Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSpitzer-IRAC GLIMPSE of high mass protostellar objects II - SED modelling of a bonafide sample
102
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We aim to estimate and analyse the physical properties of the infrared counterparts of HMPOs by comparing their spectral energy distributions (SED) with those predicted by radiative transfer accretion models of YSOs. The SED of 68 IRCs are extended beyond the GLIMPSE photometry to the possible limits, from the near-infrared to the millimetre wavelengths by using the 2MASS, GLIMPSE version 2.0 catalogs, MSX, IRAS and some single dish (and interferometric) (sub)mm data. An online SED fitting tool that uses 2D radiative transfer accretion models of YSOs is employed to fit the observed SED to obtain various physical parameters. The SED of IRCs were fitted by models of massive protostars with a range of masses between 5-42 Msun and ages between 10^3 and 10^6 years. The median mass and age are 10 Msun and 10^4 yrs. The envelopes are large with a mean size of ~ 0.2-0.3 pc and show a distribution that is very similar to the distribution of the sizes of 8 micron nebulae discussed in Paper I. The estimated envelope accretion rates are high with a mean value of 10^(-3) Msun/yr and show a power law dependence to mass with an exponent of 2, suggesting spherical accretion at those scales. Disks are found to exist in most of the sources with a mean mass of 10^(-1.4+-0.7) Msun. The observed infrared-millimetre SED of the infrared counterparts of HMPOs are successfully explained with an YSO accretion model. The modelled sources mostly represent proto-B stars although some of them could become O stars in future. We demonstrate that many of these results may represent a realistic picture of massive star formation, despite some of the results which may be an effect of the assumptions within the models.
rate research
Read More
The Spitzer-GLIMPSE point source catalog and images have been used to study a sample of 381 massive protostellar candidates. IRAC-Point source photometry was used to analyse colours, magnitudes and spectral indicies of the infrared counterparts (IRCs) of massive protostellar objects and a bonafide sample of 50 point sources was obtained. Spectral energy distributions (SEDs) of these 50 sources was extended to the near-infrared and millimeter range by using 2MASS and millimeter data from the literature. An online SED fitter tool based on Monte-Carlo radiative transfer of an accretion model involving star,disk and envelope was used to fit the SEDs of the 50 sources. The IRCs to massive protostellar objects are found to successfully imitate the SEDs of evolutionary phases similar to low mass star formation. Envelope accretion, rather than disk accretion is found to be dominant in building the most massive stars. Unresolved centimeter continuum emission is associated with 27 IRCs classified as massive protostars suggesting that ionised accretion flows may play an important role along with the molecular component. The morphology of the infrared nebulae surrounding the IRCs have an unusual resemblance to the morphologies of ultra-compact HII regions suggesting that these infrared nebulae are possible precursors to the UCHII regions.
The GLIMPSE archive was used to obtain 3.6--8.0micron, point source photometry and images for 381 massive protostellar candidates lying in the Galactic mid-plane. The colours, magnitudes and spectral indicies of sources in each of the 381 target fields were analysed and compared with the predictions of 2D radiative transfer model simulations. Although no discernable embedded clusters were found in any targets, multiple sources or associations of redenned young stellar objects were found in many sources indicating multiplicity at birth. The spectral index ($alpha$) of these point sources in 3.6--8.0mum bands display large values of $alpha$=2--5. A color-magnitude analog plot was used to identify 79 infrared counterparts to the HMPOs. Compact nebulae are found in 75% of the detected sources with morphologies that can be well described by core-halo, cometary, shell-like and bipolar geometries similar to those observed in ultra-compact HII regions. The IRAC band SEDs of the IR counterparts of HMPOs are best described to represent YSOs with a mass range of 8--20msun in their Class I stages when compared with 2D radiative transfer models. They also suggest that the high $alpha$ values represent reprocessed star/star+disk emission that is arising in the dense envelopes. Thus we are witnessing the luminous envelopes around the protostars rather than their photospheres or disks. We argue that the compact infrared nebulae likely reflect the underlying physical structure of the dense cores and are found to imitate the morphologies of known UCHII regions. Our results favour models of continuuing accretion involving both molecular and ionised accretion components to build the most massive stars rather than purely molecular rapid accretion flows.
The role of accretion disks in the formation of low-mass stars has been well assessed by means of high angular resolution observations at various wavelengths. These findings confirm the prediction that conservation of angular momentum during the collapse leading to the formation of a star is bound to produce flattening and rotation of the collapsing core. What about high-mass stars? At present, several authors have reported on detections of disks around high-mass YSOs. Notwithstanding these important results, the presence of disks rotating about high-mass stars is not sufficient by itself to prove unambiguously the accretion model: what is needed is iron-clad evidence of infall. Such evidence is very difficult to find, as the free-fall velocity becomes significant only very close to the accreting star, i.e., over a region of a few 0.01 pc ($sim$2000 au), which is very difficult to access and disentangle from the surrounding quiescent or rotating material. In this chapter we discuss how to characterize the infall of material in a sample of 36 high-mass accretion disk candidates covering a broad range of luminosities, from 10$^3$ $L_odot$ to 10$^6$ $L_odot$, compiled by Beltran & de Wit (2016) with the next generation Very Large Array (ngVLA).
We present the results of CS J=2-1 mapping observations towards 39 massive star-forming regions selected from the previous CO line survey of cold IRAS sources with high-velocity CO flows along the Galactic plane (Yang et al. 2002). All sources are detected in CS J=2-1 showing the existence of CS clumps around the IRAS sources. However, one-third of the sources are not deeply embedded in the dense clumps by comparison of the central powering IRAS sources and the morphologies of CS clumps. Physical parameters of the dense molecular clumps are presented. We have identified 12 high-mass protostellar object (HMPO) candidates by checking the association between the dense cores and the IRAS sources, the detection of water maser, and the radio properties towards the IRAS sources. We find that the HMPO sources are characterized by low FIR luminosity to virial mass ratios since they are in very early evolutionary stages when the massive protostars have not reached their full luminosities, which are typical for zero-age main sequence stars. Large turbulent motion in the HMPO sources may be largely due to the large kinetic energy ejected by the central protostars formed in the dense clumps. However, alternative means or undetected outflows may also be responsible for the turbulence in the clumps.
When studying the evolutionary stages of protostars that form in clusters, the role of any intracluster medium cannot be neglected. High foreground extinction can lead to situations where young stellar objects (YSOs) appear to be in earlier evolutionary stages than they actually are, particularly when using simple criteria like spectral indices. To address this issue, we have assembled detailed SED characterizations of a sample of 56 Spitzer-identified candidate YSOs in the clusters NGC 2264 and IC 348. For these, we use spectra obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope and ancillary multi-wavelength photometry. The primary aim is twofold: 1) to discuss the role of spectral features, particularly those due to ices and silicates, in determining a YSOs evolutionary stage, and 2) to perform comprehensive modeling of spectral energy distributions (SEDs) enhanced by the IRS data. The SEDs consist of ancillary optical-to-submillimeter multi-wavelength data as well as an accurate description of the 9.7 micron silicate feature and of the mid-infrared continuum derived from line-free parts of the IRS spectra. We find that using this approach, we can distinguish genuine protostars in the cluster from T Tauri stars masquerading as protostars due to external foreground extinction. Our results underline the importance of photometric data in the far-infrared/submillimeter wavelength range, at sufficiently high angular resolution to more accurately classify cluster members. Such observations are becoming possible now with the advent of the Herschel Space Observatory.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
