A compact starburst ring traced by clumpy OH megamaser emission (original) (raw)
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Masers as probes of massive star formation in the nuclear disk
2007
OH(1720 MHz) and methanol masers are now recognized to be excellent probes of the interactions of supernova remnants with molecular clouds and tracers of massive star formation, respectively. To better understand the nature of star formation activity in the central region of the Galaxy, we have used these two classes of masers combined with the IRAC and MIPS data to study prominent sites of ongoing star formation in the nuclear disk. The nuclear disk is characterized by massive GMCs with elevated gas temperatures, compared to their dust temperatures. We note an association between methanol masers and a class of mid-infrared "green sources". These highly embedded YSOs show enhanced 4.5µm emission due to excited molecular lines.
Infrared variability, maser activity, and accretion of massive young stellar objects
Proceedings of the International Astronomical Union
Methanol and water masers indicate young stellar objects. They often exhibit flares, and a fraction shows periodic activity. Several mechanisms might explain this behavior but the lack of concurrent infrared (IR) data complicates the identification of its cause. Recently, 6.7 GHz methanol maser flares were observed, triggered by accretion bursts of high-mass YSOs which confirmed the IR-pumping of these masers. This suggests that regular IR changes might lead to maser periodicity. Hence, we scrutinized space-based IR imaging of YSOs associated with periodic methanol masers. We succeeded to extract the IR light curve from NEOWISE data for the intermediate mass YSO G107.298+5.639. Thus, for the first time a relationship between the maser and IR variability could be established. While the IR light curve shows the same period of ~34.6 days as the masers, its shape is distinct from that of the maser flares. Possible reasons for the IR periodicity are discussed.
The Astrophysical Journal Supplement Series, 2017
We present a survey of 56 massive star-forming regions in the 44 GHz methanol maser transition made with the Karl G. Jansky Very Large Array (VLA); 24 of the 56 fields showed maser emission. The data allow us to demonstrate associations, at arcsecond precision, of the Class I maser emission with outflows, HII regions, and shocks traced by 4.5 micron emission. We find a total of 83 maser components with linewidths ranging from 0.17 to 3.3 km s −1 with a nearly flat distribution and a median value of 1.1 km s −1. The relative velocities of the masers with respect to the systemic velocity of the host clouds range from −2.5 to 3.1 km s −1 with a distribution peaking near zero. We also study the correlation between the masers and the so-called extended green objects (EGOs) from the GLIMPSE survey. Multiple sources in each field are revealed from IR images as well as from centimeter continuum emission from VLA archival data; in the majority of cases the 44 GHz masers are positionally correlated with EGOs which seem to trace the younger sources in the fields. We report a possible instance of a 44 GHz maser associated with a low-mass protostar. If confirmed, this region will be the fifth known star-forming region that hosts Class I masers associated with low-mass protostars. We discuss three plausible cases of maser variability.
13CH3OH Masers Associated With a Transient Phenomenon in a High-mass Young Stellar Object
The Astrophysical Journal, 2020
We report the first detection of isotopic methanol (13 CH 3 OH) maser emission in interstellar space. The emission was detected toward the high-mass young stellar object G358.93-0.03 during monitoring of a flare in the 6.7 GHz methanol (CH 3 OH) maser emission in this source. We find that the spectral and spatial distribution of the 13 CH 3 OH masers differs from the CH 3 OH masers imaged at the same epoch, contrary to expectations from similarity of their pumping. This conclusively demonstrates that isotopic methanol masers are bright under different physical conditions and suggests that they can provide additional, complementary information to the CH 3 OH masers from the same source. We detect a rapid decay of the 13 CH 3 OH maser lines suggesting that they are transient phenomena (masing for only a few months), likely associated with rapid changes in radiation field due to an accretion burst induced by massive disk fragmentation. Changes in the line flux density are faster than required to achieve equilibrium in the energy level populations, indicating that the pumping of these masers is likely variable.
Ring shaped 6.7 GHz methanol maser emission around a young high-mass star
Astronomy and Astrophysics, 2005
We report on EVN imaging of the 6.7 GHz methanol maser emission from the candidate high-mass protostar G23.657-0.127. The masers originate in a nearly circular ring of 127 mas radius and 12 mas width. The ring structure points at a central exciting object which characteristics are typical for a young massive star; its bolometric luminosity is estimated to be ≤ 3.2 × 10 4 L ⊙ and ≤ 1.2 × 10 5 L ⊙ for near (5.1 kpc) and far (10.5 kpc) kinematic distances, respectively. However, the spatial geometry of the underlying maser region remains ambiguous. We consider scenarios in which the methanol masers originate in a spherical bubble or in a rotating disc seen nearly face-on.
Interferometric Maser Observations from Outflows/Disks in Star-Forming Regions
Revista Mexicana de …, 2002
Las observaciones interferométricas de la emisión maser de diversas moléculas (p.ej., H 2 O, OH, CH 3 OH) están constituyendo una herramienta muy eficaz para estudiar con gran resolución angular el gas molecular cercano a las protoestrellas. Gracias a ellas se ha observado que, en algunos casos, los máseres parecen trazar el disco que se forma alrededor del objeto estelar joven, mientras que en otros casos parecen trazar el propio flujo de material eyectado. En este artículo resumimos algunos de esos estudios observacionales, incluyendo los realizados muy recientemente mediante la poderosa técnica de interferometría de base muy larga (VLBI). Esas observaciones VLBI permiten estudiar los máseres con resolución angular de una pocas décimas de milisegundo de arco (equivalente a una resolución lineal de unas pocas décimas de unidad astronómica para objetos que se encuentran a una distancia de 500 pc), permitiendo con ello medir sus movimientos propios.
Astronomy & Astrophysics
Context. Periodic flares of maser emission are thought to be induced either by variations of the seed photon flux in young binary systems or the pump rate regulated by stellar and accretion luminosities. Aims. We seek to study the variability of four maser transitions of three different species in G107.298+5.639 to constrain the dominant mechanism of periodic flares. Methods. Light curves of the 6.7 GHz methanol and 22.2 GHz water vapour maser were obtained with the Torun 32 m radio telescope over 39 and 34 cycles, respectively. The target was also monitored at the 1.6 GHz hydroxyl transitions with the Nançay radio telescope over 13 cycles. All these maser lines were imaged using VLBI arrays. Results. The study confirms alternating flares of the methanol and water masers with a period of 34.4 d and reveals the synchronised behaviour of the methanol and hydroxyl masers in this source. The observed spatial distribution of the methanol maser cloudlets and the measured time delays of th...
Masers associated with high-mass star formation regions in the Large Magellanic Cloud
Monthly Notices of the Royal Astronomical Society, 2010
We report the results of a sensitive search for 12.2-GHz methanol maser emission towards a sample of eight high-mass star formation regions in the Large Magellanic Clouds which have been detected in other maser transitions. We detected one source towards the star formation region N105a. This is the first detection of a 12.2-GHz methanol maser outside our Galaxy. We also made near-contemporaneous observations of the 6.7-GHz methanol and 22-GHz water masers towards these sources, resulting in the detection of water maser emission in six new sources, including one associated with the strongest 6.7-GHz maser in the Magellanic Clouds IRAS 05011-6815. The majority of the maser sources are closely associated with objects identified as likely Young Stellar Objects (YSO) on the basis of Spitzer Space Telescope observations. We find that the YSOs associated with masers tend to be more luminous and have redder infrared colours than the sample as a whole. SED modeling of the YSOs shows that the masers are associated with sources of higher central mass, total luminosity and ambient density than the majority of YSOs in the LMC. This is consistent with the well-established relationship between luminous methanol and water masers and young, high-mass objects observed in the Galaxy.
OH Megamasers, Starburst and AGN Activity in Markarian 231
Monthly Notices of the Royal Astronomical Society, 2005
We present Multi-Element Radio-Linked Interferometer Network (MERLIN) observations of OH maser and radio continuum emission within a few hundred pc of the core of the ultraluminous infrared galaxy (ULIRG) Markarian 231. This is the only known OH megamaser galaxy classed as a Seyfert 1. Maser emission is identified with the 1665-and 1667-MHz transitions over a velocity extent of 720 km s −1. Both lines show a similar position-velocity structure including a gradient of 1.7 km s −1 pc −1 from NW to SE along the 420-pc major axis. The (unresolved) inner few tens of pc possess a much steeper velocity gradient. The maser distribution is modelled as a torus rotating about an axis inclined at ∼45 • to the plane of the sky. We estimate the enclosed mass density to be 320 ± 90 M pc −3 in a flattened distribution. This includes a central unresolved mass of 8 × 10 6 M. All the maser emission is projected against a region with a radio continuum brightness temperature 10 5 K, giving a maser gain of 2.2. The 1667:1665 MHz line ratio is close to 1.8 (the value predicted for thermal emission) consistent with radiatively pumped, unsaturated masers. This behaviour and the kinematics of the torus suggest that the size of individual masing regions is in the range 0.25-4 pc with a covering factor close to unity. There are no very bright compact masers, in contrast to galaxies such as the Seyfert 2 Markarian 273, where the masing torus is viewed nearer edge-on. The comparatively modest maser amplification seen from Markarian 231 is consistent with its classification in the unification scheme for Seyfert galaxies. Most of the radio continuum emission on 50-500 pc scales is probably of starburst origin but the compact peak is 0.4 per cent polarized by a magnetic field running north-south, similar to the jet direction on these scales. There is no close correlation between maser and continuum intensity, suggesting that much of the radio continuum must originate in the foreground and indeed the relative continuum brightness is slightly greater in the direction of the approaching jet. Comparisons with other data show that the jet changes direction close the nucleus and suggest that the sub-kpc disc hosting the masers and starburst activity is severely warped.