Temperatures of dust and gas in S~140

In dense parts of interstellar clouds (> 10^5 cm^-3), dust & gas are expected to be in thermal equilibrium, being coupled via collisions. However, previous studies have shown that the temperatures of the dust & gas may remain decoupled even at higher densities. We study in detail the temperatures of dust & gas in the photon-dominated region S 140, especially around the deeply embedded infrared sources IRS 1-3 and at the ionization front. We derive the dust temperature and column density by combining Herschel PACS continuum observations with SOFIA observations at 37 $\mu$m and SCUBA at 450 $\mu$m. We model these observations using greybody fits and the DUSTY radiative transfer code. For the gas part we use RADEX to model the CO 1-0, CO 2-1, 13CO 1-0 and C18O 1-0 emission lines mapped with the IRAM-30m over a 4' field. Around IRS 1-3, we use HIFI observations of single-points and cuts in CO 9-8, 13CO 10-9 and C18O 9-8 to constrain the amount of warm gas, using the best fitting dust model derived with DUSTY as input to the non-local radiative transfer model RATRAN. We find that the gas temperature around the infrared sources varies between 35 and 55K and that the gas is systematically warmer than the dust by ~5-15K despite the high gas density. In addition we observe an increase of the gas temperature from 30-35K in the surrounding up to 40-45K towards the ionization front, most likely due to the UV radiation from the external star. Furthermore, detailed models of the temperature structure close to IRS 1 show that the gas is warmer and/or denser than what we model. Finally, modelling of the dust emission from the sub-mm peak SMM 1 constrains its luminosity to a few ~10^2 Lo. We conclude that the gas heating in the S 140 region is very efficient even at high densities, most likely due to the deep UV penetration from the embedded sources in a clumpy medium and/or oblique shocks.Comment: 15 pages, 23 figures, 4 tables, accepted for publication in A&

Far and mid-infrared studies of star forming regions:Probing their thermal balance, chemistry and evolution

The goal of this thesis is to understand the physical and chemical processes that take place in regions forming stars, in particular during the early evolutionary stages of star formation. Why do we care about stars and star forming regions? Many elements on Earth were once formed in the heart of a star. In particular the building blocks of life, known as organic molecules which contain both carbon and hydrogen, have also been found towards star forming regions. While staring at the stars, one should bear in mind, that the starlight we enjoy, shares its origin with the light that actually triggers the chemical reactions quintessential for the formation of Earth and life. In order to understand how stars form, this thesis investigates the role of various molecules in the thermal balance (heating and cooling) of star forming regions, which controls their stability. In addition I investigate the stages that a star goes through during the process of formation and the chemical composition of newly forming stars. Young stars are hidden from optical telescopes because they are deeply embedded in dense clouds of gas and dust. However, infrared radiation, can pass through allowing us to study these regions. The present thesis aims to study these places that give birth to stars of various masses using infrared instruments and to provide some answers in regard of the open questions in the field of star formation

The fine structure line deficit in S 140

We try to understand the gas heating and cooling in the S 140 star forming region by spatially and spectrally resolving the distribution of the main cooling lines with GREAT/SOFIA. We mapped the fine structure lines of [OI] (63 {\mu}m) and [CII] (158 {\mu}m) and the rotational transitions of CO 13-12 and 16-15 with GREAT/SOFIA and analyzed the spatial and velocity structure to assign the emission to individual heating sources. We measure the optical depth of the [CII] line and perform radiative transfer computations for all observed transitions. By comparing the line intensities with the far-infrared continuum we can assess the total cooling budget and measure the gas heating efficiency. The main emission of fine structure lines in S 140 stems from a 8.3'' region close to the infrared source IRS 2 that is not prominent at any other wavelength. It can be explained by a photon-dominated region (PDR) structure around the embedded cluster if we assume that the [OI] line intensity is reduced by a factor seven due to self-absorption. The external cloud interface forms a second PDR at an inclination of 80-85 degrees illuminated by an UV field of 60 times the standard interstellar radiation field. The main radiation source in the cloud, IRS 1, is not prominent at all in the fine structure lines. We measure line-to-continuum cooling ratios below 10^(-4), i.e. values lower than in any other Galactic source, rather matching the far-IR line deficit seen in ULIRGs. In particular the low intensity of the [CII] line can only be modeled by an extreme excitation gradient in the gas around IRS 1. We found no explanation why IRS 1 shows no associated fine-structure line peak, while IRS 2 does. The inner part of S 140 mimics the far-IR line deficit in ULIRGs thereby providing a template that may lead to a future model.Comment: Accepted for publication in Astronomy & Astrophysic

The RMS survey: a census of massive YSO multiplicity in the KK-band

Close to 100 per cent of massive stars are thought to be in binary systems. The multiplicity of massive stars seems to be intrinsically linked to their formation and evolution, and Massive Young Stellar Objects are key in observing this early stage of star formation. We have surveyed three samples totalling hundreds of MYSOs ($>8M_\odot$) across the Galaxy from the RMS catalogue, using UKIDSS and VVV point source data, and UKIRT $K-$band imaging to probe separations between 0.8-9 arcsec (approx 1000-100,000 au). We have used statistical methods to determine the binary statistics of the samples, and we find binary fractions of $64\pm 4$ per cent for the UKIDSS sample, $53\pm 4$ per cent for the VVV sample, and $49\pm 8$ per cent for the RMS imaging sample. Also we use the $J-$ and $K-$band magnitudes as a proxy for the companion mass, and a significant fraction of the detected systems have estimated mass ratios greater than 0.5, suggesting a deviation from the capture formation scenario which would be aligned with random IMF sampling. Finally, we find that YSOs located in the outer Galaxy have a higher binary fraction than those in the inner Galaxy. This is likely due to a lower stellar background density than observed towards the inner Galaxy, resulting in higher probabilities for visual binaries to be physical companions. It does indicate a binary fraction in the probed separation range of close to 100 per cent without the need to consider selection biases.Comment: 14 pages, 9 figures, accepted to MNRA

Fundamental Parameters of four Massive Eclipsing Binaries in Westerlund 1

We present fundamental parameters of 4 massive eclipsing binaries in the young massive cluster Westerlund 1. The goal is to measure accurate masses and radii of their component stars, which provide much needed constraints for evolutionary models of massive stars. Accurate parameters can further be used to determine a dynamical lower limit for the magnetar progenitor and to obtain an independent distance to the cluster. Our results confirm and extend the evidence for a high mass for the progenitor of the magnetar.Comment: 2 pages, to appear in the proceedings of the IAUS 282 on "From Interacting Binaries to Exoplanets:Essential Modelling Tools" (Tatranska Lomnica, July 18-22, 2011), Cambridge University Pres

Identifying Stars of Mass >150 Msun from Their Eclipse by a Binary Companion

We examine the possibility that very massive stars greatly exceeding the commonly adopted stellar mass limit of 150 Msun may be present in young star clusters in the local universe. We identify ten candidate clusters, some of which may host stars with masses up to 600 Msun formed via runaway collisions. We estimate the probabilities of these very massive stars being in eclipsing binaries to be >30%. Although most of these systems cannot be resolved at present, their transits can be detected at distances of 3 Mpc even under the contamination of the background cluster light, due to the large associated luminosities ~10^7 Lsun and mean transit depths of ~10^6 Lsun. Discovery of very massive eclipsing binaries would flag possible progenitors of pair-instability supernovae and intermediate-mass black holes.Comment: 5 pages, 1 figure, 1 table. Submitted to MNRA

Mapping the H2D+ and N2H+ emission toward prestellar cores. Testing dynamical models of the collapse using gas tracers

Context. The study of prestellar cores is critical as they set the initial conditions in star formation and determine the final mass of the stellar object. To date, several hypotheses have described their gravitational collapse. Deriving the dynamical model that fits both the observed dust and the gas emission from such cores is therefore of great importance. Aims. We perform detailed line analysis and modeling of H2D+ 110–111 and N2H+ 4–3 emission at 372 GHz, using 2′ × 2′ maps (James Clerk Maxwell Telescope; JCMT). Our goal is to test the most prominent dynamical models by comparing the modeled gas kinematics and spatial distribution (H2D+ and N2H+) with observations toward four prestellar (L1544, L183, L694-2, L1517B) and one protostellar core (L1521f). Methods. We fit the line profiles at all offsets showing emission using single Gaussian distributions. We investigate how the line parameters (VLSR, FWHM and TA*) change with offset to examine the velocity field, the degree of nonthermal contributions to the line broadening, and the distribution of the material in these cores. To assess the thermal broadening, we derive the average gas kinetic temperature toward all cores using the non-LTE radiative transfer code RADEX. We perform a more detailed non-LTE radiative transfer modeling using RATRAN, where we compare the predicted spatial distribution and line profiles of H2D+ and N2H+ with observations toward all cores. To do so, we adopt the physical structure for each core predicted by three different dynamical models taken from literature: quasi-equilibrium Bonnor–Ebert sphere (QE-BES), singular isothermal sphere (SIS), and Larson–Penston (LP) flow. In addition, we compare these results to those of a static sphere, whose density and temperature profiles are based on the observed dust continuum. Lastly, we constrain the abundance profiles of H2D+ and N2H+ toward each core. Results. We find that variable nonthermal contributions (variations by a factor of 2.5) are required to explain the observed line width of both H2D+ and N2H+, while the nonthermal contributions are found to be 50% higher for N2H+. The RADEX modeling results in average core column densities of ~9 × 1012 cm−2 for H2D+ and N2H+. The LP flow seems to be the dynamical model that can reproduce the observed spatial distribution and line profiles of H2D+ on a global scale of prestellar cores, while the SIS model systematically and significantly overestimates the width of the line profiles and underestimates the line peak intensity. We find similar abundance profiles for the prestellar cores and the protostellar core. The typical abundances of H2D+ vary between 10−9 and 10−10 for the inner 5000 au and drop by about an order of magnitude for the outer regions of the core (2 × 10−10–6 × 10−11). In addition, a higher N2H+ abundance by about a factor of 4 compared to H2D+ is found toward the two cores with detected emission. The presence of N2H+ 4–3 toward the protostellar core and toward one of the prestellar cores reflects the increasing densities as the core evolves. Conclusions. Our analysis provides an updated picture of the physical structure of prestellar cores. Although the dynamical models account for mass differences by up to a factor of 7, the velocity structure drives the shape of the line profiles, allowing for a robust comparison between the models. We find that the SIS model can be clearly excluded in explaining the gas emission toward the cores, but a larger sample is required to differentiate clearly between the LP flow, the QE-BES, and the static models. All models of collapse underestimate the intensity of the gas emission by up to several factors toward the only protostellar core in our sample, indicating that different dynamics take place in different evolutionary core stages. If the LP model is confirmed toward a larger sample of prestellar cores, it would indicate that they may form by compression or accretion of gas from larger scales. If the QE-BES model is confirmed, it means that quasi-hydrostatic cores can exist within turbulent ISM