The Evolution of Massive Young Stellar Objects in the Large Magellanic Cloud. I. Identification and Spectral Classification

Abstract

We present and categorize Spitzer infrared spectrometer spectra of 294 objects in the Large Magellanic Cloud (LMC) to create the largest and most complete catalog of massive young stellar object (YSO) spectra in the LMC. Target sources were identified from infrared photometry and multiwavelength images indicative of young, massive stars highly enshrouded in their natal gas and dust clouds. Several objects have been spectroscopically identified as non-YSOs and have features similar to more-evolved stars such as red supergiants, asymptotic giant branch (AGB), and post-AGB stars. Our sample primarily consists of 277 objects we identify as having spectral features indicative of embedded YSOs. The remaining sources are comprised of seven C-rich evolved sources, eight sources dominated by broad silicate emission, and one source with multiple broad emission features. Those with YSOlike spectra show a range of spectral features including polycyclic aromatic hydrocarbon emission, deep silicate absorption, fine-structure lines, and ice absorption features. Based upon the relative strengths of these features, we have classified the YSO candidates into several distinct categories using the widely used statistical procedure known as principal component analysis. We propose that these categories represent a spectrum of evolutionary stages during massive YSO formation. Using our catalog we put statistical constraints on the relative evolutionary timescale of processes involved in massive star formation. We conclude that massive pre-main-sequence stars spend a majority (possibly as high as 90%) of their massive, embedded lives emitting in the UV. Half of the sources in our study have features typical of compact H ii regions, suggesting that massive YSOs can create a detectable compact H ii region half-way through the formation time present in our sample. This study also provides a check on commonly used source-selection procedures including the use of photometry to identify YSOs. We determine that a high success rate (>95%) of identifying objects with YSO-like spectra can be achieved through careful use of infrared color-magnitude diagrams, spectral energy distributions, and image inspections.

Spectroscopic observations of ices around embedded young stellar objects in the Large Magellanic Cloud with AKARI

“…It is known that massive YSOs in their more evolved stage ionize their circumstellar gas by stellar UV radiation and form compact HII regions around the central star (van den Ancker et al 2000). A variation in their hydrogen line strength thus indicates that the present targets are in their various evolutionary stages and possess different radiation environments (especially as for UV) in their circumstellar region, as discussed in Seale et al (2009). We measured the equivalent width of the Brα line at 4.05 μm, which is the strongest hydrogen recombination line in the wavelength range of our observations.…”

Dust and gas environment of the young embedded cluster IRAS 18511+0146

“…Evidences point towards S11 being relatively more evolved as compared to the S7 and we find a variation in their silicate profiles where the silicate feature of the former is wider and peaks at a longer wavelength compared to the latter. Our study of massive young objects from the same cluster, although younger and more embedded than the sample considered by Seale et al (2009) show that silicate absorption profiles can be combined with other multiwavelength tracers to assess the evolutionary stage of the embedded object. However, further studies of silicate features as well as other tracers from the cluster members in different evolutionary stages are required to confirm this.…”

“…A census of silicate absorption features towards hundreds of massive young stellar objects in the Large Magellanic Cloud shows that the evolutionary stage of the young stellar objects can be categorised based on silicate features, emission lines and PAHs (Seale et al 2009). Evidences point towards S11 being relatively more evolved as compared to the S7 and we find a variation in their silicate profiles where the silicate feature of the former is wider and peaks at a longer wavelength compared to the latter.…”

The youngest massive protostars in the Large Magellanic Cloud

“…The presence of ice absorption is quite definitive; silicate absorption and aromatic emission are at least strongly suggestive (in the parsec-sized beam). To date, ∼300 sources in the LMC have been spectroscopically confirmed or supported (Shimonishi et al 2008;Oliveira et al 2009;Seale et al 2009;van Loon et al 2010). Of these, 126 lie in the HERITAGE SDP area; our list includes 58 (the remainder are too faint or or too confused).…”

“…Each of these regions has two clumps of masers, and all four sites are associated with mid-IR sources identified as YSO candidates. Three of these sources have been spectroscopically observed (Seale et al 2009). …”