their far-infrared to millimeter fluxes. We carefully estimated the temperatures and masses of these dense cores from their spectral energy distributions (SEDs). We also identified the densest pc-scale cloud structures of NGC 6334, one 2 px x 1 pc ridge and two 0.8 pc x 0.8 pc hubs, with volume-averaged densities of ~105 cm-3. A cross-correlation with high-mass star formation signposts suggests a mass threshold of 75 Mʘ for MDCs in NGC 6334. MDCs have temperatures of 9.5-40K, masses of 75-1000 Mʘ, and densities of 1 x 105- 7 x 107 cm-3. Their mid-infrared emission is used to separate 6 IR-bright and 10 IR-quiet protostellar MDCs while their 70 µm emission strength, with respect to fitted SEDs, helps identify 16 starless MDC candidates. The ability of the latter to host high-mass prestellar cores is investigated here and remains questionable. An increase in mass and density from the starless to the IR-quiet and IR-bright phases suggests that the protostars and MDCs simultaneously grow in mass. The statistical lifetimes of the high-mass prestellar and protostellar core phases, estimated to be 1-7 x 104 yr and at most 3 x 105 yr respectively, suggest a dynamical scenario of high-mass star formation. The present study provides good mass estimates for a statistically significant sample, covering

The Herschel OB young stellar objects survey (HOBYS) has observed the Rosette molecular cloud, providing an unprecedented view of its star formation activity. These new far-infrared data reveal a population of compact young stellar objects whose physical properties we aim to characterise. We compiled a sample of protostars and their spectral energy distributions that covers the near-infrared to submillimetre wavelength range. These were used to constrain key properties in the protostellar evolution, bolometric luminosity, and envelope mass and to build an evolutionary diagram. Several clusters are distinguished including the cloud centre, the embedded clusters in the vicinity of luminous infrared sources, and the interaction region. The analysed protostellar population in Rosette ranges from 0.1 to about 15?M? with luminosities between 1 and 150L??, which extends the evolutionary diagram from low-mass protostars into the high-mass regime. Some sources lack counterparts at near- to mid-infrared wavelengths, indicating extreme youth. The central cluster and the Phelps & Lada 7 cluster appear less evolved than the remainder of the analysed protostellar population. For the central cluster, we find indications that about 25% of the protostars classified as Class?I from near- to mid-infrared data are actually candidate Class?0 objects. As a showcase for protostellar evolution, we analysed four protostars of low- to intermediate-mass in a single dense core, and they represent different evolutionary stages from Class?0 to Class?I. Their mid- to far-infrared spectral slopes flatten towards the Class?I stage, and the 160 to 70??m flux ratio is greatest for the presumed Class?0 source. This shows that the Herschel observations characterise the earliest stages of protostellar evolution in detail.