Too large and overlooked? Extended free-free emission towards massive star formation regions

Longmore, Steven N.; Burton, M. G.; Keto, Eric; Kurtz, S.; Walsh, A. J.

doi:10.1111/j.1365-2966.2009.15320.x

Cited by 11 publications

(17 citation statements)

References 16 publications

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“…Several UC H ii regions have been observed to be embedded in extended radio continuum emission (Kurtz et al 1999). The recent work of Longmore et al (2009) also found a significant fraction of 6.7 GHz methanol maser sources that had no 3.6 cm radio continuum as measured by Walsh et al (1998), to have extended emission with the total flux at times being several orders of magnitude discrepant from previous limits. Missing an extended radio continuum component due to spatial filtering of an interferometer can lead to errors in the classification and inferred age of the source.…”

Section: Resultsmentioning

confidence: 65%

“…With the exception of 39.39-0.14, no extended emission was found in the sources. Although this does not rule out the presence of extended emission on scales larger than 30 , it suggests a lack of continuum sources similar to those discovered by Longmore et al (2009). In the case of 39.39-0.14, the extended emission led to the discovery of a counterpart at 3.6 cm leading to reclassification of the nature of radio continuum emission from optically thick to optically thin emission between 3.6 cm and 1.3 cm.…”

Section: Resultsmentioning

confidence: 97%

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Spectral energy distributions of 6.7 GHz methanol masers

Pandian

Momjian

et al. 2010

A&A

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Context. The 6.7 GHz maser transition of methanol has been found exclusively towards massive star forming regions. A majority of the masers have been found to lack the presence of any associated radio continuum. This could be due to the maser emission originating prior to the formation of an H ii region around the central star, or from the central object being too cool to produce a H ii region.Aims. One way to distinguish between the two scenarios is to determine and model the spectral energy distributions (SEDs) of the masers. Methods. We observed a sample of 20 6.7 GHz methanol masers selected from the blind Arecibo survey, from centimeter to submillimeter wavelengths. We combined our observations with existing data from various Galactic plane surveys to determine SEDs from centimeter to near-infrared wavelengths. Results. We find that 70% of the masers do not have any associated radio continuum, with the rest of the sources being associated with hypercompact and ultracompact H ii regions. Modeling the SEDs shows them to be consistent with rapidly accreting massive stars, with accretion rates well above 10 −3 M yr −1 . The upper limits on the radio continuum are also consistent with any ionized region being confined close to the stellar surface. Conclusions. This confirms the paradigm of 6.7 GHz methanol masers being signposts of early phases of massive star formation, mostly prior to the formation of a hypercompact H ii region.

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Section: Resultsmentioning

confidence: 65%

Section: Resultsmentioning

confidence: 97%

Spectral energy distributions of 6.7 GHz methanol masers

Pandian

Momjian

et al. 2010

A&A

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“…Longmore et al (2009) report the detection of spatially extended, optically thin freefree emission in the fields of six 6.7 GHz CH 3 OH masers previously reported as 3.6 cm continuum nondetections (Walsh et al 1998), and caution that the evolutionary state of CH 3 OH maser sources may be misconstrued if extended free-free emission is missed in highresolution interferometric observations. We believe that our cm continuum study of EGOs is robust against such misclassification for several reasons.…”

Section: (See Also C11)mentioning

confidence: 99%

“…(3) In several cases, the extended HII regions reported by Longmore et al (2009) are offset from the CH 3 OH maser positions, and the masers are more closely associated with HC HII regions or with 3 mm emission attributed to dust. So, as Longmore et al (2009) note, the presence of more evolved HII regions in the vicinity of 6.7 GHz CH 3 OH masers may simply be due to the clustered nature of massive star formation. In fact, we see several potential examples of this in our data.…”

Section: (See Also C11)mentioning

confidence: 99%

DEEP VERY LARGE ARRAY RADIO CONTINUUM SURVEYS OF GLIMPSE EXTENDED GREEN OBJECTS (EGOs)

Cyganowski

Brogan

Hunter

et al. 2011

ApJ

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We present the results of deep, high angular resolution Very Large Array (VLA) surveys for radio continuum emission towards a sample of 14 GLIMPSE Extended Green Objects (EGOs). Identified as massive young stellar object (MYSO) outflow candidates based on their extended 4.5 µm emission in Spitzer images, the EGOs in our survey sample are also associated with 6.7 GHz Class II and/or 44 GHz Class I CH 3 OH masers. No continuum is detected at 3.6 or 1.3 cm towards the majority (57%) of our targets (median rms ∼0.03 and 0.25 mJy beam −1 ). Only two EGOs are associated with optically thin emission consistent with ultracompact/compact HII regions. Both of these sources exhibit cm−λ multiplicity, with evidence that one of the less-evolved members may be driving the 4.5 µm outflow. Most of the other cm-λ EGO counterparts are weak ( 1 mJy), unresolved, undetected at 1.3 cm, and characterized by intermediate spectral indices consistent with hypercompact (HC) HII regions or ionized winds or jets. One EGO cm counterpart, likely an optically thick HC HII region, is detected only at 1.3 cm and is associated with hot core line emission and H 2 O and 6.7 GHz CH 3 OH masers. The results of our exceptionally sensitive survey indicate that EGOs signify an early stage of massive star formation, before photoionizing feedback from the central MYSO significantly influences the (proto)cluster environment. Actively driving outflows (and so, presumably, actively accreting), the surveyed EGOs are associated with significant clump-scale gas reservoirs, providing sufficient material for sustained, rapid accretion.

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“…In one of these rotating cores, G24 A1, the presence of an early-type star is witnessed by an embedded hypercompact (HC) Hii region, first detected at 1.3 cm by Codella et al (1997) and later studied at very high-angular resolution at 1.3 and 0.7 cm by Beltrán et al (2007). The properties of the ionized gas have also been studied through continuum emission and recombination lines by Galván-Madrid et al (2008) and Longmore et al (2009). This HC Hii region, which has a diameter of < ∼ 0.…”

Section: Introductionmentioning

confidence: 99%

Molecular outflows and hot molecular cores in G24.78+0.08 at sub-arcsecond angular resolution

Beltrán

Cesaroni

Zhang

et al. 2011

A&A

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Context. This study is part of a large project to study the physics of accretion and molecular outflows towards a selected sample of high-mass star-forming regions that show evidence of infall and rotation from previous studies. Aims. We wish to make a thorough study at high-angular resolution of the structure and kinematics of the HMCs and corresponding molecular outflows in the high-mass star-forming region G24.78+0.08. Methods. We carried out SMA and IRAM PdBI observations at 1.3 and 1.4 mm, respectively, of dust and of typical high-density and molecular outflow tracers with resolutions of <1 . Complementary IRAM 30-m 12 CO and 13 CO observations were carried out to recover the short spacing information of the molecular outflows. Results. The millimeter continuum emission towards cores G24 A1 and A2 has been resolved into three and two cores, respectively, and named A1, A1b, A1c, A2, and A2b. All these cores are aligned in a southeast-northwest direction coincident with that of the molecular outflows detected in the region, which suggests a preferential direction for star formation in this region. The masses of the cores range from 7 to 22 M , and the rotational temperatures from 128 to 180 K. The high-density tracers have revealed the existence of two velocity components towards A1. One of them peaks close to the position of the millimeter continuum peak and of the HC Hii region and is associated with the velocity gradient seen in CH 3 CN towards this core, while the other one peaks southwest of core A1 and is not associated with any millimeter continuum emission peak. The position-velocity plots along outflow A and the 13 CO (2-1) averaged blueshifted and redshifted emission indicate that this outflow is driven by core A2. Core A1 apparently does not drive any outflow. The knotty appearance of the highly collimated outflow C and the 12 CO position-velocity plot suggest an episodic outflow, where the knots are made of swept-up ambient gas.

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Too large and overlooked? Extended free-free emission towards massive star formation regions

Cited by 11 publications

References 16 publications

Spectral energy distributions of 6.7 GHz methanol masers

Spectral energy distributions of 6.7 GHz methanol masers

DEEP VERY LARGE ARRAY RADIO CONTINUUM SURVEYS OF GLIMPSE EXTENDED GREEN OBJECTS (EGOs)

Molecular outflows and hot molecular cores in G24.78+0.08 at sub-arcsecond angular resolution

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