[Paper Review] Spitzer-IRAC GLIMPSE of high mass protostellar objects. I Infrared point sources and nebulae

M. S. N. Kumar, J. M. C. Grave|ArXiv.org|May 30, 2007

Astrophysics and Star Formation Studies28 references20 citations

TL;DR

This study analyzes 381 high-mass protostellar object (HMPO) candidates using Spitzer-IRAC GLIMPSE data to identify infrared counterparts and nebular morphologies. It finds that high spectral indices (α = 2–5) and compact, structured nebulae—reminiscent of ultra-compact HII regions—indicate ongoing accretion from both molecular and ionized components, supporting a continuous accretion model for massive star formation rather than rapid, isolated molecular inflows.

ABSTRACT

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 ($α$) of these point sources in 3.6--8.0mum bands display large values of $α$=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--20\msun in their Class I stages when compared with 2D radiative transfer models. They also suggest that the high $α$ 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.

Motivation & Objective

• To identify infrared counterparts of high-mass protostellar object (HMPO) candidates in the Galactic mid-plane using Spitzer-IRAC GLIMPSE data.
• To investigate the mid-infrared (MIR) properties of HMPOs, including point source colors, magnitudes, and spectral indices, to constrain their evolutionary stages.
• To search for embedded clusters or multiplicity indicators around HMPOs via point source clustering and morphological analysis.
• To characterize compact infrared nebulae associated with HMPOs and compare their morphologies to known ultra-compact HII regions.
• To evaluate the role of ongoing accretion—both molecular and ionized—in the formation of massive stars by analyzing SEDs and spatial correlations with millimeter and centimeter data.

Proposed method

• Utilized the publicly available GLIMPSE point source photometry catalog and image cutouts for 381 HMPO candidates from four major surveys (Mol96, Sri02, Fon02, Faun04).
• Performed color-color diagram analysis on point sources in target fields and 40 control fields to identify reddened Class I and II young stellar objects (YSOs).
• Computed spectral indices (α) from IRAC 3.6–8.0 μm fluxes to assess the embedded nature and evolutionary stage of infrared counterparts.
• Constructed absolute magnitude vs. α-magnitude (AM product) diagrams to estimate stellar masses of detected YSOs, comparing with 2D radiative transfer models.
• Analyzed nebular morphologies in 8 μm images to classify compact nebulae into core-halo, cometary, shell-like, and bipolar structures.
• Correlated GLIMPSE results with existing millimeter and centimeter (VLA) data to assess the presence of ionized regions and accretion signatures.

Experimental results

Research questions

• RQ1 What are the mid-infrared properties (colors, magnitudes, spectral indices) of infrared counterparts associated with HMPO candidates?
• RQ2 Do the observed infrared counterparts and nebulae indicate ongoing multiplicity or embedded cluster formation at the birth of massive stars?
• RQ3 How do the morphologies of compact infrared nebulae compare to those of known ultra-compact HII regions, and what do they reveal about the physical structure of dense cores?
• RQ4 What is the nature of the high spectral indices (α = 2–5) observed in the IRAC bands, and what do they imply about the emission mechanisms (e.g., envelope vs. disk vs. photosphere)?
• RQ5 Is the observed data consistent with a continuous accretion model involving both molecular and ionized gas, or does it favor rapid, isolated molecular accretion?

Key findings

• 79 infrared counterparts to HMPOs were identified that are bright at 8 μm, centered on millimeter peaks, and exhibit spectral indices (α) exceeding 2, indicating deep embedding in dense envelopes.
• Spectral indices (α) of 3.6–8.0 μm point sources range from 2 to 5, consistent with reprocessed emission from protostellar envelopes rather than photospheres or disks.
• 60% of HMPO targets are associated with compact infrared nebulae displaying core-halo, cometary, shell-like, and bipolar morphologies similar to ultra-compact HII regions.
• The size distribution of nebulae in the youngest subsets (Sri02 and Mol96) peaks at 0.1–1 pc with a mean of 0.5 pc, indicating spatial confinement within millimeter-determined dense cores.
• The observed correlation between GLIMPSE infrared counterparts and centimeter continuum emission supports the presence of ionized regions near the central stars, indicating ongoing ionized accretion.
• The data favor a massive star formation scenario involving continuous accretion with both molecular and ionized components, rather than rapid, isolated molecular accretion flows.