On the infrared void in the Lupus dark clouds

Strömgren uvbyβ photometry observations obtained for 205 stars in the general direction of a void in the IRAS 100-μm emission from the Lupus dark cloud complex are presented and analysed. The colour excess versus distance diagram confirms the existence of a region depleted from interstellar material, which is also seen in the ROSAT soft X-ray background emission map. The distance to the surrounding material is estimated as being within the interval from 60 to 100 pc. This result is in disagreement with previous distance estimates to the supposed supernova that has been suggested as responsible for clearing the region from dust. As an alternative, the data presented support the suggestion that the void may have been produced by the detachment of material from the interface between Loop I and the Local Bubble as a consequence of hydromagnetic instabilities. Moreover, the distribution of colour excess as a function of distance supports a value of ∼150 pc as the most probable distance to the dark cloud known as Lupus 1.     

A SCUBA survey of Orion – the low-mass end of the core mass function

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,  ∼1.3 M_⊙  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  ∼0.1 M_⊙  . 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  ∼0.3 M_⊙  .     

Internal extinction of disc galaxies – I. High-resolution extinction map of NGC 6946

Traditionally, it has been believed that extinction effects due to dust within the interstellar medium of external galaxies are rather small and can largely be ignored. Over the last 10 years, however, considerable doubt has been cast over the evidence to support this comfortable idea, and it has become clear that a more detailed analysis is required. Here, a new technique for mapping the extinction in disc galaxies with high resolution is presented.
This technique has been applied to the Sc galaxy NGC 6946. The results show that dust extinction significantly affects both the overall brightness and appearance of the galaxy. The total extinction is found to be A_B =0.45 – somewhat larger than the value of A_B =0.2 usually quoted for an Sc galaxy. When corrected for dust the morphology more closely resembles that of an Sb galaxy rather than an Sc galaxy.
The most surprising result of this work is finding interarm regions that suffer high extinction. It appears that these regions appear faint because of the high extinction and not as a result of low stellar density. There are also interarm regions that suffer little extinction; these are therefore truly regions of low stellar density.     

The role of the geometry and dust environment in stationary models of translucent interstellar clouds

A full radiative transfer model is presented for the ultraviolet (UV) radiation impinging on an interstellar cloud of spherical or finite plane-parallel slab geometry containing gas and dust. The penetration of the UV photons is coupled to detailed chemical processes. Photodestruction rates of atomic and molecular species are calculated from the corresponding cross-sections. We show that CO line intensities are quite sensitive to geometrical effects and to the extinction curve in the far-UV.     

ISO observations of 3–200 μm emission by three dust populations in an isolated local translucent cloud

We present isophot spectrophotometry of three positions within the isolated high-latitude cirrus cloud G 300.2−16.8, spanning from the near- to far-infrared (NIR to FIR). The positions exhibit contrasting emission spectrum contributions from the unidentified infrared bands (UIBs), very small grains (VSGs) and large classical grains, and both semi-empirical and numerical models are presented. At all three positions, the UIB spectrum shapes are found to be similar and the large grain emission may be fitted by an equilibrium temperature of  ∼17.5 K  . The energy requirements of both the observed emission spectrum and optical scattered light are shown to be satisfied by the incident local interstellar radiation field (ISRF). The FIR emissivity of dust in G 300.2−16.8 is found to be lower than in globules or dense clouds and is even lower than model predictions for dust in the diffuse interstellar medium (ISM). The results suggest physical differences in the ISM mixtures between positions within the cloud, possibly arising from grain coagulation processes.     

A SCUBA survey of L1689 – the dog that didn't bark

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.     

The far-infrared signature of dust in high-latitude regions

We present ISOPHOT observations of eight interstellar regions in the 60–200 μm wavelength range. The regions belong to mostly quiescent high-latitude clouds and have optical extinction peaks from   A_V ∼1–6 mag  . From the 150- and 200-μm emission, we derived colour temperatures for the classical big grain component which show a clear trend of decreasing temperature with increasing 200-μm emission. The 200-μm emission per unit   A_V   , however, does not drop at lower temperatures. This fact can be interpreted in terms of an increased far-infrared (FIR) emissivity of the big grains. We developed a two-component model including warm dust with the temperature of the diffuse interstellar medium (ISM) of   T = 17.5 K  , and cold dust with   T = 13.5 K  and FIR emissivity increased by a factor of >4. A mixture of the two components can reproduce the observed colour variations and the ratios   I ₂₀₀/ A_V   and  τ₂₀₀/ A_V   . The relative abundance of small grains with respect to the big grains shows significant variations from region to region at low column densities. However, in lines of sight of higher column density, our data indicate the disappearance of small grains, perhaps a signature of adsorption/coagulation of dust. The larger size and porous structure could also explain the increased FIR emissivity. Our results from eight independent regions suggest that these grains might be ubiquitous in the galactic ISM.     

Limited diversity of the interstellar extinction law

We have applied the method of investigating extinction curves using statistically meaningful samples that was proposed by us 25 years ago. The extensive data sets of the ANS (Astronomical Netherlands Satellite) and 2MASS (Two Micron All Sky Survey) were used, together with UBV photometry to create average extinction curves for samples of OB stars. Our results demonstrate that in the vast majority of cases the extinction curves are very close to the mean galactic extinction curve. Only a few objects were found to be obviously discrepant from the average. The latter phenomenon may be related to nitrogen chemistry in translucent interstellar clouds (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Gas cooling by dust during dynamical fragmentation

We suggest that the abrupt switch, from hierarchical clustering on scales ≳ 0.04 pc, to binary (and occasionally higher multiple) systems on smaller scales, which Larson has deduced from his analysis of the grouping of pre-main-sequence stars in Taurus, arises because pre-protostellar gas becomes thermally coupled to dust at sufficiently high densities. The resulting change — from gas cooling by molecular lines at low densities to gas cooling by dust at high densities — enables the matter to radiate much more efficiently, and hence to undergo dynamical fragmentation. We derive the domain in which gas cooling by dust facilitates dynamical fragmentation. Low-mass (∼ M⊙) clumps — those supported mainly by thermal pressure — can probably access this domain spontaneously, albeit rather quasi-statically, provided that they exist in a region in which external perturbations are few and far between. More massive clumps probably require an impulsive external perturbation, for instance a supersonic collision with another clump, in order for the gas to reach sufficiently high density to couple thermally to the dust. Impulsive external perturbations should promote fragmentation, by generating highly non-linear substructures which can then be amplified by gravity during the subsequent collapse.     

Study of star formation in RCW 106 using far-infrared observations

High-resolution far-infrared observations of a large area of the star-forming complex RCW 106 obtained using the TIFR 1-m balloon-borne telescope are presented. Intensity maps have been obtained simultaneously in two bands centred around 150 and 210 μm. Intensity maps have also been obtained in the four IRAS bands using HIRES-processed IRAS data. From the 150- and 210-μm maps, reliable maps of dust temperature and optical depth have been generated. The star formation in this complex has occurred in five linear sub-clumps. Using the map at 210 μm, which has a spatial resolution superior to that of IRAS at 100 μm, 23 sources have been identified. The spectral energy distribution (SED) and luminosity of these sources have been determined using the associations with the IRAS maps. The luminosity distribution of these sources has been obtained. Assuming these embedded sources to be zero-age main-sequence stars and using the mass–luminosity relation for these, the power-law slope of the initial mass function is found to be −1.73±0.5 . This index for this very young complex is about the same as that for more evolved complexes and clusters. Radiation transfer calculations in spherically symmetric geometry have been undertaken to fit the SEDs of 13 sources with fluxes in both the TIFR and the IRAS bands. From this, the r ⁻² density distribution in the envelopes is ruled out. Finally, a correlation is seen between the luminosity of embedded sources and the computed dust masses of the envelopes.     

The initial conditions of isolated star formation – VII. Spitzer mapping of pre-stellar cores

We have retrieved Spitzer archive data of pre-stellar cores taken with the Multiband Imaging Photometer for Spitzer (MIPS) at a wavelength of 160 μm. Seventeen images, containing 18 cores, were constructed. Flux densities were measured for each core, and background estimates were made. Mean off-source backgrounds were found to be 48 ± 10 MJy sr⁻¹ in Taurus and 140 ± 55 MJy sr⁻¹ in Ophiuchus. Consistency was found between the MIPS 170-μm and ISOPHOT 160-μm calibrations. Fourteen cores were detected both by MIPS and by our previous submillimetre surveys. Spectral energy distributions were made for each core, using additional 24- and 70-μm data from the Spitzer data archive, as well as previous infrared and submillimetre data. Previous temperature estimates were refined, and new temperature estimates were made where no Infrared Space Observatory ( ISO ) data exist. A temperature range of 8–18 K was found for the cores, with most lying in the range 10–13 K. We discount recent claims that a large number of pre-stellar cores may have been misclassified and in fact contain low-luminosity protostars detectable only by Spitzer . We find no new protostars in our sample other than that previously reported in L1521F. It is shown that this has a negligible effect on pre-stellar lifetime estimates.