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Register a new userYoung Stellar Clusters Containing Massive Young Stellar Objects in the VVV Survey
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The purpose of this research is to study the connection of global properties of eight young stellar clusters projected in the Vista Variables in the Via Lactea (VVV) ESO Large Public Survey disk area and their young stellar object population. The analysis in based on the combination of spectroscopic parallax-based reddening and distance determinations with main sequence and pre-main sequence ishochrone fitting to determine the basic parameters (reddening, age, distance) of the sample clusters. The lower mass limit estimations show that all clusters are low or intermediate mass (between 110 and 1800 Mo), the slope Gamma of the obtained present-day mass functions of the clusters is close to the Kroupa initial mass function. On the other hand, the young stellar objects in the surrounding clusters fields are classified by low resolution spectra, spectral energy distribution fit with theoretical predictions, and variability, taking advantage of multi-epoch VVV observations. All spectroscopically confirmed young stellar objects (except one) are found to be massive (more than 8 Mo). Using VVV and GLIMPSE color-color cuts we have selected a large number of new young stellar object candidates, which are checked for variability and 57% are found to show at least low-amplitude variations. In few cases it was possible to distinguish between YSO and AGB classification on the basis of the light curves.
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Recent radio astronomical observations have revealed that HC$_{5}$N, the second shortest cyanopolyyne (HC$_{2n+1}$N), is abundant around some massive young stellar objects (MYSOs), which is not predicted by classical carbon-chain chemistry. For example, the observed HC$_{5}$N abundance toward the G28.28$-$0.36 MYSO is higher than that in L1527, which is one of the warm carbon chain chemistry (WCCC) sources, by more than one order of magnitude (Taniguchi et al., 2017). In this paper, we present chemical simulations of hot-core models with a warm-up period using the astrochemical code Nautilus. We find that the cyanopolyynes are formed initially in the gas phase and accreted onto the bulk and surface of granular ice mantles during the lukewarm phase, which occurs at $25 < T < 100$ K. In slow warm-up period models, the peak abundances occur as the cyanopolyynes desorb from dust grains after the temperature rises above 100 K. The lower limits of the abundances of HC$_{5}$N, CH$_{3}$CCH, and CH$_{3}$OH observed in the G28.28$-$0.36 MYSO can be reproduced in our hot-core models, after their desorption from dust grains. Moreover, previous observations suggested chemical diversity in envelopes around different MYSOs. We discuss possible interpretations of relationships between stages of the star-formation process and such chemical diversity, such as the different warm-up timescales. This timescale depends not only on the mass of central stars but also on the relationship between the size of warm regions and their infall velocity.
Near-infrared H- and K-band spectra are presented for 247 objects, selected from the Red MSX Source (RMS) survey as potential young stellar objects (YSOs). 195 (~80%) of the targets are YSOs, of which 131 are massive YSOs (L_BOL > 5x10^3 L_solar), M > 8M_solar. This is the largest spectroscopic study of massive YSOs to date, providing a valuable resource for the study of massive star formation. In this paper we present our exploratory analysis of the data. The YSOs observed have a wide range of embeddedness (2.7 < A_V < 114), demonstrating that this study covers minimally obscured objects right through to very red, dusty sources. Almost all YSOs show some evidence for emission lines, though there is a wide variety of observed properties. The most commonly detected lines are Brgamma, H_2, fluorescent FeII, CO bandhead, [FeII] and HeI 2-1 2^1S-2^1P, in order of frequency of occurrence. In total, ~40% of the YSOs display either fluorescent FeII 1.6878um or CO bandhead emission (or both), indicative of a circumstellar disc; however, no correlation of the strength of these lines with bolometric luminosity was found. We also find that ~60% of the sources exhibit [FeII] or H_2 emission, indicating the presence of an outflow. Three quarters of all sources have Brgamma in emission. A good correlation with bolometric luminosity was observed for both the Brgamma and H_2 emission line strengths, covering 1 L_solar< L_BOL < 3.5x10^5 L_solar. This suggests that the emission mechanism for these lines is the same for low-, intermediate-, and high-mass YSOs, i.e. high-mass YSOs appear to resemble scaled-
The ESO public survey VISTA Variables in the Via Lactea (VVV) has contributed with deep multi-epoch photometry of the Galactic bulge and the adjacent part of the disk over 526 square degrees. More than a hundred cluster candidates have been reported thanks to this survey. We present the fifth article in a series of papers focused on young and massive clusters discovered in the VVV survey. In this paper, we present the physical characterization of five clusters with a spectroscopically confirmed OB-type stellar population. To characterize the clusters, we used near-infrared photometry ($J$, $H,$ and $K_S$) from the VVV survey and near-infrared $K$-band spectroscopy from ISAAC at VLT, following the methodology presented in the previous articles of the series. All clusters in our sample are very young (ages between 1-20 Myr), and their total mass are between $(1.07^{+0.40}_{-0.30})cdot10^2$ $M_{odot}$ and $(4.17^{+4.15}_{-2.08})cdot10^3$ $M_{odot}$. We observed a relation between the clusters total mass $M_{ecl}$ and the mass of their most massive stellar member $m_{max}$, even for clusters with an age $<$ 10 Myr.
Episodic accretion-driven outbursts are an extreme manifestation of accretion variability. It has been proposed that the development of gravitational instabilities in the proto-circumstellar medium of massive young stellar objects (MYSOs) can lead to such luminous bursts, when clumps of fragmented accretion discs migrate onto the star. We simulate the early evolution of MYSOs formed by the gravitational collapse of rotating 100 M pre-stellar cores and analyze the characteristics of the bursts that episodically accompany their strongly time-variable protostellar lightcurve. We predict that MYSOs spend ~ 10^3 yr (~ 1.7%) of their modelled early 60 kyr experiencing eruptive phases, during which the peak luminosity exceeds the quiescent pre-burst values by factors from 2.5 to more than 40. Throughout these short time periods, they can acquire a substential fraction (up to ~ 50 %) of their zero-age-main sequence mass. Our findings show that fainter bursts are more common than brighter ones. We discuss our results in the context of the known bursting MYSOs, e.g. NGC6334I-MM1 and S255IR-NIRS3, and propose that these monitored bursts are part of a long-time ongoing series of eruptions, which might, in the future, be followed by other luminous flares.
We present the results of ammonia observations towards 66 massive star forming regions identified by the Red MSX source survey. We have used the Green Bank Telescope and the K-band focal plane array to map the ammonia NH3 (1,1) and (2,2) inversion emission at a resolution of 30 arcsec in 8 arcmin regions towards the positions of embedded massive star formation. We have identified a total of 115 distinct clumps, approximately two-thirds of which are associated with an embedded massive young stellar object or compact HII region, while the others are classified as quiescent. There is a strong spatial correlation between the peak NH3 emission and the presence of embedded objects. We derive the spatial distribution of the kinetic gas temperatures, line widths, and NH$_3$ column densities from these maps, and by combining these data with dust emission maps we estimate clump masses, H$_2$ column densities and ammonia abundances. The clumps have typical masses of ~1000 Msun and radii ~0.5 pc, line widths of ~2 km/s and kinetic temperatures of ~16-20 K. We find no significant difference between the sizes and masses of the star forming and quiescent subsamples; however, the distribution maps reveal the presence of temperature and line width gradients peaking towards the centre for the star forming clumps while the quiescent clumps show relatively uniform temperatures and line widths throughout. Virial analysis suggests that the vast majority of clumps are gravitationally bound and are likely to be in a state of global free fall in the absence of strong magnetic fields. The similarities between the properties of the two subsamples suggest that the quiescent clumps are also likely to form massive stars in the future, and therefore provide a excellent opportunity to study the initial conditions of massive pre-stellar and protostellar clumps.
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