[Paper Review] A Class I and Class II Methanol Maser Survey of Extended Green Objects (EGOs) from the GLIMPSE Survey

C. J. Cyganowski, C. L. Brogan+2 more|arXiv (Cornell University)|Jul 6, 2009

Astrophysics and Star Formation Studies6 references148 citations

TL;DR

This study conducts a high-resolution VLA survey of Class I (44 GHz) and Class II (6.7 GHz) CH3OH masers toward 20 Extended Green Objects (EGOs) from the GLIMPSE survey, finding high detection rates of 64% for Class II masers and 89% for Class I masers in sources with Class II emission. The results confirm EGOs as young, massive YSOs with active outflows powered by ongoing accretion.

ABSTRACT

We present the results of a high angular resolution Very Large Array (VLA) Class I 44 GHz and Class II 6.7 GHz methanol maser survey of a sample of ~20 massive young stellar object (MYSO) outflow candidates selected on the basis of extended 4.5 micron emission in Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) images. These 4.5 micron-selected candidates are referred to as extended green objects (EGOs), for the common coding of this band as green in three-color IRAC images. The detection rate of 6.7 GHz Class II methanol masers, which are associated exclusively with massive YSOs, towards EGOs is greater than ~64%--nearly double the detection rate of surveys using other MYSO selection criteria. The detection rate of Class I 44 GHz methanol masers, which trace molecular outflows, is ~89% towards EGOs associated with 6.7 GHz methanol masers. The two types of methanol masers exhibit different spatial distributions: 6.7 GHz masers are centrally concentrated and usually coincide with 24 micron emission, while 44 GHz masers are widely distributed and generally trace diffuse 4.5 micron features. We also present results of a complementary James Clerk Maxwell Telescope (JCMT) single-pointing molecular line survey of EGOs in the outflow tracers HCO+(3-2) and SiO(5-4). The HCO+ line profiles and high SiO detection rate (90%) are indicative of the presence of active outflows. No 44 GHz continuum emission is detected at the 5 mJy/beam (5 sigma) level towards 95% of EGOs surveyed, excluding bright ultracompact HII regions as powering sources for the 4.5 micron outflows. The results of our surveys constitute strong evidence that EGOs are young, massive YSOs, with active outflows, presumably powered by ongoing accretion.

Motivation & Objective

• To identify and characterize massive young stellar objects (MYSOs) with active accretion and outflows using infrared-selected EGOs.
• To test whether EGOs—identified by extended 4.5 μm emission in Spitzer GLIMPSE images—host masers indicative of ongoing star formation.
• To determine the spatial and kinematic relationship between masers, outflows, and infrared emission in EGOs.
• To rule out ultracompact H ii regions as the source of 4.5 μm emission by searching for 44 GHz continuum emission.
• To confirm the presence of molecular outflows via complementary JCMT line surveys of HCO+ (3-2) and SiO (5-4).

Proposed method

• Conducted high angular resolution Very Large Array (VLA) observations at 6.7 GHz (Class II CH3OH masers) and 44 GHz (Class I CH3OH masers) toward a sample of ~20 EGOs.
• Used Spitzer GLIMPSE 4.5 μm images to identify EGOs as extended green objects with morphology distinct from point sources.
• Performed complementary single-pointing JCMT observations of HCO+ (3-2) and SiO (5-4) lines to trace molecular outflows.
• Conducted 44 GHz continuum observations to search for emission from ultracompact H ii regions as alternative power sources of 4.5 μm emission.
• Analyzed spatial correlation between maser positions, 24 μm, 4.5 μm, and molecular line emission to infer outflow structure and excitation mechanisms.
• Applied statistical analysis to maser detection rates and line profile diagnostics to infer outflow activity and source evolution.

Experimental results

Research questions

• RQ1 What is the detection rate of 6.7 GHz Class II CH3OH masers toward EGOs, and how does it compare to other MYSO selection methods?
• RQ2 What is the spatial distribution of Class I (44 GHz) and Class II (6.7 GHz) CH3OH masers relative to 4.5 μm and 24 μm emission in EGOs?
• RQ3 Are ultracompact H ii regions responsible for the 4.5 μm extended emission in EGOs, as suggested by some models?
• RQ4 Do the HCO+ and SiO line profiles confirm the presence of active molecular outflows in EGOs?
• RQ5 What is the relationship between maser emission and the physical conditions of the outflowing gas in EGOs?

Key findings

• The detection rate of 6.7 GHz Class II CH3OH masers toward EGOs is ≥64%, nearly double the typical detection rate in other MYSO surveys.
• The detection rate of 44 GHz Class I CH3OH masers is ∼89% in EGOs that host 6.7 GHz masers, indicating strong association with outflow activity.
• 6.7 GHz masers are centrally concentrated and spatially coincident with 24 μm emission, consistent with emission from the central protostar.
• 44 GHz masers are widely distributed and trace diffuse 4.5 μm features, indicating they trace the extended outflow lobes.
• The SiO(5-4) line was detected in 90% of EGOs surveyed, with complex line profiles indicating active, high-velocity outflows.
• No 44 GHz continuum emission was detected at the 5 mJy beam⁻¹ (5σ) level toward 95% of EGOs, excluding ultracompact H ii regions as the source of 4.5 μm emission.