Search

We present a comparison of Submillimetre Common User Bolometer Array-2 (SCUBA-2) 850-μm and Herschel 70–500-μm observations of the L1495 filament in the Taurus Molecular Cloud with the goal of characterizing the SCUBA-2 Gould Belt Survey (GBS) data set. We identify and characterize starless cores in three data sets: SCUBA-2 850-μm, Herschel 250-μm, and Herschel 250-μm spatially filtered to mimic the SCUBA-2 data. SCUBA-2 detects only the highest-surface-brightness sources, principally detecting protostellar sources and starless cores embedded in filaments, while Herschel is sensitive to most of the cloud structure, including extended low-surface-brightness emission. Herschel detects considerably more sources than SCUBA-2 even after spatial filtering. We investigate which properties of a starless core detected by Herschel determine its detectability by SCUBA-2, and find that they are the core's temperature and column density (for given dust properties). For similar-temperature cores, such as those seen in L1495, the surface brightnesses of the cores are determined by their column densities, with the highest-column-density cores being detected by SCUBA-2. For roughly spherical geometries, column density corresponds to volume density, and so SCUBA-2 selects the densest cores from a population at a given temperature. This selection effect, which we quantify as a function of distance, makes SCUBA-2 ideal for identifying those cores in Herschel catalogues that are closest to forming stars. Our results can now be used by anyone wishing to use the SCUBA-2 GBS data set. [ABSTRACT FROM AUTHOR]

We present Herschel observations from the Herschel Gould Belt Survey and SCUBA-2 science verification observations from the JCMT Gould Belt Survey of the B1 clump in the Perseus molecular cloud.We determined the dust emissivity index using four different techniques to combine the Herschel PACS+SPIRE data at 160-500μm with the SCUBA-2 data at 450μm and 850μm. Of our four techniques, we found that the most robust method was filtering out the large-scale emission in the Herschel bands to match the spatial scales recovered by the SCUBA-2 reduction pipeline. Using this method, we find β ≈ 2 toward the filament region and moderately dense material and lower β values (β ≈ 1.6) toward the dense protostellar cores, possibly due to dust grain growth. We find that β and temperature are more robust with the inclusion of the SCUBA-2 data, improving estimates from Herschel data alone by factors of ~2 for β and by ~40% for temperature. Furthermore, we find core mass differences of ≲30% compared to Herschel-only estimates with an adopted β = 2, highlighting the necessity of long-wavelength submillimeter data for deriving accurate masses of prestellar and protostellar cores. [ABSTRACT FROM AUTHOR]

We present optical and submillimetre polarimetry data of the Bok globule CB3. We also present optical polarimetry and submillimetre continuum data of the Bok globule CB246. We use each set of polarimetry data to infer the magnetic field orientation in each of the clouds. The optical data can only trace the field orientation in the low-density edge regions of clouds, because if the extinction is too high then no optical emission is transmitted. The submillimetre data can only trace the field orientation in the high-density central regions of the clouds, because current submillimetre polarimeters are only sensitive to high column densities. It has previously been found that near-infrared polarization mapping of background stars does not accurately trace the magnetic field in dense cloud regions, and hence that the grains responsible for near-infrared polarization are under-represented in those regions. This may be due to a lack of aligned grains in dense regions. We test this by comparing the field orientations measured by our two independent methods of optical and submillimetre polarimetry. We find that the field orientation deduced from the optical data matches up well with the orientation estimated from the submillimetre data. We therefore claim that both methods are accurately tracing the same magnetic field in CB3. Hence, in this case, there must be significant numbers of aligned dust grains in the high-density region, and they do indeed trace the magnetic field in the submillimetre. We find an offset of 14° between the magnetic field orientation and the short axis of the globule. This is consistent with the mean value of 3° found in our previous work on pre-stellar cores, even though CB3 is a protostellar core. CB246 is a pre-stellar core, and in this case the offset between the magnetic field orientation inferred from the optical polarization data and the short axis of the core inferred from the submillimetre continuum data is 20°. Taken together, the six pre-stellar cores that we have now studied in this way show a mean offset between magnetic field orientation and core short axis of , in apparent contradiction with some models of magnetically dominated star formation. [ABSTRACT FROM AUTHOR]

We present observations of L1155 and L1148 in the Cepheus molecular cloud, taken using the Far Infrared Surveyor (FIS) instrument on the Akari satellite. We compare these data to submillimetre data taken using the Submillimetre Common-User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope, and far-infrared data taken with the imaging photo-polarimeter (ISOPHOT) camera on board the Infrared Space Observatory ( ISO) satellite. The Akari data cover a similar spectral window and are consistent with the ISO data. All of the data show a relation between the position of the peak of emission and the wavelength for the core of L1155. We interpret this as a temperature gradient. We fit modified blackbody curves to the spectral energy distributions at two positions in the core and see that the central core in L1155 (L1155C) is approximately 2° warmer at one edge than it is in the centre. We consider a number of possible heating sources and conclude that the A6V star BD+67 1263 is the most likely candidate. This star is at a distance of 0.7 pc from the front of L1155C in the plane of the sky. We carry out radiative transfer modelling of the L1155C core including the effects from the nearby star. We find that we can generate a good fit to the observed data at all wavelengths, and demonstrate that the different morphologies of the core at different wavelengths can be explained by the observed 2° temperature gradient. The L1148 core exhibits a similar morphology to that of L1155C, and the data are also consistent with a temperature gradient across the core. In this case, the most likely heating source is the star BD197053. Our findings illustrate very clearly that the apparent observed morphology of a pre-stellar core can be highly dependent on the wavelength of the observation, and that temperature gradients must be taken into account before converting images into column density distributions. This is important to note when interpreting Akari and Spitzer data and will also be significant for Herschel data. [ABSTRACT FROM AUTHOR]

We present continuum data from the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT), and the multiband imaging Photometer for Spitzer (MIPS) on the Spitzer Space Telescope, at submillimetre and infrared wavelengths, respectively. We study the Taurus molecular cloud 1 (TMC1) and, in particular, the region of the Taurus Molecular Ring (TMR). In the continuum data, we see no real evidence for a ring, but rather we see one side of it only, appearing as a filament. We name the filament ‘the bull's tail’. The filament is seen in emission at 850, 450 and 160 μm, and in absorption at 70 μm. We compare the data with archive data from the Infrared Astronomical Satellite ( IRAS) at 12, 25, 60, 100 μm, in which the filament is also seen in absorption. We find that the emission from the filament consists of two components: a narrow, cold (∼8 K), central core, and a broader, slightly warmer (∼12 K), shoulder of emission. We use a radiative transfer code to model the filament's appearance, either in emission or absorption, simultaneously at each of the different wavelengths. Our best-fitting model uses a Plummer-like density profile and a homogeneous interstellar dust grain population. Unlike previous work on a similar, but different filament in Taurus, we require no grain coagulation to explain our data. [ABSTRACT FROM AUTHOR]

We have re-analysed all of the Submillimetre Common User Bolometer Array (SCUBA) archive data of the Orion star-forming regions. We have put together all of the data taken at different times by different groups. Consequently, we have constructed the deepest submillimetre maps of these regions ever made. There are four regions that have been mapped: Orion A North and South, and Orion B North and South. We find that two of the regions, Orion A North and Orion B North, have deeper sensitivity and completeness limits, and contain a larger number of sources, so we concentrate on these two. We compare the data with archive data from the Spitzer Space Telescope to determine whether or not a core detected in the submillimetre is pre-stellar in nature. We extract all of the pre-stellar cores from the data and make a histogram of the core masses. This can be compared to the stellar initial mass function (IMF). We find the high-mass core mass function (CMF) follows a roughly Salpeter-like slope, just like the IMF, as seen in previous work. Our deeper maps allow us to see that the CMF turns over at, about a factor of 4 higher than our completeness limit. This turnover has never previously been observed, and is only visible here due to our much deeper maps. It mimics the turnover seen in the stellar IMF at . The low-mass side of the CMF is a power law with an exponent of, 0.35 ± 0.2 which is consistent with the low-mass slope of the young cluster IMF of 0.3 ± 0.1. This shows that the CMF continues to mimic the shape of the IMF all the way down to the lower completeness limit of these data at . [ABSTRACT FROM AUTHOR]

We present observations taken with the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) of the Horsehead nebula in Orion (B33), at wavelengths of 450 and 850 μm. We see bright emission from that part of the cloud associated with the photon-dominated region (PDR) at the ‘top’ of the horse's head, which we label B33-SMM1. We characterize the physical parameters of the extended dust responsible for this emission, and find that B33-SMM1 contains a more dense core than was previously suspected, with a mass of in a region of , and a peak volume density of . We compare the SCUBA data with data from the Infrared Space Observatory ( ISO) and find that the emission at 6.75 μm is offset towards the west, indicating that the mid-infrared emission is tracing the PDR while the submillimetre emission comes from the molecular cloud core behind the PDR. We calculate the virial balance of this core and find that it is not gravitationally bound but is being confined by the external pressure from the H ii region IC434, and that it will either be destroyed by the ionizing radiation, or else may undergo triggered star formation. Furthermore, we find evidence for a lozenge-shaped clump in the ‘throat’ of the horse, which is not seen in emission at shorter wavelengths. We label this source B33-SMM2 and find that it is brighter at submillimetre wavelengths than B33-SMM1. We calculate the physical parameters of SMM2 and find it has a mass of in a region , with a peak volume density of and peak column density of . SMM2 is seen in absorption in the 6.75-μm ISO data, from which we obtain an independent estimate of the column density in excellent agreement with that calculated from the submillimetre emission. We calculate the stability of this core against collapse and find that it is in approximate gravitational virial equilibrium. This is consistent with it being a pre-existing core in B33, possibly pre-stellar in nature, but that it may also eventually undergo collapse under the effects of the H ii region. [ABSTRACT FROM AUTHOR]

We present submillimetre data for the L1689 cloud in the ρ Ophiuchi molecular cloud complex. We detect a number of starless and pre-stellar cores and protostellar envelopes. We also detect a number of filaments for the first time in the submillimetre continuum that are parallel both to each other, and to filaments observed in the neighbouring L1688 cloud. These filaments are also seen in the 13CO observations of L1689. The filaments contain all of the star-formation activity in the cloud. L1689 lies next to the well-studied L1688 cloud that contains the ρ Oph-A core. L1688 has a much more active star-formation history than L1689 despite their apparent similarity in 13CO data. Hence, we label L1689 as the dog that didn't bark. We endeavour to explain this apparent anomaly by comparing the total mass of each cloud that is currently in the form of dense material such as pre-stellar cores. We note firstly that L1688 is more massive than L1689, but we also find that when normalized to the total mass of each cloud, the L1689 cloud has a much lower percentage of mass in dense cores than L1688. We attribute this to the hypothesis of Loren that the star formation in the ρ Ophiuchi complex is being affected and probably dominated by the external influence of the nearby Upper Scorpius OB association and predominantly by σ Sco. L1689 is further from σ Sco and is therefore less active. The influence of σ Sco appears none the less to have created the filaments that we observe in L1689. [ABSTRACT FROM AUTHOR]

We estimate the levels of turbulence in the envelopes of class 0 and I protostars using a model based on measurements of the peak separation of double-peaked asymmetric line profiles. We use observations of 20 protostars of both class 0 and I taken in theline that show the classic double-peaked profile. We find that some class 0 sources show high levels of turbulence, whilst others demonstrate much lower levels. In class I protostars, we find predominantly low levels of turbulence. The observations are consistent with a scenario in which class 0 protostars form in a variety of environments and subsequently evolve into class I protostars. The data do not appear to be consistent with a recently proposed scenario in which class 0 protostars can only form in extreme environments. [ABSTRACT FROM AUTHOR]

Prestellar cores represent the stage of star formation immediately prior to protostellar collapse. We present polarisation maps of three prestellar cores, L183, L1544 and L43. In each case the magnetic field lines are uniform but not parallel to the semi-minor axis of the core. This suggests that magnetic and thermal pressure support alone are inconsistent with the data. We also calculate the magnetic field strength using the Chandrasekhar-Fermi technique and find that all three cores are magnetically supercritical by a factor of ∼ 2. This is consistent with the observation that the magnetic field is not dominating the evolution of these cores. [ABSTRACT FROM AUTHOR]