ArticlePublisher preview available

A heatwave of accretion energy traced by masers in the G358-MM1 high-mass protostar

May 2020
Nature Astronomy 4(5):1-5

Authors:

National Astronomical Observatory of Japan

High-mass stars are thought to accumulate much of their mass via short, infrequent bursts of disk-aided accretion1,2. Such accretion events are rare and difficult to observe directly but are known to drive enhanced maser emission3–6. In this Letter we report high-resolution, multi-epoch methanol maser observations toward G358.93-0.03, which reveal an interesting phenomenon: the subluminal propagation of a thermal radiation ‘heatwave’ emanating from an accreting high-mass protostar. The extreme transformation of the maser emission implies a sudden intensification of thermal infrared radiation from within the inner (40-mas, 270-au) region. Subsequently, methanol masers trace the radial passage of thermal radiation through the environment at ≥4% of the speed of light. Such a high translocation rate contrasts with the ≤10 km s−1 physical gas motions of methanol masers typically observed using very-long-baseline interferometry (VLBI). The observed scenario can readily be attributed to an accretion event in the high-mass protostar G358.93-0.03-MM1. While being the third case in its class, G358.93-0.03-MM1 exhibits unique attributes hinting at a possible ‘zoo’ of accretion burst types. These results promote the advantages of maser observations in understanding high-mass-star formation, both through single-dish maser monitoring campaigns and via their international cooperation as VLBI arrays. A ring of maser emission seemingly expanding at 0.05 c is actually tracing the propagation of heat through the circumstellar medium around a high-mass protostar rather than subluminal motion. The heatwave is a manifestation of an accretion burst.

Spectral profiles of the 6.7-GHz methanol maser emission in G358-MM1 Solid areas and lines indicate the auto- and cross-correlation spectra, respectively. Magnifications are shown in the lower insets to display low-flux-density components. The dashed line indicates the source systemic velocity. Source data

…

Methanol maser distributions in G358-MM1 Zeroth- (contours) and first- (colours) moment maps of the 6.7-GHz methanol maser emission in G358-MM1. Left, the distribution of emission during the vx026a epoch; right, that of vx026c, taken 26 days later. Moment image cubes were produced for emission above a 5σ cut-off and contours increase by factors of 2 multiples of the first contour at 2 Jy beam⁻¹ km s⁻¹. The white cross indicates the position of the brightest millimetre continuum peak of the G358-MM1 region¹⁶.

…

Schematic illustration of the observational data Left: a schematic model of the maser distribution and evolution in an accreting star–disk system. Right: spot maps of emission above 5σ detailing the evolution of methanol maser emission in G358-MM1. Colours indicate the velocity in the frame of the local standard of rest, and symbol sizes are arbitrary. The upper and lower panels illustrate the data of vx026a (2 February 2019) and vx026c (28 February 2019), respectively. Directional offsets are stated with respect to the coordinates (RA, dec.) = (17 h 43 min 10.1014 s, −29° 51′ 45.693″), which correspond to the position of G358-MM1 where the symbol size indicates the 40-mas absolute positional uncertainty in the continuum source position from ref. ¹⁶. The dark-grey rings delineate the fits to each epoch, while the light-grey ring indicates the extent of the vx026a masers at the epoch of vx026c. Source data

…

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Letters

https://doi.org/10.1038/s41550-019-0989-3

1Mizusawa VLBI Observatory, National Astronomical Observatory of Japan, Mitaka, Tokyo, Japan. 2Korea Astronomy and Space Science Institute, Daejeon,

Republic of Korea. 3NARIT, Chiang Mai, Thailand. 4University of Science and Technology, Korea (UST), Daejeon, Republic of Korea. 5Ural Federal University,

Ekaterinburg, Russia. 6Thüringer Landessternwarte, Tautenburg, Germany. 7The University of Western Ontario, London, Ontario, Canada. 8Hartebeesthoek

Radio Astronomy Observatory, Krugersdorp, South Africa. 9Center for Astronomy, Ibaraki University, Ibaraki, Japan. 10Centre for Astronomy, Faculty

of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Torun, Poland. 11School of Natural Sciences, University of Tasmania, Hobart,

Tasmania, Australia. 12Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi, Xinjiang, China. 13Dublin Institute for Advanced Studies,

Astronomy & Astrophysics Section, Dublin, Ireland. 14NRAO, Charlottesville, VA, USA. 15Australia Telescope National Facility, CSIRO, Epping, New South

Wales, Australia. 16Max Planck Institute for Astronomy, Heidelberg, Germany. 17INAF Osservatorio Astronomico di Cagliari, Selargius, Italy. 18Space

Research Unit, Physics Department, North West University, Potchefstroom, South Africa. 19Department of Physics and Astronomy, Faculty of Physical

Sciences, University of Nigeria, Nsukka, Nigeria. 20Netherlands Institute for Radio Astronomy, Dwingeloo, The Netherlands. 21Max-Planck-Institut für

Radioastronomie, Bonn, Germany. *e-mail: ross.burns@nao.ac.jp

High-mass stars are thought to accumulate much of their

mass via short, infrequent bursts of disk-aided accretion1,2.

Such accretion events are rare and difficult to observe directly

but are known to drive enhanced maser emission3–6. In this

Letter we report high-resolution, multi-epoch methanol maser

observations toward G358.93-0.03, which reveal an interest-

ing phenomenon: the subluminal propagation of a thermal

radiation ‘heatwave’ emanating from an accreting high-mass

protostar. The extreme transformation of the maser emission

implies a sudden intensification of thermal infrared radiation

from within the inner (40-mas, 270-au) region. Subsequently,

methanol masers trace the radial passage of thermal radiation

through the environment at ≥4% of the speed of light. Such

a high translocation rate contrasts with the ≤10 km s−1 physi-

cal gas motions of methanol masers typically observed using

very-long-baseline interferometry (VLBI). The observed sce-

nario can readily be attributed to an accretion event in the

high-mass protostar G358.93-0.03-MM1. While being the

third case in its class, G358.93-0.03-MM1 exhibits unique

attributes hinting at a possible ‘zoo’ of accretion burst types.

These results promote the advantages of maser observations

in understanding high-mass-star formation, both through

single-dish maser monitoring campaigns and via their inter-

national cooperation as VLBI arrays.

Masers provide a novel approach to investigating accretion

bursts3–6, the 51 → 60 A+ methanol transition at 6.7 GHz being of par-

ticular suitability as it arises in the presence of far-infrared radiation

from warm (>100-K) dust and high gas densities (105–8 cm−3)7, mak-

ing it a signpost of high-mass-star formation8. This maser has been

seen to trace rotating disks and tori9–11, and its emission actively

responds to changes in its local environment12.

G358.93-0.03 was discovered by its 6.7-GHz methanol maser

in the Methanol Multibeam survey13, conducted in 2006. The early

6.7-GHz spectrum showed several <10 Jy peaks in the velocity range

of −22.0 to −14.5 km s−1. In January 2019 a flare of the 6.7-GHz

methanol maser at −15.9 km s−1 was identified14 using the Hitachi

32-m telescope15, prompting intensive monitoring and follow-up

observations across a wide range of facilities. These observations,

coordinated by the Maser Monitoring Organisation (M2O, a global

cooperative of maser monitoring programmes—MaserMonitoring.

org), constitute the first intensive observational campaign con-

ducted during the onset of an accretion burst in a high-mass star.

Early results from target-of-opportunity observations with the

Submillimeter Array, the Atacama Large Millimeter/submillimeter

Array (ALMA)16, the Australia Telescope Compact Array (ATCA)17,

the NSF’s Karl G. Jansky Very Large Array (O. S. Bayandina etal.,

manuscript in preparation; X. Chen etal., manuscript in prepara-

tion) and the Stratospheric Observatory for Infrared Astronomy

(SOFIA) (B. S. etal., manuscript in preparation) have already been

established. These contemporary works have revealed striking

temporal behaviour16,18, rich and dynamic hot-core chemistry16,17,

complex maser emission16 (O. S. Bayandina etal., manuscript in

preparation; X. Chen etal., manuscript in preparation) and a kine-

matic signature indicating possible expansion16. The (sub)millime-

tre dust continuum uncovered a cluster environment of bolometric

luminosity Lbol= 5,700–22,000 L⊙ with the most luminous source,

G358.93-0.03-MM1 (hereafter ‘G358-MM1’), being the counterpart

to the aforementioned flare activity16. A comparison of 160-μm flux

densities measured before (Hi-GAL19) and during the burst with

FIFI-LS20 aboard SOFIA showed an increase from 111.728 ± 0.690 Jy

to 295.7 ± 13.7 Jy (B. S. etal., manuscript in preparation), roughly

tripling, and verifying the occurrence of an accretion burst

in G358-MM1.

Using the systemic line-of-sight velocity of G358-MM1 with

respect to the local standard of rest, vLSR = −16.5 ± 0.3 km s−1 (ref. 16),

gives a kinematic distance of

D¼

6:75þ0:

�

kpc via the Revised

Kinematic Galactic distance estimation tool provided by the

BeSSeL project21. Visible stars within a 0.25-arcmin field around

G358 observed as part of the Gaia mission have distance estimates

of ≤5 kpc (ref. 22). Extinction from the G358 star-forming region

A heatwave of accretion energy traced by masers

in the G358-MM1 high-mass protostar

R. A. Burns 1,2*, K. Sugiyama1,3, T. Hirota1, Kee-Tae Kim2,4, A. M. Sobolev5, B. Stecklum6,

G. C. MacLeod7,8, Y. Yonekura9, M. Olech10, G. Orosz11,12, S. P. Ellingsen 11, L. Hyland11,

A. Caratti o Garatti 13, C. Brogan14, T. R. Hunter14, C. Phillips 15, S. P. van den Heever8, J. Eislöffel6,

H. Linz16, G. Surcis 17, J. O. Chibueze18,19, W. Baan20 and B. Kramer 3,21

NATURE ASTRONOMY | VOL 4 | MAY 2020 | 506–510 | www.nature.com/natureastronomy

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Variabilities of Methanol Maser and Thermal Molecular Lines from an Accretion Burst Source G358.93-0.03

Article

Full-text available

Oct 2024

From 2019 January to June, the high-mass young stellar object G358.93-0.03 underwent a remarkable accretion burst. Detecting variations in the physical and kinematic environments during episodic accretion is essential for exploring such events. Searching for new maser or molecular lines and monitoring their variability during the accretion burst and postburst stages is feasible for such a study. Using the Purple Mountain Observatory 13.7 m telescope, we carried out monitoring of methanol masers and molecular lines toward G358.93-0.03 in the 3 mm band during the burst (2019 March to June) and postburst stages (2022 October). In addition to the previously abundant detection of the methanol maser lines in the burst stage, eight new methanol maser lines were detected, identified from transitions at 85.57, 93.20, 94.54, 94.82, 99.77, 102.96, 104.35, and 104.41 GHz. Their integrated intensity exhibited an exponential decline during the burst stage, likely associated with the decay process of the accretion burst. None of the eight methanol maser lines were detected in the postburst stage. Besides maser lines, 12 thermal molecular lines were detected in both the burst and postburst stages. Variability of some of them was observed in both stages, but most notably, the ratio of HCO ⁺ /N 2 H ⁺ increased significantly in the burst stage compared to the postburst stage. It is likely caused by the changes of both molecular lines during the burst and postburst stages due to the heating process induced by episodic accretion. Therefore, the comparison of their line intensity ratios might offer a new method for tracing episodic accretion bursts.

The accretion burst of the massive young stellar object G323.46 -0.08

Preprint

Full-text available

May 2024

Accretion bursts from low-mass young stellar objects (YSOs) are known for many decades. In recent years, the first accretion bursts of massive YSOs (MYSOs) were observed. These phases of intense protostellar growth are of particular importance for studying massive star formation. Bursts of MYSOs are accompanied by flares of Class II methanol masers (hereafter masers), caused by an increase in exciting mid-infrared (MIR) emission. The G323.46

-

0.08 (hereafter G323) event extends the small sample of known MYSO bursts. Maser observations of the MYSO G323 show evidence of a flare, which was presumed to be caused by an accretion burst. This should be verified with IR data. We used time-dependent radiative transfer (TDRT) to characterize the heating and cooling timescales for eruptive MYSOs and to infer the main burst parameters. The G323 accretion burst is confirmed. It reached its peak in late 2013/early 2014 with a Ks-band increase of 2.5mag. TDRT indicates that the duration of the thermal afterglow in the far-infrared (FIR) can exceed the burst duration by years. The latter was proved by SOFIA observations, which indicate a flux increase of

(14.2\pm4.6)

% at

70\, \rm \mu m

and

(8.5\pm6.1)

% at

160\, \mu

m in 2022 (2 years after the burst end). A one-sided light echo emerged that was propagating into the interstellar medium. The G323 burst is probably the most energetic MYSO burst observed so far. Within

8.4 \rm \, yrs

, an energy of

(0.9\pm_{0.8}^{2.5}) \times 10^{47}\,\rm erg

was released. The short timescale points to the accretion of a compact body, while the burst energy corresponds to an accumulated mass of at least

(7\pm_{6}^{20})\,M_{Jup}

and possibly even more if the protostar is bloated. In this case, the accretion event might have triggered protostellar pulsations, which give rise to the observed maser periodicity.

Thermal and maser molecular emission in infrared dark clouds

Conference Paper

Jan 2024

Nadya Shakhvorostova

In this paper, we briefly discuss the observational manifestations of the star formation process in infrared dark clouds in the Galactic interstellar medium. Examples of such manifestations are presented, and a brief overview of the characteristics of molecular clouds is given. The results of observations of 37 molecular cores in infrared dark clouds in the methanol maser transition at 44 GHz and in the thermal molecular lines at 85 and 97 GHz are presented.

Sample and Statistical Analysis on NEOWISE Variability of ATLASGAL Sources

Article

Full-text available

Jun 2024

This study focuses on the analysis of mid-infrared variability in a sample of high-mass young stellar objects (YSOs) associated with the cataloged sources from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). The Near-Earth Object Wide-field Infrared Survey Explorer Reactivation Mission (NEOWISE) database was used to explore the long-term mid-infrared variability of these high-mass YSOs at a half-year scale. After matching with NEOWISE photometric measurements, a total of 2230 ATLASGAL sources were selected for the variability analysis, out of which 717 were identified as variables. The derived proportions of variables at different evolutionary stages show that the variability rate of high-mass YSOs is highest during the YSO stage and decreases with evolution toward the H ii region stage, resembling the behavior of low-mass YSOs. The variables can be classified into six types based on their light curves, divided into two categories: secular (linear, sin, sin+linear) and stochastic variables (burst, drop, and irregular). The magnitude–color variations observed in ∼160 secular variables can be mainly divided into “bluer when brighter/redder when dimming” and “redder when brighter/bluer when dimming,” likely originating from changes in accretion rate or the effect of extinction due to obscuration. Moreover, several episodic accretion candidates were selected for further observational studies.

Masers and Star Formation Activities in W51A

Article

Full-text available

Jun 2024

Concurrently with the maser flare observed in W51-North during the 3 month monitoring of H 2 O, NH 3 , and CH 3 OH maser variability from 2020 January to April using the Tianma 65 m Radio Telescope, we conducted Very Large Array mappings for these three maser species across the entirety of W51A region. After finding the ring-shaped H 2 O maser which might trace the disk surrounding the protostar residing in W51-North, the NH 3 (9,6) maser delineated a jet which might be illuminated in the luminosity outburst possibly caused by the infalling streamer’s interaction with the protostar’s disk. An analysis of the comprehensive distribution of maser spots allowed us to affirm that W51N4 or ALMAmm31 serves as the primary source of the Lacy jet. Furthermore, we observed that class I methanol maser spots may extend beyond the locations of the H 2 O and NH 3 (9,6) masers within the outflow. Additionally, emissions from other NH 3 maser transitions coincided with specific 1.3 mm continuum sources. The arrangement of H 2 O maser spots in the vicinity of W51e2-E potentially indicates episodic accretions in this source. Combining the data from the Atacama Large Millimeter/submillimeter Array data archive for W51-North, W51e2, and W51e8, we have discovered that the H 2 O, NH 3 , and CH 3 OH masers, as well as the HC 3 N and SiO emissions are found to be good tools for tracing outflow in this work for W51A.

Identifying the mechanisms of water maser variability during the accretion burst in NGC6334I

Article

Full-text available

Oct 2024

Context. High-mass young stellar objects gain most of their mass in short intense bursts of accretion. Maser emission is an invaluable tool in discovering and probing these accretion bursts. Aims. Our aim was to observe the 22 GHz water maser response induced by the accretion burst in NGC6334I-MM1B and to identify the underlying maser variability mechanisms. Methods. We report seven epochs of very long baseline interferometry (VLBI) observations of 22 GHz water masers in NGC6334I with the VLBI Exploration of Radio Astrometry (VERA) array, from 2014 to 2016, spanning the onset of the accretion burst in 2015.1. We also report 2019 Atacama Large Millimeter/submillimeter Array (ALMA) observations of 321 GHz water masers and 22 GHz single-dish maser monitoring by the Hartebeesthoek Radio Astronomical Observatory (HartRAO). We analysed long-term variability patterns and used proper motions with the 22 GHz to 321 GHz line ratio to distinguish between masers in non-dissociative C-shocks and dissociative J-shocks. We also calculated the burst-to-quiescent variance ratio of the single-dish time series. Results. We detected a water maser distribution resembling a bipolar outflow morphology. The constant mean proper motion before and after the burst indicates that maser variability is due to excitation effects from variable radiation rather than jet ejecta. For the whole region, we find that the flux density variance ratio in the single-dish time series can identify maser efficiency variations in 22 GHz masers. The northern region, CM2-W2, is excited in C-shocks and showed long-term flaring with velocity-dependent excitation of new maser features after the onset of the burst. We propose that radiative heating of H 2 due to high-energy radiation from the accretion burst be the main mechanism for the flaring in CM2-W2. The southern regions are excited by J-shocks, which have shown short-term flaring and dampening of water masers. We attribute the diverse variability patterns in the southern regions to the radiative transfer of the burst energy in the complex source geometry. Conclusions. Our results indicate that the effects of source geometry, shock type, and incident radiation spectrum are fundamental factors affecting 22 GHz maser variability. Investigating water masers in irradiated shocks will improve their use as a diagnostic in time-variable radiation environments, such as accretion bursting sources.

Radiative transfer modeling of outbursts of massive young stellar objects

Preprint

Full-text available

May 2024

Verena Wolf

Young stellar objects (YSOs) accrete up to half of their material in short periods of enhanced mass accretion. For massive YSOs (MYSOs with more than 8 solar masses), accretion outbursts are of special importance, as they serve as diagnostics in highly obscured regions. Within this work, two outbursting MYSOs within different evolutionary stages, the young source G358.93-0.03 MM1 (G358) and the more evolved one G323.46-0.08 (G323), are investigated, and the major burst parameters are derived. For both sources, follow-up observations with the airborne SOFIA observatory were performed to detect the FIR afterglows. All together, we took three burst-/post-observations in the far infrared. The burst parameters are needed to understand the accretion physics and to conclude on the possible triggering mechanisms behind it. Up to today, G323s burst is the most energetic one ever observed for a MYSO. G358s burst was about two orders of magnitude weaker and shorter (2 months instead of 8 years). We suggest that G358s burst was caused by the accretion of a spiral fragment (or a small planet), where G323 accreted a heavy object (a planet or even a potential companion). To model those sources, we use radiative transfer (RT) simulations (static and time-dependent). G323s accretion burst is the first astrophysical science case, that is modeled with time-dependent RT (TDRT). We incorporate a small TDRT parameter-study and develop a time-depending fitting tool (the TFitter) for future modeling.

The accretion burst of the massive young stellar object G323.46-0.08

Article

Full-text available

May 2024

Context. Accretion bursts from low-mass young stellar objects (YSOs) have been known for many decades. In recent years, the first accretion bursts of massive YSOs (MYSOs) have been observed. These phases of intense protostellar growth are of particular importance for studying massive star formation. Bursts of MYSOs are accompanied by flares of Class II methanol masers (hereafter masers), which are caused by an increase in exciting mid-infrared (MIR) emission. They can lead to long-lasting thermal afterglows of the dust continuum radiation visible at infrared (IR) and (sub)millimeter (hereafter (sub)mm) wavelengths. Furthermore, they might cause a scattered light echo. The G323.46−0.08 (hereafter G323) event, which shows all these features, extends the small sample of known MYSO bursts. Aims. Maser observations of the MYSO G323 show evidence of a flare, which was presumed to be caused by an accretion burst. This should be verified with IR data. We used time-dependent radiative transfer (TDRT) to characterize the heating and cooling timescales for eruptive MYSOs and to infer the main burst parameters. Methods. Burst light curves, as well as the pre-burst spectral energy distribution (SED) were established from archival IR data. The properties of the MYSO, including its circumstellar disk and envelope, were derived by using static radiative transfer modeling of pre-burst data. For the first time, TDRT was used to predict the temporal evolution of the SED. Observations with SOFIA/HAWC+ were performed to constrain the burst energy from the strength of the thermal afterglow. Image subtraction and ratioing were applied to reveal the light echo. Results. The G323 accretion burst is confirmed. It reached its peak in late 2013/early 2014 with a K s -band increase of ∼2.5 mag. Both K s -band and integrated maser flux densities follow an exponential decay. TDRT indicates that the duration of the thermal afterglow in the far-infrared (FIR) can exceed the burst duration by years. The latter was proved by SOFIA observations, which indicate a flux increase of (14.2 ± 4.6)% at 70 μm and (8.5 ± 6.1)% at 160 μm in 2022 (2 yr after the burst ended). A one-sided light echo emerged that was propagating into the interstellar medium. Conclusions. The burst origin of the G323 maser flare has been verified. TDRT simulations revealed the strong influence of the burst energetics and the local dust distribution on the strength and duration of the afterglow. The G323 burst is probably the most energetic MYSO burst that has been observed so far. Within 8.4 yr, an energy of (0.9 −0.8 +2.5 ) × 10 ⁴⁷ erg was released. The short timescale points to the accretion of a compact body, while the burst energy corresponds to an accumulated mass of at least (7 −6 ⁺²⁰ ) M Jup and possibly even more if the protostar is bloated. In this case, the accretion event might have triggered protostellar pulsations, which give rise to the observed maser periodicity. The associated IR light echo is the second observed from a MYSO burst.

Radio outburst from a massive (proto)star: III. Unveiling the bipolarity of the radio jet from S255IR NIRS 3

Article

Full-text available

Mar 2024

We report new Very Large Array high-resolution observations of the radio jet from the outbursting high-mass star S255IR NIRS 3. The images at 6, 10, and 22.2 GHz confirm the existence of a new lobe emerging to the SW and expanding at a mean speed of ∼285 km s ⁻¹ , about half as fast as the NE lobe. The new data allow us to reproduce both the morphology and the continuum spectrum of the two lobes with the model already adopted in our previous studies. We conclude that in all likelihood both lobes are powered by the same accretion outburst. We also find that the jet is currently fading down, recollimating, and recombining.

The influence of accretion bursts on methanol and water in massive young stellar objects

Article

Full-text available

Feb 2024

Context . The effect of accretion bursts on massive young stellar objects (MYSOs) represents a new research field in the study of young stars and their environment. The impact of such bursts on the disk and envelope has been observed and plays the role of a “smoking gun” providing information about the properties of the burst itself. Aims . We aim to investigate the impact of an accretion burst on massive disks with different types of envelopes and to study the effects of an accretion burst on the temperature structure and the chemistry of the disk. We focus on water and methanol as chemical species for this paper. Methods . The thermochemical code of ProDiMo (PROtoplanetary DIsk MOdel) is used to perform simulations of high-mass protoplanetary-disk models with different types of envelopes in the presence of an accretion burst. The models in question represent different evolutionary stages of protostellar objects. We calculated and show the chemical abundances in three phases of the simulation (pre-burst, burst, and post-burst). Results . More heavily embedded disks show higher temperatures. The impact of the accretion burst is mainly characterized by the desorption of chemical species present in the disk and envelope from the dust grains to the gas phase. When the post-burst phase starts, the sublimated species freeze out again. The degree of sublimation depends strongly on the type of envelope the disk is embedded in. An accretion burst in more massive envelopes produces stronger desorption of the chemical species. However, our models show that the timescale for the chemistry to reach the pre-burst state is independent of the type of envelope. Conclusions . The study shows that the disk’s temperature increases with a more massive envelope enclosing it. Thus, the chemistry of MYSOs in earlier stages of their evolution reacts stronger to an accretion burst than at later stages where the envelope has lost most of its mass or has been dissipated. The study of the impact of accretion bursts could also provide helpful theoretical context to the observation of methanol masers in massive disks.

Sub-arcsecond (Sub)millimeter Imaging of the Massive Protocluster G358.93−0.03: Discovery of 14 New Methanol Maser Lines Associated with a Hot Core

Article

Full-text available

Aug 2019

We present (sub)millimeter imaging at 0.″5 resolution of the massive star-forming region G358.93-0.03 acquired in multiple epochs at 2 and 3 months following the recent flaring of its 6.7 GHz CH3OH maser emission. Using the Submillimeter Array and Atacama Large Millimeter/submillimeter Array, we have discovered 14 new Class II CH3OH maser lines ranging in frequency from 199 to 361 GHz, which originate mostly from {v}t = 1 torsionally excited transitions and include one {v}t = 2 transition. The latter detection provides the first observational evidence that Class II maser pumping involves levels in the {v}t = 2 state. The masers are associated with the brightest continuum source (MM1), which hosts a line-rich hot core. The masers present a consistent curvilinear spatial velocity pattern that wraps around MM1, suggestive of a coherent physical structure 1200 au in extent. In contrast, the thermal lines exhibit a linear pattern that crosses MM1 but at progressive position angles that appear to be a function of either increasing temperature or decreasing optical depth. The maser spectral profiles evolved significantly over one month, and the intensities dropped by factors of 3.0-7.2, with the {v}t = 2 line showing the largest change. A small area of maser emission from only the highest excitation lines closest to MM1 has disappeared. There are seven additional dust continuum sources in the protocluster, including another hot core (MM3). We do not find evidence for a significant change in (sub)millimeter continuum emission from any of the sources during the one month interval, and the total protocluster emission remains comparable to prior single-dish measurements.

Discovery of Six New Class II Methanol Maser Transitions, Including the Unambiguous Detection of Three Torsionally Excited Lines toward G 358.931–0.030

Article

Full-text available

May 2019

We present the unambiguous discovery of six new class II methanol maser transitions, three of which are torsionally excited ( v t = 1). The newly discovered 6.18 GHz 17 −2 → 18 −3 E ( v t = 1), 7.68 GHz 12 4 → 13 3 A ⁻ ( v t = 0), 7.83 GHz 12 4 → 13 3 A ⁺ ( v t = 0), 20.9 GHz 10 1 → 11 2 A ⁺ ( v t = 1), 44.9 GHz 2 0 → 3 1 E ( v t = 1), and 45.8 GHz 9 3 → 10 2 E ( v t = 0) methanol masers were detected toward G 358.931−0.030, where the known 6.68 GHz maser has recently been reported to be undergoing a period flaring. The detection of the v t = 1 torsionally excited lines corroborates one of the missing puzzle pieces in class II maser pumping, but the intensity of the detected emission provides an additional challenge, especially in the case of the very highly excited 6.18 GHz line. Together with the newly detected v t = 0 lines, these observations provide significant new information that can be utilized to improve class II methanol maser modeling. We additionally present detections of 6.68, 19.9, 23.1, and 37.7 GHz class II masers, as well as 36.2 and 44.1 GHz class I methanol masers, and provide upper limits for the 38.3 and 38.5 GHz class II lines. Near simultaneous Australia Telescope Compact Array observations confirm that all 10 of the class II methanol maser detections are co-spatial to ∼0.2 arcsec, which is within the uncertainty of the observations. We find significant levels of linearly polarized emission in the 6.18, 6.67, 7.68, 7.83, 20.9, 37.7, 44.9, and 45.8 GHz transitions, and low levels of circular polarization in the 6.68, 37.7, and 45.8 GHz transitions.

Estimating Distance from Parallaxes. IV. Distances to 1.33 Billion Stars in Gaia Data Release 2

Article

Full-text available

Jul 2018
ASTRON J

For the vast majority of stars in the second Gaia data release, reliable distances cannot be obtained by inverting the parallax. A correct inference procedure must instead be used to account for the nonlinearity of the transformation and the asymmetry of the resulting probability distribution. Here, we infer distances to essentially all 1.33 billion stars with parallaxes published in the second Gaia data release. This is done using a weak distance prior that varies smoothly as a function of Galactic longitude and latitude according to a Galaxy model. The irreducible uncertainty in the distance estimate is characterized by the lower and upper bounds of an asymmetric confidence interval. Although more precise distances can be estimated for a subset of the stars using additional data (such as photometry), our goal is to provide purely geometric distance estimates, independent of assumptions about the physical properties of, or interstellar extinction toward, individual stars. We analyze the characteristics of the catalog and validate it using clusters. The catalog can be queried using ADQL at http://gaia.ari.uni-heidelberg.de/tap.html (which also hosts the Gaia catalog) and downloaded from http://www.mpia.de/∼calj/gdr2-distances.html. © 2018. The American Astronomical Society. All rights reserved..

FIFI-LS: The Field-Imaging Far-Infrared Line Spectrometer on SOFIA

Article

Full-text available

Jun 2018

We describe the design of the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS), operated as a Facility-Class instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA). FIFI-LS is an imaging spectrometer for medium resolution spectroscopy. Since being commissioned in 2014, it has performed over 50 SOFIA commissioning and science flights. After operating as a principal investigator instrument in 2014 and early 2015, it was accepted as a Facility Science Instrument in 2015. In addition to the description of the design, we report on the in-flight performance and the concept of operation. We also provide an overview of the science opportunities with FIFI-LS and describe how FIFI-LS observations complement and complete observations with the PACS instrument on the Herschel observatory.

A Masing Event in NGC6334I: Contemporaneous Flaring of Hydroxyl, Methanol and Water Masers

Article

Full-text available

Apr 2018
MON NOT R ASTRON SOC

As a product of the maser monitoring program with the 26m telescope of the Hartebeesthoek Radio Astronomy Observatory (HartRAO), we present an unprecedented, contemporaneous flaring event of 10 maser transitions in hydroxyl, methanol, and water that began in 2015 January in the massive star-forming region NGC6334I in the velocity range -10 to -2 km/s. The 6.7 GHz methanol and 22.2 GHz water masers began flaring within 22 days of each other, while the 12.2 GHz methanol and 1665 MHz hydroxyl masers flared 80 and 113 days later respectively. The 1665 MHz, 6.7 GHz, and 22.2 GHz masers have all remained in their flared state for nearly 3 years. The brightest flaring components increased by factors of 66, 21, 26, and 20 in the 12.2 and 6.7 GHz methanol, 1665 MHz hydroxyl and 22.2 GHz water maser transitions respectively; some weaker components increased by up to a factor of 145. We also report new maser emission in the 1720, 6031, and 6035 MHz OH lines and the 23.1 GHz methanol line, along with the detection of only the fifth 4660 MHz OH maser. We note the correlation of this event with the extraordinary (sub)millimeter continuum outburst from the massive protostellar system NGC6334I-MM1 and discuss the implications of the observed time lags between different maser velocity components on the nature of the outburst. Finally, we identify two earlier epoch maser flaring events likely associated with this object, which suggest a recurring accretive phenomenon that generates powerful radiative outbursts.

Extended CH3OH maser flare excited by a bursting massive YSO

Article

Full-text available

Mar 2017

Aims. Recently, substantial flaring in the 6.7 GHz methanol maser line has been observed toward the high-mass young stellar object (YSO) S255 NIRS 3, where an accretion burst was also detected in the IR. Our goal is to study the change in the properties of the 6.7 GHz masers between the pre- and outburst phases, and investigate the connection between the maser and the accretion burst. Methods. With the Karl G. Jansky Very Large Array (JVLA) and the European VLBI Network (EVN), we performed observations of the 6.7 GHz masers (covering a range in angular resolution from a few milliarcseconds to ≈ 1′′) during the burst phase and compared these observations with pre-burst measurements at similar spatial scales. Results. The accretion burst and the subsequent increase in IR luminosity are very likely the origin of the 6.7 GHz maser flare. Since most maser centers operate in the unsaturated regime, a change by a relatively small factor (≈ 5) in the flux of pumping photons has produced an exponential growth in the maser intensity. The main pre-burst maser cluster is no longer detected during the burst. Compared to the pre-burst phase, flaring 6.7 GHz masers emit across a different VLSR range that is more strongly redshifted, and the emission extends over a larger area at larger separation from the high-mass YSO. In particular, the outburst peak emission originates from a remarkably extended (0.′ 2-0.′ 3) maser plateau at a radial distance of 500-1000 AU from the source. Conclusions. Both the maser flare and the extraordinarily large extent of the maser structure can be a natural consequence of the burst in the accretion luminosity of the high-mass YSO. Our results strongly support models that predict IR radiative pumping for the 6.7 GHz CH3OH masers.

Detection of new methanol maser transitions associated with G358.93−0.03

Article

Nov 2019
MON NOT R ASTRON SOC

We report the detection of new 12.178, 12.229, 20.347, and 23.121 GHz methanol masers in the massive star-forming region G358.93−0.03, which are flaring on similarly short time-scales (days) as the 6.668 GHz methanol masers also associated with this source. The brightest 12.178 GHz channel increased by a factor of over 700 in just 50 d. The masers found in the 12.229 and 20.347 GHz methanol transitions are the first ever reported and this is only the fourth object to exhibit associated 23.121 GHz methanol masers. The 12.178 GHz methanol maser emission appears to have a higher flux density than that of the 6.668 GHz emission, which is unusual. No associated near-infrared flare counterpart was found, suggesting that the energy source of the flare is deeply embedded.

Giant burst of methanol maser in S255IR-NIRS3

Article

Jul 2018

Context. High-mass young stellar objects (HMYSOs) can undergo accretion episodes that strongly affect the star evolution, the dynamics of the disk, and its chemical evolution. Recently reported extraordinary bursts in the methanol maser emission may be the observational signature of accretion events in deeply embedded HMYSOs. Aims. We analyze the light curve of 6.7 GHz methanol masers in S255IR-NIRS3 during the 2015–2016 burst. Methods. 8.5-yr monitoring data with an average sampling interval of 5 days were obtained with the Torun 32 m radio telescope. Archival data were added, extending the time series to ~27 yr. Results. The maser emission showed moderate (25–30%) variability on timescales of months to years over ~23 yr since its discovery. The main burst was preceded by a 1 yr increase of the total flux density by a factor of 2.5, then it grew by a factor of 10 over ~0.4 yr and declined by a factor of 8 during the consecutive 2.4 yr. The peak maser luminosity was a factor of 24.5 higher than the pre-burst quiescent value. The light curves of individual features showed considerable diversity but indicated a general trend of suppression of the maser emission at blueshifted (<4.7 km s ⁻¹ ) velocities when the redshifted emission rapidly grew and new emission features appeared at velocities >5.8 km s ⁻¹ . This new emission provided a contribution of about 80% to the maser luminosity around the peak of the burst. The duration of the burst at the extreme redshifted velocities of 7.1–8.7 km s ⁻¹ was from 0.9 to 1.9 yr, and its lower limit for the other features was ~3.9 yr. Conclusions. The onset of the maser burst exactly coincides with that of the infrared burst estimated from the motion of the light echo. This strongly supports the radiative pumping scheme of the maser transition. The growth of the maser luminosity is the result of an increasing volume of gas where the maser inversion is achieved.

The bursting variability of 6.7 GHz methanol maser of G33.641-0.228

Article

Sep 2017

From 2014 to 2015, we conducted a total of 469 days observation of the 6.7 GHz methanol maser in a star forming region G33.641-0.228, known to be a bursting maser source. As a result, eleven bursts were detected. On MJD 57364, the flux density grew by more than six times w.r.t the day before. Moreover, during the largest burst, the flux density repeatedly increased and decreased rapidly with time-scale as short as 0.24 day. Since these characteristics of the burst are similar to the solar burst, we speculate that the burst of the 6.7 GHz methanol maser in G33.641-0.228 might occur with a similar mechanism of the solar burst.

The Extraordinary Outburst in the Massive Protostellar System NGC6334I-MM1: Emergence of Strong 6.7 GHz Methanol Masers

Article

Jan 2018
ASTROPHYS J

We report the first sub-arcsecond VLA imaging of 6 GHz continuum, methanol maser, and excited-state hydroxyl maser emission toward the massive protostellar cluster NGC6334I following the recent 2015 outburst in (sub)millimeter continuum toward MM1, the strongest (sub)millimeter source in the protocluster. In addition to detections toward the previously known 6.7 GHz Class II methanol maser sites in the hot core MM2 and the UCHII region MM3 (NGC6334F), we find new maser features toward several components of MM1, along with weaker features

\sim1''

north, west, and southwest of MM1, and toward the non-thermal radio continuum source CM2. None of these areas have heretofore exhibited Class II methanol maser emission in three decades of observations. The strongest MM1 masers trace a dust cavity, while no masers are seen toward the strongest dust sources MM1A, 1B and 1D. The locations of the masers are consistent with a combination of increased radiative pumping due to elevated dust grain temperature following the outburst, the presence of infrared photon propagation cavities, and the presence of high methanol column densities as indicated by ALMA images of thermal transitions. The non-thermal radio emission source CM2 (

2''

north of MM1) also exhibits new maser emission from the excited 6.035 and 6.030 GHz OH lines. Using the Zeeman effect, we measure a line-of-sight magnetic field of +0.5 to +3.7 mG toward CM2. In agreement with previous studies, we also detect numerous methanol and excited OH maser spots toward the UCHII region MM3, with predominantly negative line-of-sight magnetic field strengths of -2 to -5 mG and an intriguing south-north field reversal.

A heatwave of accretion energy traced by masers in the G358-MM1 high-mass protostar

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