arXiv subjects

S. P. Ellingsen

Publications and source records attributed to S. P. Ellingsen.

At least 19 recordsLinked to original sources

arXiv2023

ATLASGAL: 3-mm class I methanol masers in high-mass star formation regions

We analyzed the 3-mm wavelength spectral line survey of 408 ATLASGAL clumps observed with the IRAM 30m-telescope, focusing on the class I methanol masers with frequencies near 84, 95 and 104.3 GHz. We detect narrow, maser-like features towards 54, 100 and 4 sources in the maser lines near 84, 95 and 104.3 GHz, respectively. Among them, fifty 84 GHz masers, twenty nine 95 GHz masers and four rare 104.3 GHz masers are new discoveries. The new detections increase the number of known 104.3 GHz masers from 5 to 9. The 95 GHz class I methanol maser is generally stronger than the 84 GHz maser counterpart. We find 9 sources showing class I methanol masers but no SiO emission, indicating that class I methanol masers might be the only signpost of protostellar outflow activity in extremely embedded objects at the earliest evolutionary stage. Class I methanol masers that are associated with sources that show SiO line wings are more numerous and stronger than those without such wings. The total integrated intensity of class I methanol masers is well correlated with the integrated intensity and velocity coverage of the SiO (2--1) emission. The properties of class I methanol masers are positively correlated with the bolometric luminosity, clump mass, peak H$_2$ column density of their associated clumps but uncorrelated with the luminosity-to-mass ratio, dust temperature, and mean H$_2$ volume density. We suggest that the properties of class I masers are related to shocks traced by SiO. Based on our observations, we conclude that class I methanol masers at 84 and 95 GHz can trace a similar evolutionary stage as H$_2$O maser, and appear prior to 6.7 and 12.2 GHz methanol and OH masers. Despite their small number, the 104.3 GHz class I masers appear to trace a short and more evolved stage compared to the other class I masers. [abridged]

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arXiv2023

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

High-mass stars are thought to accumulate much of their mass via short, infrequent bursts of disk-aided accretion. Such accretion events are rare and difficult to observe directly but are known to drive enhanced maser emission. In this Letter we report high-resolution, multi-epoch methanol maser observations toward G358.93-0.03 which reveal an interesting phenomenon; the sub-luminal propagation of a thermal radiation "heat-wave" emanating from an accreting high-mass proto-star. 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 $\geq$ 4-8\% the speed of light. Such a high translocation rate contrasts with the $\leq$ 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 proto-star 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.

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arXiv2023

A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar

High-mass protostars (M$_{\star} >$ 8 M$_{\odot}$) are thought to gain the majority of their mass via short, intense bursts of growth. This episodic accretion is thought to be facilitated by gravitationally unstable and subsequently inhomogeneous accretion disks. Limitations of observational capabilities, paired with a lack of observed accretion burst events has withheld affirmative confirmation of the association between disk accretion, instability and the accretion burst phenomenon in high-mass protostars. Following its 2019 accretion burst, a heat-wave driven by a burst of radiation propagated outward from the high-mass protostar G358.93-0.03-MM1. Six VLBI (very long baseline interferometry) observations of the raditively pumped 6.7 GHz methanol maser were conducted during this period, tracing ever increasing disk radii as the heat-wave propagated outward. Concatenating the VLBI maps provided a sparsely sampled, milliarcsecond view of the spatio-kinematics of the accretion disk covering a physical range of $\sim$ 50 - 900 AU. We term this observational approach `heat-wave mapping'. We report the discovery of a Keplerian accretion disk with a spatially resolved four-arm spiral pattern around G358.93-0.03-MM1. This result positively implicates disk accretion and spiral arm instabilities into the episodic accretion high-mass star formation paradigm.

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arXiv2023

Ammonia masers toward G358.931-0.030

We report the detection of ammonia masers in the non-metastable (6, 3), (7, 5) and (6, 5) transitions, the latter is the first unambiguous maser detection of that transition ever made. Our observations include the first VLBI detection of ammonia maser emission, which allowed effective constrain of the (6, 5) maser brightness temperature. The masers were detected towards G358.931-0.030, a site of 6.7-GHz class~II methanol maser emission that was recently reported to be undergoing a period of flaring activity. These ammonia masers appear to be flaring contemporaneously with the class~II methanol masers during the accretion burst event of G358.931-0.030. This newly detected site of ammonia maser emission is only the twelfth such site discovered in the Milky Way. We also report the results of an investigation into the maser pumping conditions, for all three detected masing transitions, through radiative transfer calculations constrained by our observational data. These calculations support the hypothesis that the ammonia (6, 5) maser transition is excited through high colour temperature infrared emission, with the (6, 5) and (7, 5) transition line-ratio implying dust temperatures >400K. Additionally, we detect significant linearly polarised emission from the ammonia (6, 3) maser line. Alongside our observational and radiative transfer calculation results, we also report newly derived rest frequencies for the ammonia (6, 3) and (6, 5) transitions.

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arXiv2023

The Co-Ordinated Radio and Infrared Survey for High-Mass Star Formation. V. The CORNISH-South Survey and Catalogue

We present the first high spatial resolution radio continuum survey of the southern Galactic plane. The CORNISH project has mapped the region defined by $295^{\circ} < l < 350^{\circ}$; $|b| < 1^{\circ}$ at 5.5-GHz, with a resolution of 2.5$^{''}$ (FWHM). As with the CORNISH-North survey, this is designed to primarily provide matching radio data to the Spitzer GLIMPSE survey region. The CORNISH-South survey achieved a root mean square noise level of $\sim$ 0.11 mJy beam$^{-1}$, using the 6A configuration of the Australia Telescope Compact Array (ATCA). In this paper, we discuss the observations, data processing and measurements of the source properties. Above a 7$σ$ detection limit, 4701 sources were detected, and their ensemble properties show similar distributions with their northern counterparts. The catalogue is highly reliable and is complete to 90 per cent at a flux density level of 1.1 mJy. We developed a new way of measuring the integrated flux densities and angular sizes of non-Gaussian sources. The catalogue primarily provides positions, flux density measurements and angular sizes. All sources with IR counterparts at 8$μm$ have been visually classified, utilizing additional imaging data from optical, near-IR, mid-IR, far-IR and sub-millimetre galactic plane surveys. This has resulted in the detection of 524 H II regions of which 255 are ultra-compact H II regions, 287 planetary nebulae, 79 radio stars and 6 massive young stellar objects. The rest of the sources are likely to be extra-galactic. These data are particularly important in the characterization and population studies of compact ionized sources such as UCHII regions and PNe towards the Galactic mid-plane.

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arXiv2022

A sensitive search for water masers associated with star formation regions in the Local Group galaxy NGC 6822

We report the results of a sensitive search for water maser emission in the Local Group Galaxy NGC 6822 with the Karl G. Jansky Very Large Array. The observations provide tentative single-epoch detections of four candidates, associated with two infrared-bright star formation regions (Hubble I/III and Hubble IV). The candidate maser detections are all offset from the velocity range where strong emission from HI neutral gas is observed toward NGC 6822, with the closest offset by 40 kms$^{-1}$. Our observations include the location of NL1K, a previous tentative water maser detection in NGC 6822. We do not detect any emission from this location with a sensitivity limit approximately a factor of 5 better than the original Sardina Radio Telescope observations.

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arXiv2022

SPLASH: The Southern Parkes Large-Area Survey in Hydroxyl -- Data Description & Release

We present the full data release for the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH), a sensitive, unbiased single-dish survey of the Southern Galactic Plane in all four ground-state transitions of the OH radical at 1612, 1665, 1667 and 1720 MHz. The survey covers the inner Galactic Plane, Central Molecular Zone and Galactic Centre over the range $|b|<$ 2$^{\circ}$, 332$^{\circ}$ $< l <$ 10$^{\circ}$, with a small extension between 2$^{\circ}$ $< b <$ 6$^{\circ}$, 358$^{\circ}$ $< l <$ 4$^{\circ}$. SPLASH is the most sensitive large-scale survey of OH to-date, reaching a characteristic root-mean-square sensitivity of $\sim15$ mK for an effective velocity resolution of $\sim0.9$ km/s. The spectral line datacubes are optimised for the analysis of extended, quasi-thermal OH, but also contain numerous maser sources, which have been confirmed interferometrically and published elsewhere. We also present radio continuum images at 1612, 1666 and 1720 MHz. Based on initial comparisons with $^{12}$CO(J=1-0), we find that OH rarely extends outside CO cloud boundaries in our data, but suggest that large variations in CO-to-OH brightness temperature ratios may reflect differences in the total gas column density traced by each. Column density estimation in the complex, continuum-bright Inner Galaxy is a challenge, and we demonstrate how failure to appropriately model sub-beam structure and the line-of-sight source distribution can lead to order-of-magnitude errors. Anomalous excitation of the 1612 and 1720 MHz satellite lines is ubiquitous in the inner Galaxy, but is disabled by line overlap in and around the Central Molecular Zone.

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arXiv2022

Redshifted methanol absorption tracing infall motions of high-mass star formation regions

Gravitational collapse is one of the most important processes in high-mass star formation. Compared with the classic blue-skewed profiles, redshifted absorption against continuum emission is a more reliable method to detect inward motions within high-mass star formation regions. We aim to test if methanol transitions can be used to trace infall motions within high-mass star formation regions. Using the Effelsberg-100 m, IRAM-30 m, and APEX-12 m telescopes, we carried out observations of 37 and 16 methanol transitions towards two well-known collapsing dense clumps, W31C (G10.6-0.4) and W3(OH), to search for redshifted absorption features or inverse P-Cygni profiles. Redshifted absorption is observed in 14 and 11 methanol transitions towards W31C and W3(OH), respectively. The infall velocities fitted from a simple two-layer model agree with previously reported values derived from other tracers, suggesting that redshifted methanol absorption is a reliable tracer of infall motions within high-mass star formation regions. Our observations indicate the presence of large-scale inward motions, and the mass infall rates are roughly estimated to be $\gtrsim$10$^{-3}$ $M_{\odot}$yr$^{-1}$, which supports the global hierarchical collapse and clump-fed scenario. With the aid of bright continuum sources and the overcooling of methanol transitions leading to enhanced absorption, redshifted methanol absorption can trace infall motions within high-mass star formation regions hosting bright H{\scriptsize II} regions.

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arXiv2020

ATLASGAL -- Relationship between dense star forming clumps and interstellar masers

We have used catalogues from several Galactic plane surveys and dedicated observations to investigate the relationship between various maser species and Galactic star forming clumps, as identified by the ATLASGAL survey. The maser transitions of interest are the 6.7 & 12.2 GHz methanol masers, 22.2 GHz water masers, and the masers emitting in the four ground-state hyperfine structure transitions of hydroxyl. We find clump association rates for the water, hydroxyl and methanol masers to be 56, 39 and 82 per cent respectively, within the Galactic longitude range of 60° > $l$ > -60°. We investigate the differences in physical parameters between maser associated clumps and the full ATLASGAL sample, and find that clumps coincident with maser emission are more compact with increased densities and luminosities. However, we find the physical conditions within the clumps are similar for the different maser species. A volume density threshold of $n$(H$_{2}$) > 10$^{4.1}$ cm$^{-3}$ for the 6.7 GHz methanol maser found in our previous study is shown to be consistent across for all maser species investigated. We find limits that are required for the production of maser emission to be 500 L$_{\odot}$ and 6 M$_{\odot}$ respectively. The evolutionary phase of maser associated clumps is investigated using the L/M ratio of clumps coincident with maser emission, and these have similar L/M ranges (~10$^{0.2}$ - 10$^{2.7}$ L$_{\odot}$/M$_{\odot}$) regardless of the associated transitions. This implies that the conditions required for the production of maser emission only occur during a relatively narrow period during a star's evolution. Lower limits of the statistical lifetimes for each maser species are derived, ranging from ~0.4 - 2 x 10$^{4}$ yrs and are in good agreement with the "straw man" evolutionary model previously presented.

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arXiv2020

Interstellar Scintillation of an Extreme Scintillator: PKS B1144-379

The University of Tasmania Ceduna radio telescope has been used to investigate rapid variability in the radio flux density of the BL Lac object PKS B1144-379 at 6.7 GHz. High-cadence monitoring of this extreme scintillator was carried out over a period of approximately nine years, between 2003 and 2011. We have used structure functions created from the intensity time series to determine the characteristic timescale of the variability. The characteristic timescale is consistently observed to increase during certain periods of each year, demonstrating the annual cycle expected for scintillation through an interstellar scattering screen. The best-fitting annual cycle model for each year suggests that the scintillation pattern has an anisotropic structure and that the upper limit of its scattering screen is at a distance of ~0.84 kpc. Higher anisotropy in some of the annual cycle fits suggests that changes in the intrinsic source structure might be influencing the variability timescale. We found a prominent annual cycle is only present in the variability timescale for certain years where other evidence suggests that the core is compact. From our measurements, we calculated that the core angular size varied between 5.65-15.90 $μ$as (0.05-0.13 pc). The core component was found to be at its most compact during two flares in the total flux density, which were observed in 2005 and 2008. We conclude that the long-term variability in the radio flux density of PKS B1144-379 is due to intrinsic changes in the source and that these affect our ability to measure an annual cycle in its variability time scale.

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arXiv2020

Excited-state hydroxyl maser catalogue from the methanol multibeam survey -- I. Positions and Variability

We present the results of the first complete unbaised survey of the Galactic Plane for 6035-MHz excited-state hydroxyl masers undertaken as part of the Methanol Multibeam Survey. These observations cover the Galactic longitude ranges $186^{\circ}< l < 60^{\circ}$ including the Galactic Centre. We report the detection of 127 excited-state hydroxyl masers within the survey region, 47 being new sources. The positions of new detections were determined from interferometric observations with the Australia Telescope Compact Array. We discuss the association of 6035-MHz masers in our survey with the 6668-MHz masers from the MMB Survey, finding 37 likely methanol-excited-state hydroxyl masers maser pairs with physical separations of <=0.03pc and 55 pairings separated by <=0.1pc. Using these we calculate for the first time an excited-state hydroxyl maser life time of between 3.3x10^3 and 8.3x10^3 years. We also discuss the variability of the 6035-MHz masers and detection rates of counterpart 6030-MHz excited-state hydroxyl masers (28% of our sample having detection at both frequencies).

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arXiv2020

The Evolutionary Status of Protostellar Clumps Hosting Class II Methanol Masers

The Methanol MultiBeam survey (MMB) provides the most complete sample of Galactic massive young stellar objects (MYSOs) hosting 6.7GHz class II methanol masers. We characterise the properties of these maser sources using dust emission detected by the Herschel Infrared Galactic Plane Survey (Hi-GAL) to assess their evolutionary state. Associating 731 (73%) of MMB sources with compact emission at four Hi-GAL wavelengths, we derive clump properties and define the requirements of a MYSO to host a 6.7GHz maser. The median far-infrared (FIR) mass and luminosity are 630M$_{\odot}$ and 2500L$_{\odot}$ for sources on the near side of Galactic centre and 3200M$_{\odot}$ and 10000L$_{\odot}$ for more distant sources. The median luminosity-to-mass ratio is similar for both at $\sim$4.2L$_{\odot}/$M$_{\odot}$. We identify an apparent minimum 70$μ$m luminosity required to sustain a methanol maser of a given luminosity (with $L_{70} \propto L_{6.7}^{0.6}$). The maser host clumps have higher mass and higher FIR luminosities than the general Galactic population of protostellar MYSOs. Using principal component analysis, we find 896 protostellar clumps satisfy the requirements to host a methanol maser but lack a detection in the MMB. Finding a 70$μ$m flux density deficiency in these objects, we favour the scenario in which these objects are evolved beyond the age where a luminous 6.7GHz maser can be sustained. Finally, segregation by association with secondary maser species identifies evolutionary differences within the population of 6.7GHz sources.

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arXiv2019

36 GHz methanol lines from nearby galaxies: maser or quasi-thermal emission?

Methanol (CH3OH) is one of the most abundant interstellar molecules, offering a vast number of transitions to be studied, including many maser lines. While the strongest Galactic CH3OH lines, the so-called class II masers, show no indications for the presence of superluminous counterparts in external galaxies, the less luminous Galactic class I sources appear to be different. Here we report class I 36GHz CH3OH 4(-1) - 3(0) E line emission from the nearby galaxies Maffei2 and IC342, measured with the 100-m telescope at Effelsberg at three different epochs within a time span of about five weeks. The 36GHz methanol line of Maffei2 is the second most luminous among the sources detected with certainty outside the Local Group of galaxies. This is not matched by the moderate infrared luminosity of Maffei2. Higher resolution data are required to check whether this is related to its prominent bar and associated shocks. Upper limits for M82, NGC4388, NGC5728 and Arp220 are also presented. The previously reported detection of 36GHz maser emission in Arp220 is not confirmed. Non-detections are reported from the related class I 44GHz methanol transition towards Maffei2 and IC342, indicating that this line is not stronger than its 36GHz counterpart. In contrast to the previously detected 36GHz CH3OH emission in NGC253 and NGC4945, our 36GHz profiles towards Maffei2 and IC342 are similar to those of previously detected non-masing lines from other molecular species. However, by analogy to our Galactic center region, it may well be possible that the 36GHz methanol lines in Maffei~2 and IC~342 are composed of a large number of faint and narrow maser features that remain spatially unresolved. In view of this, a search for a weak broad 36GHz line component would also be desirable in NGC253 and NGC4945.

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arXiv2019

ATLASGAL -- physical parameters of dust clumps associated with 6.7 GHz methanol masers

We have constructed the largest sample of dust-associated class II 6.7 GHz methanol masers yet obtained. New measurements from the the Methanol MultiBeam (MMB) Survey were combined with the 870 $μ$m APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) and the 850 $μ$m JCMT Plane Survey (JPS). Together with two previous studies we have now identified the host clumps for 958 methanol masers across the Galactic Plane, covering approximately 99% of the MMB catalogue and increasing the known sample of dust-associated masers by over 30%. We investigate correlations between the physical properties of the clumps and masers using distances and luminosities drawn from the literature. Clumps hosting methanol masers are significantly more compact and have higher volume densities than the general population of clumps. We determine a minimum volume density threshold of $n$(H$_2$) $\geq 10^4$ cm$^{-3}$ for the efficient formation of intermediate- and high-mass stars. We find 6.7 GHz methanol masers are associated with a distinct part of the evolutionary process ($L_{\rm bol}$/$M_{\rm fwhm}$ ratios of between 10$^{0.6}$ and 10$^{2.2}$) and have well defined turning on and termination points. We estimate the lower limit for the mass of embedded objects to be $\geq$ 6 M$_{\odot}$ and the statistical lifetime of the methanol maser stage to be $\sim$ 3.3$\times$10$^{4}$ yrs. This suggests that methanol masers are indeed reliable tracers of high mass star formation, and indicates that the evolutionary period traced by this marker is relatively rapid.

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arXiv2019

Detection statistics of the RadioAstron AGN survey

The largest Key Science Program of the RadioAstron space VLBI mission is a survey of active galactic nuclei (AGN). The main goal of the survey is to measure and study the brightness of AGN cores in order to better understand the physics of their emission while taking interstellar scattering into consideration. In this paper we present detection statistics for observations on ground-space baselines of a complete sample of radio-strong AGN at the wavelengths of 18, 6, and 1.3 cm. Two-thirds of them are indeed detected by RadioAstron and are found to contain extremely compact, tens to hundreds of $μ$as structures within their cores.

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arXiv2019

Weak and Compact Radio Emission in Early High-Mass Star Forming Regions: II. The Nature of the Radio Sources

In this study we analyze 70 radio continuum sources associated with dust clumps and considered to be candidates for the earliest stages of high-mass star formation. The detection of these sources was reported by Rosero et al. (2016), who found most of them to show weak (${\scriptstyle <}$1 mJy) and compact (${\scriptstyle <}\,$0.6$^{\prime \prime}$) radio emission. Herein, we used the observed parameters of these sources to investigate the origin of the radio continuum emission. We found that at least $\sim 30\%$ of these radio detections are most likely ionized jets associated with high-mass protostars, but for the most compact sources we cannot discard the scenario that they represent pressure-confined HII regions. This result is highly relevant for recent theoretical models based on core accretion that predict the first stages of ionization from high-mass stars to be in the form of jets. Additionally, we found that properties such as the radio luminosity as a function of the bolometric luminosity of ionized jets from low and high-mass stars are extremely well-correlated. Our data improve upon previous studies by providing further evidence of a common origin for jets independently of luminosity.

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arXiv2019

Discovery of six new class II methanol maser transitions, including the unambiguous detection of three torsionally excited lines toward G358.931-0.030

We present the unambiguous discovery of six new class II methanol maser transitions, three of which are torsionally excited (vt=1). The newly discovered 6.18-GHz 17_-2 -> 18_-3 E (vt=1), 7.68-GHz 12_4 -> 13_3 A- (vt=0), 7.83-GHz 12_4 -> 13_3 A+ (vt = 0), 20.9-GHz 10_1 -> 11_2 A+ (vt=1), 44.9-GHz 2_0 -> 3_1 E (vt=1) and 45.8-GHz 9_3 -> 10_2 E (vt=0) methanol masers were detected towards G358.931-0.030, where the known 6.68-GHz maser has recently been reported to be undergoing a period flaring. The detection of the vt=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 vt=0 lines, these observations provide significant new information which can be utilised to improve class II methanol maser modelling. 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 (ATCA) 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 polarised 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 polarisation in the 6.68-, 37.7- and 45.8-GHz transitions.

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arXiv2017

Interferometry of class I methanol masers, statistics and the distance scale

The Australia Telescope Compact Array (ATCA) participated in a number of survey programs to search for and image common class I methanol masers (at 36 and 44 GHz) with high angular resolution. In this paper, we discuss spatial and velocity distributions revealed by these surveys. In particular, the number of maser regions is found to fall off exponentially with the linear distance from the associated young stellar object traced by the 6.7-GHz maser, and the scale of this distribution is 263+/-15 milliparsecs. Although this relationship still needs to be understood in the context of the broader field, it can be utilised to estimate the distance using methanol masers only. This new technique has been analysed to understand its limitations and future potential. It turned out, it can be very successful to resolve the ambiguity in kinematic distances, but, in the current form, is much less accurate (than the kinematic method) if used on its own.

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