Search

Nitrogen-bearing complex organic molecules have been commonly detected in the gas phase but not yet in interstellar ices. This has led to the long-standing question of whether these molecules form in the gas phase or in ices. $\textit{James Webb}$ Space Telescope ($\textit{JWST}$) offers the sensitivity, spectral resolution, and wavelength coverage needed to detect them in ices and investigate whether their abundance ratios are similar in gas and ice. We report the first tentative detection of CH$_3$CN, C$_2$H$_5$CN, and the simple molecule, N$_2$O, based on the CN-stretch band in interstellar ices toward three (HOPS 153, HOPS 370, and IRAS 20126+4104) out of the five protostellar systems observed as part of the Investigating Protostellar Accretion (IPA) GO program with $\textit{JWST}$-NIRSpec. We also provide upper limits for the two other sources with smaller luminosities in the sample. We detect OCN$^-$ in the ices of all sources with typical CH$_3$CN/OCN$^-$ ratios of around 1. Ice and gas column density ratios of the nitrogen-bearing species with respect to each other are better matched than those with respect to methanol, which are a factor of ${\sim}5$ larger in the ices than the gas. We attribute the elevated ice column densities with respect to methanol to the difference in snowline locations of nitrogen-bearing molecules and of methanol, biasing the gas-phase observations toward fewer nitrogen-bearing molecules. Moreover, we find tentative evidence for enhancement of OCN$^-$, CH$_3$CN, and C$_2$H$_5$CN in warmer ices, although formation of these molecules likely starts along with methanol in the cold prestellar phase. Future surveys combining NIRSpec and MIRI, and additional laboratory spectroscopic measurements of C$_2$H$_5$CN ice, are necessary for robust detection and conclusions on the formation history of complex cyanides., Comment: Accepted for publication in A&A

We review the use of young low mass stars and protostars, or young stellar objects (YSOs), as tracers of star formation. Observations of molecular clouds at visible, infrared, radio and X-ray wavelengths can identify and characterize the YSOs populating these clouds, with the ability to detect deeply embedded objects and all evolutionary stages. Surveys with the Spitzer, Herschel, XMM-Newton and Chandra space telescopes have measured the spatial distribution of YSOs within a number of nearby (< 2.5 kpc) molecular clouds, showing surface densities varying by more than three orders of magnitude. These surveys have been used to measure the spatially varying star formation rates and efficiencies within clouds, and when combined with maps of the molecular gas, have led to the discovery of star-forming relations within clouds. YSO surveys can also characterize the structures, ages, and star formation histories of embedded clusters, and they illuminate the relationship of the clusters to the networks of filaments, hubs and ridges in the molecular clouds from which they form. Measurements of the proper motions and radial velocities of YSOs trace the evolving kinematics of clusters from the deeply embedded phases through gas dispersal, providing insights into the factors that shape the formation of bound clusters. On 100 pc scales that encompass entire star-forming complexes, Gaia is mapping the young associations of stars that have dispersed their natal gas and exist alongside molecular clouds. These surveys reveal the complex structures and motions in associations, and show evidence for supernova driven expansions. Remnants of these associations have now been identified by Gaia, showing that traces of star-forming structures can persist for a few hundred million years., Comment: Review article accepted to PASP

We use hydrodynamical simulations of star-forming gas with stellar feedback and sink particles (proxies for young stellar objects, i.e., YSOs) to produce and analyze synthetic 1.1mm continuum observations at different distances (150 - 1000pc) and ages (0.49 - 1.27 Myr). We characterize how the inferred core properties, including mass, size, and clustering with respect to diffuse natal gas structure, change with distance, cloud evolution, and the presence of YSOs. We find that atmospheric filtering and core segmentation treatments have distance-dependent impacts on the resulting core properties for d < 300pc and 500pc, respectively, which dominate over evolutionary differences. Concentrating on synthetic observations at further distances (650-1000pc), we find a growing separation between the inferred sizes and masses of cores with and without YSOs in the simulations, which is not seen in recent observations of the Mon R2 cloud at 860pc. We find that the synthetic cores cluster in smaller groups, and their mass densities are correlated with gas column density over a much narrower range, than the Mon R2 observations. Such differences limit applicability of the evolutionary predictions we report here and motivate future efforts to adapt our synthetic observation and analysis framework to next generation simulations such as STARFORGE. These predictions and systematic characterizations will help guide analysis of cores for the upcoming TolTEC Clouds to Cores Legacy Survey on the Large Millimeter Telescope Alfonso Serrano (LMT)., Comment: Accepted for publication in ApJ

Context: The study of dust emission at millimeter wavelengths is important to shed light on the dust properties and physical structure of pre-stellar cores, the initial conditions in the process of star and planet formation. Aims: Using two new continuum facilities, AzTEC at the LMT and MUSTANG-2 at the GBO, we aim to detect changes in the optical properties of dust grains as a function of radius for the well-known pre-stellar core L1544. Methods: We determine the emission profiles at 1.1 and 3.3 mm and examine whether they can be reproduced in terms of the current best physical models for L1544. We also make use of various tools to determine the radial distributions of the density, temperature, and the dust opacity in a self-consistent manner. Results: We find that our observations cannot be reproduced without invoking opacity variations. With the new data, new temperature and density profiles, as well as opacity variations across the core, have been derived. The opacity changes are consistent with the expected variations between uncoagulated bare grains, toward the outer regions of the core, and grains with thick ice mantles, toward the core center. A simple analytical grain growth model predicts the presence of grains of ~3-4 um within the central 2000 au for the new density profile., Comment: Accepted for publication in A&A

We present a 1.1~mm census of dense cores in the Mon~R2 Giant Molecular Cloud with the AzTEC instrument on the Large Millimeter Telescope (LMT). We detect 295 cores (209 starless, and 86 with protostars) in a two square degree shallow survey. We also carry out a deep follow-up survey of 9 regions with low to intermediate ($3$10~$M_{\odot}$ are exclusively found in regions with high core number densities and that the CMF of the starless cores has an excess of low-mass cores ($<$5~$M_{\odot}$) compared to the CMF of protostellar cores. We report a power law correlation of index $1.99 \pm 0.03$ between local core mass density and gas column density (as traced by Herschel) over a wide range of size scales (0.3-5~pc). This power law is consistent with that predicted for thermal fragmentation of a self-gravitating sheet., Comment: 22 pages, 18 figures, accepted for publications in MNRAS

We present a study of the kinematics and structure of the Cep OB3b cluster based on new spectra obtained with the Hectoschelle spectrograph on the MMT and data from Spitzer, Chandr}, and Gaia. At a distance of 819+/-16 pc, Cep OB3b is one of the closest examples of a young (~3 - 5 Myr), large (~3000 total members) cluster at the late stages of gas dispersal. The cluster is broken into two sub-clusters surrounded by a lower density halo. We fit the empirical density law of King (1962) to each sub-cluster to constrain their sizes and structure. The richer eastern sub-cluster has circular symmetry, a modest central density, and lacks molecular gas toward its core suggesting it has undergone expansion due to gas dispersal. In contrast, the western sub-cluster deviates from circular symmetry, has a smaller core size, and contains significant molecular gas near its core, suggesting that it is in an earlier phase of gas dispersal. We present posterior probability distributions for the velocity dispersions from the Hectoschelle spectra. The east will continue to expand and likely form a bound cluster with ~35% of stars remaining. The west is undergoing slower gas dispersal and will potentially form a bound cluster with ~75% of stars remaining. If the halo dissipates, this will leave two independent clusters with ~300 members; proper motions suggest that the two sub-clusters are not bound to each other., Comment: 44 pages, 15 figures, 9 tables, accepted in ApJ

Sensitive, imaging observations of the 1.1 mm dust continuum emission from a 1 deg^2 area collected with the AzTEC bolometer camera on the Large Millimeter Telescope are presented. A catalog of 1545 compact sources is constructed based on a Wiener-optimization filter. These sources are linked to larger clump structures identified in the Bolocam Galactic Plane Survey. Hydrogen column densities are calculated for all sources and mass and mean volume densities are derived for the subset of sources for which kinematic distances can be assigned. The AzTEC sources are localized, high density peaks within the massive clumps of molecular clouds and comprise 5-15% of the clump mass. We examine the role of the gravitational instability in generating these fragments by comparing the mass of embedded AzTEC sources to the Jeans' mass of the parent BGPS object. For sources with distances less than 6 kpc the fragment masses are comparable to the clump Jeans' mass, despite having isothermal Mach numbers between 1.6 and 7.2. AzTEC sources linked to ultra-compact HII regions have mass surface densities greater than the critical value implied by the mass-size relationship of infrared dark clouds with high mass star formation while AzTEC sources associated with Class II methanol masers have mass surface densities greater than 0.7 g cm^{-2} that approaches the proposed threshold required to form massive stars., Comment: 14 pages, 9 figures, 6 tables. Accepted for publication in MNRAS

Filamentary structures are ubiquitous in molecular clouds, and have been recently argued to play an important role in regulating the size and mass of embedded clumps through fragmentation and mass accretion. Here, we reveal the dynamical state and fragmentation of filamentary molecular gas associated with the Serpens South protocluster through analysis of wide (~4 x 4 pc) observations of NH3 (1,1) and (2,2) inversion transitions with the Green Bank Telescope. Detailed modeling of the NH3 lines reveals that the kinematics of the cluster and surrounding filaments are complex. We identify hierarchical structure using a dendrogram analysis of the NH3 emission. The distance between neighbour structures that are embedded within the same parent structure is generally greater than expected from a spherical Jeans analysis, and is in better agreement with cylindrical fragmentation models. The NH3 line width-size relation is flat, and average gas motions are sub- or trans-sonic over all physical scales observed. Subsonic regions extend far beyond the typical 0.1 pc scale previously identified in star-forming cores. As a result, we find a strong trend of decreasing virial parameter with increasing structure mass in Serpens South. Extremely low virial parameters on the largest scales probed by our data suggest that the previously observed, ordered magnetic field is insufficient to support the region against collapse, in agreement with large radial infall motions previously measured toward some of the filaments. A more complex magnetic field configuration in the dense gas, however, may be able to support the filaments., Comment: 20 pages, 12 figures. Accepted to ApJ

We present new deep UBVRI images and high-resolution multi-object optical spectroscopy of the young (~ 6 - 10 Myr old), relatively nearby (800 pc) open cluster IC 2395. We identify nearly 300 cluster members and use the photometry to estimate their spectral types, which extend from early B to middle M. We also present an infrared imaging survey of the central region using the IRAC and MIPS instruments on board the Spitzer Space Telescope, covering the wavelength range from 3.6 to 24 microns. Our infrared observations allow us to detect dust in circumstellar disks originating over a typical range of radii ~ 0.1 to ~ 10AU from the central star. We identify 18 Class II, 8 transitional disk, and 23 debris disk candidates, respectively 6.5%, 2.9%, and 8.3% of the cluster members with appropriate data. We apply the same criteria for transitional disk identification to 19 other stellar clusters and associations spanning ages from ~ 1 to ~ 18 Myr. We find that the number of disks in the transitional phase as a fraction of the total with strong 24 micron excesses ([8] - [24] > 1.5) increases from 8.4 +\- 1.3% at ~ 3 Myr to 46 +\- 5% at ~ 10 Myr. Alternative definitions of transitional disks will yield different percentages but should show the same trend., Comment: accepted by the Astrophysical Journal

We present a far-IR survey of the entire Mon R2 GMC with $Herschel-SPIRE$ cross-calibrated with $Planck-HFI$ data. We fit the SEDs of each pixel with a greybody function and an optimal beta value of 1.8. We find that mid-range column densities obtained from far-IR dust emission and near-IR extinction are consistent. For the entire GMC, we find that the column density histogram, or N-PDF, is lognormal below $\sim$10$^{21}$ cm$^{-2}$. Above this value, the distribution takes a power law form with an index of -2.16. We analyze the gas geometry, N-PDF shape, and YSO content of a selection of subregions in the cloud. We find no regions with pure lognormal N-PDFs. The regions with a combination of lognormal and one power law N-PDF have a YSO cluster and a corresponding centrally concentrated gas clump. The regions with a combination of lognormal and two power law N-PDF have significant numbers of typically younger YSOs but no prominent YSO cluster. These regions are composed of an aggregate of closely spaced gas filaments with no concentrated dense gas clump. We find that for our fixed scale regions, the YSO count roughly correlates with the N-PDF power law index. The correlation appears steeper for single power law regions relative to two power law regions with a high column density cut-off, as a greater dense gas mass fraction is achieved in the former. A stronger correlation is found between embedded YSO count and the dense gas mass among our regions., Comment: 15 pages, 15 figures, 3 tables. Accepted in MNRAS

Newborn stars form within the localized, high density regions of molecular clouds. The sequence and rate at which stars form in dense clumps and the dependence on local and global environments are key factors in developing descriptions of stellar production in galaxies. We seek to observationally constrain the rate and latency of star formation in dense massive clumps that are distributed throughout the Galaxy and to compare these results to proposed prescriptions for stellar production. A sample of 24 micron-based Class~I protostars are linked to dust clumps that are embedded within molecular clouds selected from the APEX Telescope Large Area Survey of the Galaxy. We determine the fraction of star-forming clumps, f*, that imposes a constraint on the latency of star formation in units of a clump's lifetime. Protostellar masses are estimated from models of circumstellar environments of young stellar objects from which star formation rates are derived. Physical properties of the clumps are calculated from 870 micron dust continuum emission and NH_3 line emission. Linear correlations are identified between the star formation rate surface density, Sigma_{SFR}, and the quantities Sigma_{H2}/tau_{ff} and Sigma_{H2}/tau_{cross}, suggesting that star formation is regulated at the local scales of molecular clouds. The measured fraction of star forming clumps is 23%. Accounting for star formation within clumps that are excluded from our sample due to 24 micron saturation, this fraction can be as high as 31%. Dense, massive clumps form primarily low mass (< 1-2 msun) stars with emergent 24 micron fluxes below our sensitivity limit or are incapable of forming any stars for the initial 70% of their lifetimes. The low fraction of star forming clumps in the Galactic center relative to those located in the disk of the Milky Way is verified., Comment: Accepted for publication in Astronomy and Astrophysics, 16 pages, 10 figures, 4 tables

We analyze the spatial distribution of dusty young stellar objects (YSOs) identified in the Spitzer Survey of the Orion Molecular clouds, augmenting these data with Chandra X-ray observations to correct for incompleteness in dense clustered regions. We also devise a scheme to correct for spatially varying incompleteness when X-ray data are not available. The local surface densities of the YSOs range from 1 pc$^{-2}$ to over 10,000 pc$^{-2}$, with protostars tending to be in higher density regions. This range of densities is similar to other surveyed molecular clouds with clusters, but broader than clouds without clusters. By identifying clusters and groups as continuous regions with surface densities $\ge10$ pc$^{-2}$, we find that 59% of the YSOs are in the largest cluster, the Orion Nebular Cluster (ONC), while 13% of the YSOs are found in a distributed population. A lower fraction of protostars in the distributed population is evidence that it is somewhat older than the groups and clusters. An examination of the structural properties of the clusters and groups show that the peak surface densities of the clusters increase approximately linearly with the number of members. Furthermore, all clusters with more than 70 members exhibit asymmetric and/or highly elongated structures. The ONC becomes azimuthally symmetric in the inner 0.1 pc, suggesting that the cluster is only $\sim 2$ Myr in age. We find the star formation efficiency (SFE) of the Orion B cloud is unusually low, and that the SFEs of individual groups and clusters are an order of magnitude higher than those of the clouds. Finally, we discuss the relationship between the young low mass stars in the Orion clouds and the Orion OB 1 association, and we determine upper limits to the fraction of disks that may be affected by UV radiation from OB stars or by dynamical interactions in dense, clustered regions., Comment: Accepted to AJ. Higher resolution and two column version of paper as well as photometry data available at http://astro1.physics.utoledo.edu/~megeath/Orion/The_Spitzer_Orion_Survey.html

We present mid-IR (19 - 37 microns) imaging observations of S106 from SOFIA/FORCAST, complemented with IR observations from Spitzer/IRAC (3.6 - 8.0 microns), IRTF/MIRLIN (11.3 and 12.5 microns), and Herschel/PACS (70 and 160 microns). We use these observations, observations in the literature, and radiation transfer modeling to study the heating and composition of the warm (~ 100 K) dust in the region. The dust is heated radiatively by the source S106 IR, with little contributions from grain-electron collisions and Ly-alpha radiation. The dust luminosity is >~ (9.02 +/- 1.01) x 10^4 L_sun, consistent with heating by a mid- to late-type O star. We find a temperature gradient (~ 75 - 107 K) in the lobes, which is consistent with a dusty equatorial geometry around S106 IR. Furthermore, the SOFIA observations resolve several cool (~ 65 - 70 K) lanes and pockets of warmer (~ 75 - 90 K) dust in the ionization shadow, indicating that the environment is fragmented. We model the dust mass as a composition of amorphous silicates, amorphous carbon, big grains, very small grains, and PAHs. We present the relative abundances of each grain component for several locations in S106., Comment: 18 pages, 11 figures, 4 tables, accepted by ApJ

As part of the Young Stellar Object VARiability (YSOVAR) program, we monitored NGC 1333 for ~35 days at 3.6 and 4.5 um using the Spitzer Space Telescope. We report here on the mid-infrared variability of the point sources in the ~10x~20arcmin area centered on 03:29:06, +31:19:30 (J2000). Out of 701 light curves in either channel, we find 78 variables over the YSOVAR campaign. About half of the members are variable. The variable fraction for the most embedded SEDs (Class I, flat) is higher than that for less embedded SEDs (Class II), which is in turn higher than the star-like SEDs (Class III). A few objects have amplitudes (10-90th percentile brightness) in [3.6] or [4.5]>0.2 mag; a more typical amplitude is 0.1-0.15 mag. The largest color change is >0.2 mag. There are 24 periodic objects, with 40% of them being flat SED class. This may mean that the periodic signal is primarily from the disk, not the photosphere, in those cases. We find 9 variables likely to be 'dippers', where texture in the disk occults the central star, and 11 likely to be 'bursters', where accretion instabilities create brightness bursts. There are 39 objects that have significant trends in [3.6]-[4.5] color over the campaign, about evenly divided between redder-when-fainter (consistent with extinction variations) and bluer-when-fainter. About a third of the 17 Class 0 and/or jet-driving sources from the literature are variable over the YSOVAR campaign, and a larger fraction (~half) are variable between the YSOVAR campaign and the cryogenic-era Spitzer observations (6-7 years), perhaps because it takes time for the envelope to respond to changes in the central source. The NGC 1333 brown dwarfs do not stand out from the stellar light curves in any way except there is a much larger fraction of periodic objects (~60% of variable brown dwarfs are periodic, compared to ~30% of the variables overall)., Comment: 84 pages, 43 figures. Accepted by AJ

We present an IR-monitoring survey with the $Spitzer$ Space Telescope of the star forming region GGD 12-15. Over 1000 objects were monitored including about 350 objects within the central 5 arcminutes which is found to be especially dense in cluster members. The monitoring took place over 38 days and is part of the Young Stellar Object VARiability (YSOVAR) project. The region was also the subject of a contemporaneous 67ks $Chandra$ observation. The field includes 119 previously identified pre-main sequence star candidates. X-rays are detected from 164 objects, 90 of which are identified with cluster members. Overall, we find that about half the objects in the central 5 arcminutes are young stellar objects based on a combination of their spectral energy distribution, IR variability and X-ray emission. Most of the stars with IR excess relative to a photosphere show large amplitude (>0.1 mag) mid-IR variability. There are 39 periodic sources, all but one of these is found to be a cluster member. Almost half of the periodic sources do not show IR excesses. Overall, more than 85% of the Class I, flat spectrum, and Class II sources are found to vary. The amplitude of the variability is larger in more embedded young stellar objects. Most of the Class~I/II objects exhibit redder colors in a fainter state, compatible with time-variable extinction. A few become bluer when fainter, which can be explained with significant changes in the structure of the inner disk. A search for changes in the IR due to X-ray events is carried out, but the low number of flares prevented an analysis of the direct impact of X-ray flares on the IR lightcurves. However, we find that X-ray detected Class II sources have longer timescales for change in the mid-IR than a similar set of non-X-ray detected Class IIs., Comment: Accepted by AJ 47 (large print) pages including 20 figures and 4 tables

We present a time-variability study of young stellar objects in the cluster IRAS 20050+2720, performed at 3.6 and 4.5 micron with the Spitzer Space Telescope; this study is part of the Young Stellar Object VARiability project (YSOVAR). We have collected light curves for 181 cluster members over 40 days. We find a high variability fraction among embedded cluster members of ca. 70%, whereas young stars without a detectable disk display variability less often (in ca. 50% of the cases) and with lower amplitudes. We detect periodic variability for 33 sources with periods primarily in the range of 2-6 days. Practically all embedded periodic sources display additional variability on top of their periodicity. Furthermore, we analyze the slopes of the tracks that our sources span in the color-magnitude diagram (CMD). We find that sources with long variability time scales tend to display CMD slopes that are at least partially influenced by accretion processes, while sources with short variability time scales tend to display extinction-dominated slopes. We find a tentative trend of X-ray detected cluster members to vary on longer time scales than the X-ray undetected members., Comment: Accepted for publication in The Astronomical Journal. Data from Table 2 is given as a machine-readable file in the source of this arXiv submission

We present SCUBA-2 450micron and 850micron observations of the Serpens MWC 297 region, part of the JCMT Gould Belt Survey of nearby star-forming regions. Simulations suggest that radiative feedback influences the star-formation process and we investigate observational evidence for this by constructing temperature maps. Maps are derived from the ratio of SCUBA-2 fluxes and a two component model of the JCMT beam for a fixed dust opacity spectral index of beta = 1.8. Within 40 of the B1.5Ve Herbig star MWC 297, the submillimetre fluxes are contaminated by free-free emission with a spectral index of 1.03+-0.02, consistent with an ultra-compact HII region and polar winds/jets. Contamination accounts for 73+-5 per cent and 82+-4 per cent of peak flux at 450micron and 850micron respectively. The residual thermal disk of the star is almost undetectable at these wavelengths. Young Stellar Objects are confirmed where SCUBA-2 850micron clumps identified by the fellwalker algorithm coincide with Spitzer Gould Belt Survey detections. We identify 23 objects and use Tbol to classify nine YSOs with masses 0.09 to 5.1 Msun. We find two Class 0, one Class 0/I, three Class I and three Class II sources. The mean temperature is 15+-2K for the nine YSOs and 32+-4K for the 14 starless clumps. We observe a starless clump with an abnormally high mean temperature of 46+-2K and conclude that it is radiatively heated by the star MWC 297. Jeans stability provides evidence that radiative heating by the star MWC 297 may be suppressing clump collapse., Comment: 24 pages, 13 figures, 7 tables

The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope observing program obtained the first extensive mid-infrared (3.6 & 4.5 um) time-series photometry of the Orion Nebula Cluster plus smaller footprints in eleven other star-forming cores (AFGL490, NGC1333, MonR2, GGD 12-15, NGC2264, L1688, Serpens Main, Serpens South, IRAS 20050+2720, IC1396A, and Ceph C). There are ~29,000 unique objects with light curves in either or both IRAC channels in the YSOVAR data set. We present the data collection and reduction for the Spitzer and ancillary data, and define the "standard sample" on which we calculate statistics, consisting of fast cadence data, with epochs about twice per day for ~40d. We also define a "standard sample of members", consisting of all the IR-selected members and X-ray selected members. We characterize the standard sample in terms of other properties, such as spectral energy distribution shape. We use three mechanisms to identify variables in the fast cadence data--the Stetson index, a chi^2 fit to a flat light curve, and significant periodicity. We also identified variables on the longest timescales possible of ~6 years, by comparing measurements taken early in the Spitzer mission with the mean from our YSOVAR campaign. The fraction of members in each cluster that are variable on these longest timescales is a function of the ratio of Class I/total members in each cluster, such that clusters with a higher fraction of Class I objects also have a higher fraction of long-term variables. For objects with a YSOVAR-determined period and a [3.6]-[8] color, we find that a star with a longer period is more likely than those with shorter periods to have an IR excess. We do not find any evidence for variability that causes [3.6]-[4.5] excesses to appear or vanish within our data; out of members and field objects combined, at most 0.02% may have transient IR excesses., Comment: Accepted to AJ; 38 figures, 93 pages

The emission from young stellar objects (YSOs) in the mid-IR is dominated by the inner rim of their circumstellar disks. We present an IR-monitoring survey of about 800 objects in the direction of the Lynds 1688 (L1688) star forming region over four visibility windows spanning 1.6 years using the \emph{Spitzer} space telescope in its warm mission phase. Among all lightcurves, 57 sources are cluster members identified based on their spectral-energy distribution and X-ray emission. Almost all cluster members show significant variability. The amplitude of the variability is larger in more embedded YSOs. Ten out of 57 cluster members have periodic variations in the lightcurves with periods typically between three and seven days, but even for those sources, significant variability in addition to the periodic signal can be seen. No period is stable over 1.6 years. Non-periodic lightcurves often still show a preferred timescale of variability which is longer for more embedded sources. About half of all sources exhibit redder colors in a fainter state. This is compatible with time-variable absorption towards the YSO. The other half becomes bluer when fainter. These colors can only be explained with significant changes in the structure of the inner disk. No relation between mid-IR variability and stellar effective temperature or X-ray spectrum is found., Comment: accepted by ApJ, 24 pages, 17 figures

The Cygnus-X star-forming complex is one of the most active regions of low and high mass star formation within 2 kpc of the Sun. Using mid-infrared photometry from the IRAC and MIPS Spitzer Cygnus-X Legacy Survey, we have identified over 1800 protostar candidates. We compare the protostellar luminosity functions of two regions within Cygnus-X: CygX-South and CygX-North. These two clouds show distinctly different morphologies suggestive of dissimilar star-forming environments. We find the luminosity functions of these two regions are statistically different. Furthermore, we compare the luminosity functions of protostars found in regions of high and low stellar density within Cygnus-X and find that the luminosity function in regions of high stellar density is biased to higher luminosities. In total, these observations provide further evidence that the luminosities of protostars depend on their natal environment. We discuss the implications this dependence has for the star formation process., Comment: Accepted to AJ. Pages 1-42 are the main text, pages 43-83 are the table of Spitzer photometry. High resolution version available at: http://astro1.physics.utoledo.edu/~megeath/cygnus_x/ms_final.pdf Full photometry table at: http://astro1.physics.utoledo.edu/~megeath/cygnus_x/photometry_table.pdf

We have detected bright HC7N J = 21-20 emission toward multiple locations in the Serpens South cluster-forming region using the K-Band Focal Plane Array at the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold filamentary structures that have yet to form stars, largely avoiding the dense gas associated with small protostellar groups and the main central cluster of Serpens South. Where detected, the HC7N abundances are similar to those found in other nearby star forming regions. Toward some HC7N `clumps', we find consistent variations in the line centroids relative to NH3 (1,1) emission, as well as systematic increases in the HC7N non-thermal line widths, which we argue reveal infall motions onto dense filaments within Serpens South with minimum mass accretion rates of M ~ 2-5 M_sun Myr^-1. The relative abundance of NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n ~ 10^4 cm^-3, for timescales t < 1-2 x 10^5 yr. Since HC7N emission peaks are rarely co-located with those of either NH3 or continuum, it is likely that Serpens South is not particularly remarkable in its abundance of HC7N, but instead the serendipitous mapping of HC7N simultaneously with NH3 has allowed us to detect HC7N at low abundances in regions where it otherwise may not have been looked for. This result extends the known star-forming regions containing significant HC7N emission from typically quiescent regions, like the Taurus molecular cloud, to more complex, active environments., Comment: 19 pages, 13 figures, accepted to MNRAS. Version with full resolution figures available at http://www.dunlap.utoronto.ca/~friesen/Friesen_HC7N.pdf

The formation of stars in massive clusters is one of the main modes of the star formation process. However, the study of massive star forming regions is hampered by their typically large distances to the Sun. One exception to this is the massive star forming region Cygnus OB2 in the Cygnus X region, at the distance of about 1400 pc. Cygnus OB2 hosts very rich populations of massive and low-mass stars, being the best target in our Galaxy to study the formation of stars, circumstellar disks, and planets in presence of massive stars. In this paper we combine a wide and deep set of photometric data, from the r band to 24 micron, in order to select the disk bearing population of stars in Cygnus OB2 and identify the class I, class II, and stars with transition and pre-transition disks. We selected 1843 sources with infrared excesses in an area of 1 degree x 1 degree centered on Cyg OB2 in several evolutionary stages: 8.4% class I, 13.1% flat-spectrum sources, 72.9% class II, 2.3% pre-transition disks, and 3.3% transition disks. The spatial distribution of these sources shows a central cluster surrounded by a annular overdensity and some clumps of recent star formation in the outer region. Several candidate subclusters are identified, both along the overdensity and in the rest of the association., Comment: Accepted by ApJ

We present an XMM-Newton survey of the part of Orion A cloud south of the Orion Nebula. This survey includes the Lynds 1641 (L1641) dark cloud, a region of the Orion A cloud with very few massive stars and hence a relatively low ambient UV flux, and the region around the O9 III star Iota Orionis. In addition to proprietary data, we used archival XMM data of the Orion Nebula Cluster (ONC) to extend our analysis to a major fraction of the Orion A cloud. We have detected 1060 X-ray sources in L1641 and Iota Ori region. About 94% of the sources have 2MASS & Spitzer counterparts, 204 and 23 being Class II and Class I or protostars objects, respectively. In addition, we have identified 489 X-ray sources as counterparts to Class III candidates, given they are bright in X-rays and appear as normal photospheres at mid-IR wavelengths. The remaining 205 X-ray sources are likely distant AGNs or other galactic sources not related to Orion A. We find that Class III candidates appear more concentrated in two main clusters in L1641. The first cluster of Class III stars is found toward the northern part of L1641, concentrated around Iota Ori. The stars in this cluster are more evolved than those in the Orion Nebula. We estimate a distance of 300-320 pc for this cluster and thus it is closer than the Orion A cloud. Another cluster rich in Class III stars is located in L1641 South and appears to be a slightly older cluster embedded in the Orion A cloud. Furthermore, other evolved Class III stars are found north of the ONC toward NGC 1977., Comment: 42 pages, 14 figures, accepted for publication in ApJ

Infrared stellar photometry from 2MASS and spectral line imaging observations of 12CO and 13CO J = 1-0 line emission from the FCRAO 14m telescope are analysed to assess the variation of the CO abundance with physical conditions throughout the Orion A and Orion B molecular clouds. Three distinct Av regimes are identified in which the ratio between the 13CO column density and visual extinction changes corresponding to the photon dominated envelope, the strongly self-shielded interior, and the cold, dense volumes of the clouds. Within the strongly self-shielded interior of the Orion A cloud, the 13CO abundance varies by 100% with a peak value located near regions of enhanced star formation activity. The effect of CO depletion onto the ice mantles of dust grains is limited to regions with AV > 10 mag and gas temperatures less than 20 K as predicted by chemical models that consider thermal-evaporation to desorb molecules from grain surfaces. Values of the molecular mass of each cloud are independently derived from the distributions of Av and 13CO column densities with a constant 13CO-to-H2 abundance over various extinction ranges. Within the strongly self-shielded interior of the cloud (Av > 3 mag), 13CO provides a reliable tracer of H2 mass with the exception of the cold, dense volumes where depletion is important. However, owing to its reduced abundance, 13CO does not trace the H2 mass that resides in the extended cloud envelope, which comprises 40-50% of the molecular mass of each cloud. The implied CO luminosity to mass ratios, M/L_{CO}, are 3.2 and 2.9 for Orion A and Orion B respectively, which are comparable to the value (2.9), derived from gamma-ray observations of the Orion region. Our results emphasize the need to consider local conditions when applying CO observations to derive H2 column densities., Comment: Accepted for publication in MNRAS. 21 pages, 14 figures

We present a survey of the Orion A and B molecular clouds undertaken with the IRAC and MIPS instruments onboard Spitzer. In total, five distinct fields were mapped covering 9 sq. degrees in five mid-IR bands spanning 3-24 microns. The survey includes the Orion Nebula Cluster, the Lynds 1641, 1630 and 1622 dark clouds, and the NGC 2023, 2024, 2068 and 2071 nebulae. These data are merged with the 2MASS point source catalog to generate a catalog of eight band photometry. We identify 3479 dusty young stellar objects (YSOs) in the Orion molecular clouds by searching for point sources with mid-IR colors indicative of reprocessed light from dusty disks or infalling envelopes. The YSOs are subsequently classified on the basis of their mid-IR colors and their spatial distributions are presented. We classify 2991 of the YSOs as pre-main sequence stars with disks and 488 as likely protostars. Most of the sources were observed with IRAC in 2-3 epochs over 6 months; we search for variability between the epochs by looking for correlated variability in the 3.6 and 4.5 micron bands. We find that 50% of the dusty YSOs show variability. The variations are typically small (0.2 mag.) with the protostars showing a higher incidence of variability and larger variations. The observed correlations between the 3.6, 4.5, 5.8 and 8 micron variability suggests that we are observing variations in the heating of the inner disk due to changes in the accretion luminosity or rotating accretion hot spots., Comment: Accepted to the Astronomical Journal. A full resolution version of the paper and the point source catalog can be found at http://astro1.physics.utoledo.edu/~megeath/Orion/The_Spitzer_Orion_Survey.html

IRAS 20050+2720 is young star forming region at a distance of 700 pc without apparent high mass stars. We present results of our multiwavelength study of IRAS 20050+2720 which includes observations by Chandra and Spitzer, and 2MASS and UBVRI photometry. In total, about 300 YSOs in different evolutionary stages are found. We characterize the distribution of young stellar objects (YSOs) in this region using a minimum spanning tree (MST) analysis. We newly identify a second cluster core, which consists mostly of class II objects, about 10 arcmin from the center of the cloud. YSOs of earlier evolutionary stages are more clustered than more evolved objects. The X-ray luminosity function (XLF) of IRAS 20050+2720 is roughly lognormal, but steeper than the XLF of the more massive Orion nebula complex. IRAS 20050+2720 shows a lower N_H/A_K ratio compared with the diffuse ISM., Comment: 15 pages, 12 figures, accepted by AJ

Eclipsing binaries (EBs) provide critical laboratories for empirically testing predictions of theoretical models of stellar structure and evolution. Pre-main-sequence (PMS) EBs are particularly valuable, both due to their rarity and the highly dynamic nature of PMS evolution, such that a dense grid of PMS EBs is required to properly calibrate theoretical PMS models. Analyzing multi-epoch, multi-color light curves for 2400 candidateOrion Nebula Cluster (ONC) members from our Warm Spitzer Exploration Science Program YSOVAR, we have identified 12 stars whose light curves show eclipse features. Four of these 12 EBs are previously known. Supplementing our light curves with follow-up optical and near-infrared spectroscopy, we establish two of the candidates as likely field EBs lying behind the ONC. We confirm the remaining six candidate systems, however, as newly identified ONC PMS EBs. These systems increase the number of known PMS EBs by over 50% and include the highest mass (Theta1 Ori E, for which we provide a complete set of well-determined parameters including component masses of 2.807 and 2.797 solar masses) and longest period (ISOY J053505.71-052354.1, P \sim 20 days) PMS EBs currently known. In two cases (Theta1 Ori E and ISOY J053526.88-044730.7), enough photometric and spectroscopic data exist to attempt an orbit solution and derive the system parameters. For the remaining systems, we combine our data with literature information to provide a preliminary characterization sufficient to guide follow-up investigations of these rare, benchmark systems., Comment: Accepted by ApJ

We present Spitzer IRAC and MIPS observations of the star-forming region containing intermediate-mass young stellar object (YSO) AFGL 490. We supplement these data with near-IR 2MASS photometry and with deep SQIID observations off the central high extinction region. We have more than doubled the known membership of this region to 57 Class I and 303 Class II YSOs via the combined 1-24 um photometric catalog derived from these data. We construct and analyze the minimum spanning tree of their projected positions, isolating one locally over-dense cluster core containing 219 YSOs (60.8% of the region's members). We find this cluster core to be larger yet less dense than similarly analyzed clusters. Although the structure of this cluster core appears irregular, we demonstrate that the parsec-scale surface densities of both YSOs and gas are correlated with a power law slope of 2.8, as found for other similarly analyzed nearby molecular clouds. We also explore the mass segregation implications of AFGL 490's offset from the center of its core, finding that it has no apparent preferential central position relative to the low-mass members., Comment: 44 pages, 13 figures, accepted to ApJ

We identify protostars in Spitzer surveys of nine star-forming molecular clouds within 1 kpc: Serpens, Perseus, Ophiuchus, Chamaeleon, Lupus, Taurus, Orion, Cep OB3, and Mon R2, which combined host over 700 protostar candidates. Our diverse cloud sample allows us to compare protostar luminosity functions in these varied environments. We combine photometry from 2MASS J, H, and Ks bands and Spitzer IRAC and MIPS 24 micron bands to create 1 - 24 micron spectral energy distributions (SEDs). Using protostars from the c2d survey with well-determined bolometric luminosities (Lbol), we derive a relationship between Lbol, L_MIR (integrated from 1 - 24 microns), and SED slope. Estimations of Lbol for protostar candidates are combined to create luminosity functions for each cloud. Contamination due to edge-on disks, reddened Class II sources, and galaxies is estimated and removed from the luminosity functions. We find that luminosity functions for high mass star forming clouds peak near 1 Lsun and show a tail extending toward luminosities above 100 Lsun. The luminosity functions of the low mass star forming clouds do not exhibit a common peak, however the combined luminosity function of these regions peaks below 1 Lsun. Finally, we examine the luminosity functions as a function of the local surface density of YSOs. In the Orion molecular cloud, we find a significant difference between the luminosity functions of protostars in regions of high and low stellar density, the former of which is biased toward more luminous sources. This may be the result of primordial mass segregation, although this interpretation is not unique. We compare our luminosity functions to those predicted by models and find that our observed luminosity functions are best matched by models which invoke competitive accretion, although we do not find strong agreement of the high mass star forming clouds with any of the models., Comment: 76 pages, 18 figures, 7 tables. Accepted for publication in the Astronomical Journal

Molecular globules and pillars are spectacular features, found only in the interface region between a molecular cloud and an HII-region. Impacting Far-ultraviolet (FUV) radiation creates photon dominated regions (PDRs) on their surfaces that can be traced by typical cooling lines. With the GREAT receiver onboard SOFIA we mapped and spectrally resolved the [CII] 158 micron atomic fine-structure line and the highly excited 12CO J=11-10 molecular line from three objects in Cygnus X (a pillar, a globule, and a strong IRAS source). We focus here on the globule and compare our data with existing Spitzer data and recent Herschel Open-Time PACS data. Extended [CII] emission and more compact CO-emission was found in the globule. We ascribe this emission mainly to an internal PDR, created by a possibly embedded star-cluster with at least one early B-star. However, external PDR emission caused by the excitation by the Cyg OB2 association cannot be fully excluded. The velocity-resolved [CII] emission traces the emission of PDR surfaces, possible rotation of the globule, and high-velocity outflowing gas. The globule shows a velocity shift of ~2 km/s with respect to the expanding HII-region, which can be understood as the residual turbulence of the molecular cloud from which the globule arose. This scenario is compatible with recent numerical simulations that emphazise the effect of turbulence. It is remarkable that an isolated globule shows these strong dynamical features traced by the [CII]-line, but it demands more observational studies to verify if there is indeed an embedded cluster of B-stars., Comment: Letter accepted by A&A (SOFIA special issue)

We report Spitzer observations of five newly identified bow shocks in the massive star-forming region RCW 38. Four are visible at IRAC wavelengths, the fifth is visible only at 24 microns. Chandra X-ray emission indicates that winds from the central O5.5 binary, IRS~2, have caused an outflow to the NE and SW of the central subcluster. The southern lobe of hot ionised gas is detected in X-rays; shocked gas and heated dust from the shock-front are detected with Spitzer at 4.5 and 24 microns. The northern outflow may have initiated the present generation of star formation, based on the filamentary distribution of the protostars in the central subcluster. Further, the bow-shock driving star, YSO 129, is photo-evaporating a pillar of gas and dust. No point sources are identified within this pillar at near- to mid-IR wavelengths. We also report on IRAC 3.6 & 5.8 micron observations of the cluster DBS2003-124, NE of RCW 38, where 33 candidate YSOs are identified. One star associated with the cluster drives a parsec-scale jet. Two candidate HH objects associated with the jet are visible at IRAC and MIPS wavelengths. The jet extends over a distance of ~3 pc. Assuming a velocity of 100 km/s for the jet material gives an age of about 30,000 years, indicating that the star (and cluster) are likely to be very young, with a similar or possibly younger age than RCW 38, and that star formation is ongoing in the extended RCW 38 region., Comment: 27 pages, 6 figures, accepted to ApJ

We present 1.1 mm observations for a sample of 16 powerful radio galaxies at 0.5

We report the discovery and characterization of a power law correlation between the local surface densities of Spitzer-identified, dusty young stellar objects and the column density of gas (as traced by near-IR extinction) in eight molecular clouds within 1 kpc and with 100 or more known YSOs. This correlation, which appears in data smoothed over size scales of ~1 pc, varies in quality from cloud to cloud; those clouds with tight correlations, MonR2 and Ophiuchus, are fit with power laws of slope 2.67 and 1.87, respectively. The spread in the correlation is attributed primarily to local gas disruption by stars that formed there or to the presence of very young sub-regions at the onset of star formation. We explore the ratio of the number of Class II to Class I sources, a proxy for the star formation age of a region, as a function of gas column density; this analysis reveals a declining Class II to Class I ratio with increasing column density. We show that the observed star-gas correlation is consistent with a star formation law where the star formation rate per area varies with the gas column density squared. We also propose a simple picture of thermal fragmentation of dense gas in an isothermal, self-gravitating layer as an explanation for the power law. Finally, we briefly compare the star gas correlation and its implied star formation law with other recent proposed of star formation laws at similar and larger size scales from nearby star forming regions., Comment: 28 pages, 18 figures, Accepted to ApJ

The Serpens South embedded cluster, which is located at the constricted part in a long filamentary infrared dark cloud, is believed to be in very early stage of cluster formation. We present results of near-infrared (JHKs) polarization observations toward the filamentary cloud. Our polarization measurements of near-infrared point sources indicate a well-ordered global magnetic field that is perpendicular to the main filament, implying that the magnetic field is likely to have controlled the formation of the main filament. On the other hand, the sub-filaments, which converge on the central part of the cluster, tend to run along the magnetic field. The global magnetic field appears to be curved in the southern part of the main filament. Such morphology is consistent with the idea that the global magnetic field is distorted by gravitational contraction along the main filament toward the northern part that contains larger mass. Applying the Chandrasekhar-Fermi method, the magnetic field strength is roughly estimated to be a few x 100 microgauss, suggesting that the filamentary cloud is close to magnetically critical as a whole., Comment: 9 pages, 9 figures, accepted for publication in ApJ

We present initial results from time series imaging at infrared wavelengths of 0.9 sq. degrees in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 micron data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (Ic) and/or near-infrared (JKs) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence (PMS) eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 & 4.5 micron variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009, and we highlight our light curves for AA-Tau analogs - YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs., Comment: Accepted to ApJ

We present early results of our multiwavelength study of the star forming region IRAS 20050+2720. While we use X-rays and IR to classify young stellar objects, the optical data can be used to exclude foreground objects. The dataset set contains 57 class I sources, 183 class II sources and 183 X-ray sources, of which 140 are class III candidates. Within IRAS 20050+2720 four subclusters are found. Subcluster A and B are the densest regions, which contain stars of all evolutionary stages. Subcluster C is much younger than the other two. It has not formed any class III objects yet. We newly identify a fourth subcluster, which consists mostly of class II objects and is located about 10 arcmin from the center of the cloud., Comment: Poster Proceedings Cool Stars 16

We present a study of the variation of spatial structure of stellar populations within dwarf galaxies as a function of the population age. We use deep Hubble Space Telescope/Advanced Camera for Surveys imaging of nearby dwarf galaxies in order to resolve individual stars and create composite colour-magnitude diagrams (CMDs) for each galaxy. Using the obtained CMDs, we select Blue Helium Burning stars (BHeBs), which can be unambiguously age-dated by comparing the absolute magnitude of individual stars with stellar isochrones. Additionally, we select a very young (<10 Myr) population of OB stars for a subset of the galaxies based on the tip of the young main-sequence. By selecting stars in different age ranges we can then study how the spatial distribution of these stars evolves with time. We find, in agreement with previous studies, that stars are born within galaxies with a high degree of substructure which is made up of a continuous distribution of clusters, groups and associations from parsec to hundreds of parsec scales. These structures disperse on timescales of tens to hundreds of Myr, which we quantify using the two-point correlation function and the Q-parameter developed by Cartwright & Whitworth (2004). On galactic scales, we can place lower limits on the time it takes to remove the original structure (i.e., structure survives for at least this long), tevo, which varies between ~100~Myr (NGC~2366) and ~350 Myr (DDO~165). This is similar to what we have found previously for the SMC (~80~Myr) and the LMC (~175 Myr). We do not find any strong correlations between tevo and the luminosity of the host galaxy., Comment: 15 pages, 14, MNRAS in press

We present a global study of low mass, young stellar object (YSO) surface densities in nearby (< 500 pc) star forming regions based on a comprehensive collection of Spitzer Space Telescope surveys. We show that the distribution of YSO surface densities in the solar neighbourhood is a smooth distribution, being adequately described by a lognormal function from a few to 10^3 YSOs per pc^2, with a peak at 22 stars/pc^2 and a dispersion of 0.85. We do not find evidence for multiple discrete modes of star-formation (e.g. clustered and distributed). Comparing the observed surface density distribution to previously reported surface density threshold definitions of clusters, we find that the fraction of stars in clusters is crucially dependent on the adopted definitions, ranging from 40 to 90%. However, we find that only a low fraction (< 26%) of stars are formed in dense environments where their formation/evolution (along with their circumstellar disks and/or planets) may be affected by the close proximity of their low-mass neighbours., Comment: 6 pages, 2 figures, MNRAS letters, accepted

We present IRAC/MIPS Spitzer Space Telescope observations of the solar type and the low mass stellar population of the young (~5 Myr) Lambda Orionis cluster. Combining optical and 2MASS photometry, we identify 436 stars as probable members of the cluster. Given the distance (450 pc) and the age of the cluster, our sample ranges in mass from 2 solar mass to objects below the substellar limit. With the addition of the Spitzer mid-infrared data, we have identified 49 stars bearing disks in the stellar cluster. Using spectral energy distribution (SED) slopes, we place objects in several classes: non-excess stars (diskless), stars with optically thick disks, stars with ``evolved disks''( with smaller excesses than optically thick disk systems), and ``transitional disks'' candidates (in which the inner disk is partially or fully cleared). The disk fraction depends on the stellar mass, ranging from ~6% for K type stars (Rc-J<2) to ~27% for stars with spectral type M5 or later (Rc-J>4). We confirm the dependence of disk fraction on stellar mass in this age range found in other studies. Regarding clustering levels, the overall fraction of disks in the Lambda Orionis cluster is similar to those reported in other stellar groups with ages normally quoted as ~5 Myr., Comment: 37 pages, 11 Figures and 3 Tables, Accepted for publication in The Astrophysical Journal

We present new images and photometry of the massive star forming complex Cygnus X obtained with IRAC and MIPS on the Spitzer Space Telescope. A combination of IRAC, MIPS, UKIDSS, and 2MASS data are used to identify and classify young stellar objects. Of the 8,231 sources detected exhibiting infrared excess in Cygnus X North, 670 are classified as Class I and 7,249 are classified as Class II. Using spectra from the FAST spectrograph at the Fred L. Whipple Observatory and Hectospec on the MMT, we spectrally typed 536 sources in the Cygnus X complex to identify the massive stars. We find that YSOs tend to be grouped in the neighborhoods of massive B stars (spectral types B0 to B9). We present a minimal spanning tree analysis of clusters in two regions in Cygnus X North. The fraction of infrared excess sources that belong to clusters with >10 members is found to be 50--70%. Most Class II objects lie in dense clusters within blown out HII regions, while Class I sources tend to reside in more filamentary structures along the bright-rimmed clouds, indicating possible triggered star formation., Comment: 16 pages, 20 figures, 7 tables, Accepted for publication in the Astrophysical Journal

We present Chandra X-ray data of the NGC 1333 embedded cluster, combining these data with existing Chandra data, Sptizer photometry and ground based spectroscopy of both the NGC 1333 & Serpens North clusters to perform a detailed study of the X-ray properties of two of the nearest embedded clusters to the Sun. In NGC 1333, a total of 95 cluster members are detected in X-rays, of which 54 were previously identified with Spitzer. Of the Spitzer sources, we detect 23% of the Class I protostars, 53% of the Flat Spectrum sources, 52% of the Class II, and 50% of the Transition Disk YSOs. Forty-one Class III members of the cluster are identified, bringing the total identified YSO population to 178. The X-ray Luminosity Functions (XLFs) of the NGC 1333 and Serpens clusters are compared to each other and the Orion Nebula Cluster. Based on this comparison, we obtain a new distance for the Serpens cluster of 360+22/-13 pc. The X-ray luminosity was found to depend on the bolometric luminosity as in previous studies of other clusters, and that Lx depends primarily on the stellar surface area. In the NGC 1333 cluster, the Class III sources have a somewhat higher X-ray luminosity for a given surface area. We also find evidence in NGC 1333 for a jump in the X-ray luminosity between spectral types of M0 and K7, we speculate that this may result from the presence of radiative zones in the K-stars. The gas column density vs. extinction in the NGC 1333 was found to be N_H = 0.89 +/- 0.13 x 10^22 A_K, this is lower than expected of the standard ISM but similar to that found previously in the Serpens Cloud Core., Comment: 58 pages, 14 figures, accepted by AJ

Using data from the SCUBA Legacy Catalogue (850 um) and Spitzer Space Telescope (3.6 - 70 um), we explore dense cores in the Ophiuchus, Taurus, Perseus, Serpens, and Orion molecular clouds. We develop a new method to discriminate submillimeter cores found by SCUBA as starless or protostellar, using point source photometry from Spitzer wide field surveys. First, we identify infrared sources with red colors associated with embedded young stellar objects (YSOs). Second, we compare the positions of these YSO-candidates to our submillimeter cores. With these identifications, we construct new, self-consistent starless and protostellar core mass functions (CMFs) for the five clouds. We find best fit slopes to the high-mass end of the CMFs of -1.26 +/- 0.20, -1.22 +/- 0.06, -0.95 +/- 0.20, and -1.67 +/- 0.72 for Ophiuchus, Taurus, Perseus, and Orion, respectively. Broadly, these slopes are each consistent with the -1.35 power-law slope of the Salpeter IMF at higher masses, but suggest some differences. We examine a variety of trends between these CMF shapes and their parent cloud properties, potentially finding a correlation between the high-mass slope and core temperature. We also find a trend between core mass and effective size, but we are very limited by sensitivity. We make similar comparisons between core mass and size with visual extinction (for A_V >= 3) and find no obvious trends. We also predict the numbers and mass distributions of cores that future surveys with SCUBA-2 may detect in each of these clouds., Comment: 56 pages, 18 figures, fixed typo in Eq 1, results in paper remain unchanged

We present IRAC/MIPS Spitzer observations of intermediate-mass stars in the 5 Myr old Lambda Orionis cluster. In a representative sample of stars earlier than F5 (29 stars), we find a population of 9 stars with a varying degree of moderate 24um excess comparable to those produced by debris disks in older stellar groups. As expected in debris disks systems, those stars do not exhibit emission lines in their optical spectra. We also include in our study the star HD 245185, a known Herbig Ae object which displays excesses in all Spitzer bands and shows emission lines in its spectrum. We compare the disk population in the Lambda Orionis cluster with the disk census in other stellar groups studied using similar methods to detect and characterize their disks and spanning a range of ages from 3 Myr to 10 Myr. We find that for stellar groups of 5 Myr or older the observed disk frequency in intermediate mass stars (with spectral types from late B to early F) is higher than in low mass stars (with spectral types K and M). This is in contradiction with the observed trend for primordial disks evolution, in which stars with higher stellar masses dissipate their primordial disks faster. At 3 Myr the observed disk frequency in intermediate mass stars is still lower than for low mass stars indicating that second generation dusty disks start to dominate the disk population at 5 Myr for intermediate mass stars. This result agrees with recent models of evolution of solids in the region of the disk where icy objects form (>30 AU), which suggest that at 5-10 Myr collisions start to produce large amount of dust during the transition from runaway to oligarchic growth (reaching sizes of ~500 km) and then dust production peaks at 10-30 Myr, when objects reach their maximum sizes (>1000 km), Comment: 29 pages, 6 figures. Accepted in the Astrophysical Journal

We present results of optical spectroscopic and BVR_CI_C photometric observations of 77 pre-main sequence (PMS) stars in the Cepheus flare region. A total of 64 of these are newly confirmed PMS stars, originally selected from various published candidate lists. We estimate effective temperatures and luminosities for the PMS stars, and comparing the results with pre-main sequence evolutionary models we estimate stellar masses of 0.2-2.4M_sun and stellar ages of 0.1-15 Myr. Among the PMS stars, we identify 15 visual binaries with separations of 2-10 arcsec. From archival IRAS, 2MASS, and Spitzer data, we construct their spectral energy distributions and classify 5% of the stars as Class I, 10% as Flat SED, 60% as Class II, and 3% as Class III young stellar objects (YSOs). We identify 12 CTTS and 2 WTTS as members of NGC 7023, with mean age of 1.6 Myr. The 13 PMS stars associated with L1228 belong to three small aggregates: RNO 129, L1228A, and L1228S. The age distribution of the 17 PMS stars associated with L1251 suggests that star formation has propagated with the expansion of the Cepheus flare shell. We detect sparse aggregates of 6-7 Myr old PMS stars around the dark clouds L1177 and L1219, at a distance of 400 pc. Three T Tauri stars appear to be associated with the Herbig Ae star SV Cep at a distance of 600 pc. Our results confirm that the molecular complex in the Cepheus flare region contains clouds of various distances and star forming histories., Comment: 61 pages, 27 figures, 8 tables; accepted for publication by ApJS

We present a uniform mid-infrared imaging and photometric survey of 36 young, nearby, star-forming clusters and groups using {\it Spitzer} IRAC and MIPS. We have confidently identified and classified 2548 young stellar objects using recently established mid-infrared color-based methods. We have devised and applied a new algorithm for the isolation of local surface density enhancements from point source distributions, enabling us to extract the overdense cores of the observed star forming regions for further analysis. We have compiled several basic structural measurements of these cluster cores from the data, such as mean surface densities of sources, cluster core radii, and aspect ratios, in order to characterize the ranges for these quantities. We find that a typical cluster core is 0.39 pc in radius, has 26 members with infrared excess in a ratio of Class II to Class I sources of 3.7, is embedded in a $A_K$=0.8 mag cloud clump, and has a surface density of 60 pc$^{-2}$. We examine the nearest neighbor distances among the YSOs in several ways, demonstrating similarity in the spacings between Class II and Class I sources but large member clusters appear more dense than smaller clusters. We demonstrate that near-uniform source spacings in cluster cores are common, suggesting that simple Jeans fragmentation of parsec-scale cloud clumps may be the dominant process governing star formation in nearby clusters and groups. Finally, we compare our results to other similar surveys in the literature and discuss potential biases in the data to guide further interpretation., Comment: 207 pages, 54 figures, 44 tables. Accepted to ApJ Supplements. Full resolution version at http://www.cfa.harvard.edu/~rgutermuth/preprints/gutermuth_clusters_survey.pdf

We present adaptive optics (AO) near-infrared (JHKs) observations of the deeply embedded massive cluster RCW 38 using NACO on the VLT. Narrowband AO observations centered at wavelengths of 1.28, 2.12, and 2.17 micron were also obtained. The area covered by these observations is about 0.5 pc square, centered on the O star RCW 38 IRS2. We use the JHKs colors to identify young stars with infrared excess. Through a detailed comparison to a nearby control field, we find that most of the 337 stars detected in all three infrared bands are cluster members (~317), with essentially no contamination due to background or foreground sources. Five sources have colors suggestive of deeply embedded protostars, while 53 sources are detected at Ks only; their spatial distribution with respect to the extinction suggests they are highly reddened cluster members. Detectable Ks-band excess is found toward 29 +/- 3 % of the stars. For comparison to a similar area of Orion observed in the near-infrared, mass and extinction cuts are applied, and the excess fractions redetermined. The resulting excesses are then 25 +/- 5 % for RCW 38, and 42 +/- 8 % for Orion. RCW 38 IRS2 is shown to be a massive star binary with a projected separation of ~500 AU. Two regions of molecular hydrogen emission are revealed through the 2.12 micron imaging. One shows a morphology suggestive of a protostellar jet, and is clearly associated with a star only detected at H and Ks, previously identified as a highly obscured X-ray source. Three spatially extended cometary-like objects, suggestive of photoevaporating disks, are identified, but only one is clearly directly influenced by RCW 38 IRS2. A King profile provides a reasonable fit to the cluster radial density profile and a nearest neighbor distance analysis shows essentially no sub-clustering., Comment: 45 pages (17 figures), and full source table. Full resolution version available at http://www.cfa.harvard.edu/~bourke/rcw38/

We present spectral observations of 130 young stellar objects (YSOs) in the Serpens Cloud Core and NGC 1333 embedded clusters. The observations consist of near-IR spectra in the H and K-bands, from SpeX on the IRTF and far-red spectra (6000 - 9000 A) from Hectospec on the MMT. These YSOs were identified in previous Spitzer and Chandra observations, and the evolutionary classes of the YSOs were determined from the Spitzer mid-IR photometry. With these spectra, we search for corroborating evidence for the pre-main sequence nature of the objects, study the properties of the detected emission lines as a function of evolutionary class, and obtain spectral types for the observed YSOs. By comparing the positions of the YSOs in the HR diagrams with the pre-main sequence tracks of Baraffe (1998), we determine ages of the embedded sources and study the relative ages of the YSOs with and without optically thick circumstellar disks. The apparent isochronal ages of the YSOs in both clusters range from less than 1 Myr to 10 Myr, with most objects below 3 Myr. The observed distribution of ages for the Class II and Class III objects are statistically indistinguishable. We examine the spatial distribution and extinction of the YSOs as a function of their isochronal ages. We find the sources < 3 Myr to be concentrated in the molecular cloud gas while the older sources are spatially dispersed and are not deeply embedded. Nonetheless, the sources with isochronal ages > 3 Myr show all the characteristics of young stellar objects in their spectra, their IR spectral energy distributions, and their X-ray emission., Comment: 42 Pages, 14 Figures, accepted to AJ

We present an analysis of the spatial distribution of various stellar populations within the Large and Small Magellanic Clouds. We use optically selected stellar samples with mean ages between ~9 and ~1000 Myr, and existing stellar cluster catalogues to investigate how stellar structures form and evolve within the LMC/SMC. We use two statistical techniques to study the evolution of structure within these galaxies, the $Q$-parameter and the two-point correlation function (TPCF). In both galaxies we find the stars are born with a high degree of substructure (i.e. are highly fractal) and that the stellar distribution approaches that of the 'background' population on timescales similar to the crossing times of the galaxy (~80/150 Myr for the SMC/LMC respectively). By comparing our observations to simple models of structural evolution we find that 'popping star clusters' do not significantly influence structural evolution in these galaxies. Instead we argue that general galactic dynamics are the main drivers, and that substructure will be erased in approximately the crossing time, regardless of spatial scale, from small clusters to whole galaxies. This can explain why many young Galactic clusters have high degrees of substructure, while others are smooth and centrally concentrated. We conclude with a general discussion on cluster 'infant mortality', in an attempt to clarify the time/spatial scales involved., Comment: 6 pages, conference contribution to IAU Symposium 256, van Loon J.T. & Oliviera J.M., eds

We present new Spitzer Space Telescope observations of stars in the young ~5 Myr gamma Velorum stellar cluster. Combining optical and 2MASS photometry, we have selected 579 stars as candidate members of the cluster. With the addition of the Spitzer mid-infrared data, we have identified 5 debris disks around A-type stars, and 5-6 debris disks around solar-type stars, indicating that the strong radiation field in the cluster does not completely suppress the production of planetesimals in the disks of cluster members. However, we find some evidence that the frequency of circumstellar primordial disks is lower, and the IR flux excesses are smaller than for disks around stellar populations with similar ages. This could be evidence for a relatively fast dissipation of circumstellar dust by the strong radiation field from the highest mass star(s) in the cluster. Another possibility is that gamma Velorum stellar cluster is slightly older than reported ages and the the low frequency of primordial disks reflects the fast disk dissipation observed at ~5 Myr., Comment: Accepted for publication in the Astrophysical Journal: 32 pages; 11 Figures