Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userHighly variable young massive stars in ATLASGAL clumps
113
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
High-amplitude variability in Young Stellar Objects (YSOs) is usually associated with episodic accretion events. It has not been observed so far in massive YSOs. Here, the high-amplitude variable star sample of ContrerasPe~{n}a et al.(2016) has been used to search for highly-variable($Delta$K$ge$1,mag) sources coinciding with dense clumps mapped using the 850mum continuum emission by the ATLASGAL survey. 18 variable sources are centred on the sub-mm clump peaks, and coincide ($<$1) with a 24$mu$m point or compact ($<$10) source. 13 of these 18 sources can be fit by YSO models. The 13 variable YSOs(VYSO) have luminosities of $sim$10$^3$ L$_{odot}$, an average mass of 8 M$_{odot}$ and a range of ages up to 10$^6$ yr. 11 of these 13 VYSOs are located in the midst of infrared dark clouds. 9 of the 13 sources have $Delta$K$>$2 mag, significantly higher compared to the mean variability of the entire VVV sample. The light curves of these objects sampled between 2010-2015 display rising, declining, or quasi-periodic behaviour but no clear periodicity. Light-curve analysis using Plavchan method show that the most prominent phased signals have periods of a few hundred days. The nature and time-scale of variations found in 6.7 Ghz methanol maser emission (MME) in massive stars are similar to that of the VYSO light curves. We argue that the origin of the observed variability is episodic accretion. We suggest that the timescale of a few hundred days may represent the frequency at which a spiralling disk feeds dense gas to the young massive star.
rate research
Read More
We have undertaken the largest survey for outflows within the Galactic Plane using simultaneously observed 13CO and C18O data. 325 out of a total of 919 ATLASGAL clumps have data suitable to identify outflows, and 225 (69+-3%) of them show high velocity outflows. The clumps with detected outflows show significantly higher clump masses (M_{clump}), bolometric luminosities (L_{bol}), luminosity-to-mass ratios (L_{bol}/M_{clump}) and peak H_2 column densities (N_{H_2}) compared to those without outflows. Outflow activity has been detected within the youngest quiescent clump (i.e.,70um weak) in this sample and we find that the outflow detection rate increases with M_{clump},L_{bol},L_{bol}/M_{clump} and N_{H_2},approaching 90% in some cases(uchii regions=93+-3%;masers=86+-4%;hchii regions=100%). This high detection rate suggests that outflows are ubiquitous phenomena of massive star formation. The mean outflow mass entrainment rate implies a mean accretion rate of ~10^{-4}M_odot,yr^{-1}, in full agreement with the accretion rate predicted by theoretical models of massive star formation. Outflow properties are tightly correlated with M_{clump},L_{bol} and L_{bol}/M_{clump},and show the strongest relation with the bolometric clump luminosity. This suggests that outflows might be driven by the most massive and luminous source within the clump. The correlations are similar for both low-mass and high-mass outflows over 7 orders of magnitude, indicating that they may share a similar outflow mechanism. Outflow energy is comparable to the turbulent energy within the clump, however, we find no evidence that outflows increase the level of clump turbulence as the clumps evolve. This implies that the origin of turbulence within clumps is fixed before the onset of star formation.
We obtained K-band spectroscopy of the brightest members of the cluster VVV CL074 in order to identify the massive star population. We also determined the stellar properties of the clusters massive stars to better quantify the evolutionary sequences linking different types of massive stars. We collected integral field spectroscopy of selected fields in the cluster VVV CL074 with SINFONI on the ESO/VLT. We performed a spectral classification based on the K-band spectra and comparison to infrared spectral atlases. We determined the stellar parameters of the massive stars from analysis with atmosphere models computed with the code CMFGEN. We uncover a population of 25 early-type (OB and Wolf-Rayet) stars, 19 being newly discovered by our observations out of which 15 are likely cluster members. The clusters spectrophotometric distance is 10.2+/-1.6 kpc, placing it close to the intersection of the galactic bar and the Norma arm, beyond the galactic center. This makes VVV CL074 one the farthest young massive clusters identified so far. Among the massive stars population, three objects are Wolf-Rayet stars, the remaining are O and B stars. From the Hertzsprung-Russell diagram we find that most stars have an age between 3 and 6 Myr according to the Geneva evolutionary tracks. WN8 and WC8-9 stars are the descendants of stars with initial masses between 40 and 60 Msun. The massive star population of VVV CL074 is very similar to that of the cluster DBS2003-179 and to a lesser extent to that of the Quintuplet cluster, indicating the same age. The central cluster of the Galaxy is ~3 Myr older. From the comparison of the massive stars populations in these four clusters, one concludes that galactic stars with an initial mass in the range 40 to 60 Msun likely go through a WN8-9 phase.
Deuteration has been used as a tracer of the evolutionary phases of low- and high-mass star formation. The APEX Telescope Large Area Survey (ATLASGAL) provides an important repository for a detailed statistical study of massive star-forming clumps in the inner Galactic disc at different evolutionary phases. We study the amount of deuteration using NH2D in a representative sample of high-mass clumps discovered by the ATLASGAL survey covering various evolutionary phases of massive star formation. Unbiased spectral line surveys at 3 mm were thus conducted towards ATLASGAL clumps between 85 and 93 GHz with the Mopra telescope and from 84 to 115 GHz using the IRAM 30m telescope. A subsample was followed up in the NH2D transition at 74 GHz with the IRAM 30m telescope. We determined the deuterium fractionation from the column density ratio of NH2D and NH3 and measured the NH2D excitation temperature for the first time from the simultaneous modelling of the 74 and 110 GHz line using MCWeeds. We find a large range of the NH2D to NH3 column density ratio up to 1.6+/-0.7 indicating a high degree of NH3 deuteration in a subsample of the clumps. Our analysis yields a clear difference between NH3 and NH2D rotational temperatures for a fraction of the sources. We therefore advocate observation of the NH2D transitions at 74 and 110 GHz simultaneously to determine the NH2D temperature directly. We determine a median ortho-to-para column density ratio of 3.7+/-1.2. The high detection rate of NH2D confirms a high deuteration previously found in massive star-forming clumps. Using the excitation temperature of NH2D instead of NH3 is needed to avoid an overestimation of deuteration. We measure a higher detection rate of NH2D in sources at early evolutionary stages. The deuterium fractionation shows no correlation with evolutionary tracers such as the NH3 (1,1) line width, or rotational temperature.
An unsettled question concerning the formation and distribution of massive stars is whether they must be born in massive clusters and, if found in less dense environments, whether they must have migrated there. With the advent of wide-area digital photometric surveys, it is now possible to identify massive stars away from prominent Galactic clusters without bias. In this study we consider 40 candidate OB stars found in the field around the young massive cluster, Westerlund 2, by Mohr-Smith et al (2017): these are located inside a box of 1.5x1.5 square degrees and are selected on the basis of their extinctions and K magnitudes. We present VLT/X-shooter spectra of two of the hottest O stars, respectively 11 and 22 arcmin from the centre of Westerlund 2. They are confirmed as O4V stars, with stellar masses likely to be in excess of 40 Msun. Their radial velocities relative to the non-binary reference object, MSP 182, in Westerlund 2 are -29.4 +/- 1.7 and -14.4 +/- 2.2 km/s, respectively. Using Gaia DR2 proper motions we find that between 8 and 11 early O/WR stars in the studied region (including the two VLT targets, plus WR 20c and WR 20aa) could have been ejected from Westerlund 2 in the last one million years. This represents an efficiency of massive-star ejection of up to 25%. On sky, the positions of these stars and their proper motions show a near N--S alignment. We discuss the possibility that these results are a consequence of prior sub-cluster merging combining with dynamical ejection.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
