arXiv subjects

Peter Hofner

Publications and source records attributed to Peter Hofner.

At least 19 recordsLinked to original sources

arXiv2026

Unveiling Ionized Jet Morphologies: Sub-arcsecond VLA Observations of Compact Radio Sources

We present sub-arcsecond ($\theta\sim0.1^{\prime\prime}$) resolution VLA 1.3 cm continuum and 22.2 GHz H$_2$O maser observations toward 15 compact radio continuum sources with rising spectral index and 8 string-like radio continuum structures in the Rosero et al. (2016, 2019) survey. Three different morphologies are observed: elongated or double-peak string-like structure (6 out of 23 cases), a collection of distinct continuum peaks (4 out of 23 cases), and single compact sources (13 out of 23 cases). The majority of H$_2$O maser spots detected are within a sky-projected distance of $\sim5,000$ au from the radio continuum peaks and tend to be well aligned and distributed in an elongated structure when more than three spots are observed. We generally recover less emission than Rosero et al. (2016, 2019), which together with the fact that more than half of the jet candidates in our survey appear mostly compact, suggest core/halo shock structures even on small scales. We also detected proper motion in 10 cases and measured an average projected velocity of approximately 120 km s$^{-1}$. Radio brightness variability is detected in at least two cases, possibly due to weak accretion bursts. This work, together with our previous molecular jet study, provides further evidence that support the main source of ionization in the studied sources is shocks, yet collimation is only observed in 4 cases. We conclude that the available data supports the thermal jet classification of 7 sources, and the ionized jet interpretation is further supported in 16 sources.

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arXiv2024

Radio Continuum and Water Maser Observations of the High-Mass Protostar IRAS 19035+0641 A

We present Very Large Array (VLA) 1.3 cm continuum and 22.2 GHz H$_2$O maser observations of the high-mass protostellar object IRAS 19035+0641 A. Our observations unveil an elongated bipolar 1.3 cm continuum structure at scales $\lesssim500\,$au which, together with a rising in-band spectral index, strongly suggests that the radio emission toward IRAS 19035+0641 A arises from an ionized jet. In addition, eight individual water maser spots well aligned with the jet axis were identified. The Stokes V spectrum of the brightest H$_2$O maser line ($\sim100\,$Jy) shows a possible Zeeman splitting and is well represented by the derivatives of two Gaussian components fitted to the Stokes I profile. The measured $B_{\mathrm{los}}$ are $123\,(\pm27)$ and $156\,(\pm8)\,$mG, translating to a pre-shock magnetic field of $\approx7\,$mG. Subsequent observations to confirm the Zeeman splitting showed intense variability in all the water maser spots, with the brightest maser completely disappearing. The observed variability in a one-year time scale could be the result of an accretion event. These findings strengthen our interpretation of IRAS 19035+0641 A as a high-mass protostar in an early accretion/outflow evolutionary phase.

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arXiv2023

Searching for Molecular Jets from High-Mass Protostars

We report Very Large Array (VLA) observations in the Q-band toward 10 ionized jet candidates to search for SiO emission, a well-known shocked gas tracer. We detected 7 mm continuum counterparts toward 90% of the jet candidates. In most cases, the jet candidate is located toward the center of the 7 mm core, and the high masses ($\approx 100\,M_\odot$) and densities ($\approx 10^7\, \text{cm}^{-3}$) of the cores suggest that the central objects are very young high-mass protostars. We detected SiO $J=1-0$ emission associated with 6 target sources. In all cases, the morphology and spectrum of the emission is consistent with what is expected for molecular jets along an outflow axis, thus confirming the jet nature of 60% of our sample. Our data suggest a positive correlation between the SiO luminosity $L_{SiO}$, and both the bolometric luminosity $L_{Bol}$ and the radio luminosity $S_\nu d^2$ of the driving sources.

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arXiv2018

Particle acceleration in the Herbig-Haro objects HH 80 and HH 81

We present an analysis of radio (Karl G. Jansky Very Large Array (VLA)), optical (HST), and X-ray (Chandra and XMM-Newton) observations and archival data of the Herbig-Haro objects HH 80 and HH 81 in the context of jet-cloud interactions. Our radio images are the highest angular resolution to date of these objects, allowing to spatially resolve the knots and compare the regions emitting in the different spectral ranges. We found that soft X-ray thermal emission is located ahead of the non-thermal radio peak. This result is consistent with a radiative forward shock that heats the shocked gas up to 10 6 K, and an adiabatic reverse shock able to accelerate particles and produce synchrotron radiation detectable at radio frequencies. These high angular resolution radio images also reveal a bow shock structure in the case of HH 80N, being the first time this morphology is detected in a Herbig-Haro object at these frequencies.

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arXiv2018

A Chandra X-ray and Infrared Study of the Stellar Population in the High-Mass Star Forming Region IRAS 16562-3959

We present the results from Chandra X-ray observations, and near- and mid-infrared analysis, using VISTA/VVV and Spitzer/GLIMPSE catalogs, of the high-mass star-forming region IRAS 16562-3959, which contains a candidate for a high mass protostar. We detected 249 X-ray sources within the ACIS-I field-of-view. The majority of the X-ray sources have low count rates (< 0.638 cts/ks), and hard X-ray spectra. The search for YSOs in the region using VISTA/VVV and Spitzer/GLIMPSE catalogs resulted in a total of 799 YSOs, with 66 Class I and 733 Class II YSOs. The search for near- and mid-infrared counterparts of the X-ray sources led to a total of 165 VISTA/VVV counterparts, and a total of 151 Spitzer/GLIMPSE counterparts. The infrared analysis of the X-ray counterparts allowed us to add an extra 123 YSOs associated with the region. We conclude that a total of 922 YSOs are associated with the region, with 67 Class I, 764 Class II, and 91 Class III YSOs. We also found that the region is composed of 11 subclusters. In the vicinity of the high-mass protostar, the stellar distribution has a core-halo structure. The subcluster containing the high-mass protostar is the densest and the youngest in the region, and the high-mass protostar is located at its center. The YSOs in this cluster appear to be substantially older than the high mass protostar.

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arXiv2017

Unveiling the Hot Molecular Core in the Ultracompact \hii Region with Extended Emission G12.21--0.10

We present a multiwavelength study of the cometary \hii region G12.21--0.10 using the VLA and OVRO. Both radio continuum (0.3, 0.7, 2 and 3.6~cm) and spectral lines of H41$\alpha$, $^{13}$CS(2$-$1) \& (1$-$0), and NH$_{\rm 3}$(2,2) \& (4,4) observations are included. We find two 3~mm continuum peaks toward G12.21--0.10, one of them is spatially coincident with the UC~H~II region, while the other coincides spatially with a molecular clump. We also find that the 0.7, 2 and 3.6~cm continuum and H41$\alpha$ line are only detected toward the UC~H~II region, while the $^{13}$CS, and NH$_{\rm 3}$ is spatially associated with the molecular clump. Based on the morphology, kinetic temperature ($\sim$86~K), volumetric density ($\sim$1.5$\times$10$^6$~cm$^{-3}$) and linear size ($\sim$0.22~pc) of the molecular clump, we suggest this source is consistent with a hot molecular core.

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arXiv2017

Interferometric and single-dish observations of 44, 84 and 95 GHz Class I methanol masers

We present observations of massive star-forming regions selected from the IRAS Point Source Catalog. The observations were made with the Very Large Array and the Large Millimeter Telescope to search for Class I methanol masers. We made interferometric observations of 125 massive star-forming regions in the 44 GHz methanol maser transition; 53 of the 125 fields showed 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 $\mu$m emission. We made single-dish observations toward 38 of 53 regions with 44 GHz masers detected to search for methanol transitions at 84.5, 95.1, 96.7, 107.0, and 108.8 GHz. We find detection rates of 74, 55, 100, 3, and 45%, respectively. We used a wide-band receiver which revealed many other spectral lines that are common in star-forming regions.

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arXiv2017

A catalog of 44 GHz methanol masers in massive star-forming regions. IV. The high-mass protostellar object sample

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.

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arXiv2015

X-ray and Radio Observations of the Massive Star Forming Region IRAS 20126+4104

We present results of Chandra ACIS-I and Karl G. Jansky Very Large Array (VLA) 6 cm continuum observations of the IRAS 20126+4104 massive star forming region. We detect 150 X-ray sources within the 17 arcmin x 17 arcmin ACIS-I field, and a total of 13 radio sources within the 9'.2 primary beam at 4.9 GHz. Among these are the first 6 cm detections of the central sources reported by Hofner et al. (2007), namely I20N1, I20S, and I20var. A new variable radio sources is also reported. Searching the 2MASS archive we identified 88 NIR counterparts to the X-ray sources. Only 4 of the X-ray sources had 6 cm counterparts. Based on an NIR color-color analysis, and on the Besancon simulation of Galactic stellar populations (Robin et al. 2003), we estimate that about 90 X-ray sources are associated with this massive star forming region. We detect an increasing surface density of X-ray sources toward the massive protostar and infer the presence of a cluster of at least 46 YSOs within a distance of 1.2 pc from the massive protostar.

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arXiv2012

Resolving the Circumstellar Disk Around the Massive Protostar Driving the HH 80-81 Jet

We present new high-angular resolution observations toward the driving source of the HH 80-81 jet (IRAS 18162-2048). Continuum emission was observed with the Very Large Array at 7 mm and 1.3 cm, and with the Submillimeter Array at 860 microns, with angular resolutions of ~0"1 and ~0"8 respectively. Submillimeter observations of the sulfur oxide (SO) molecule are reported as well. At 1.3 cm the emission traces the well-known radio jet, while at 7 mm the continuum morphology is quadrupolar and seems to be produced by a combination of free-free and dust emission. An elongated structure perpendicular to the jet remains in the 7 mm image after subtraction of the free-free contribution. This structure is interpreted as a compact accretion disk of ~200 AU radius. Our interpretation is favored by the presence of rotation in our SO observations observed at larger scales. The observations presented here add to the small list of cases where the hundred-AU scale emission from a circumstellar disk around a massive protostar has been resolved.

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arXiv2011

X-Ray Emission from Young Stars in the Massive Star Forming Region IRAS 20126+4104

We present a $40\,$ks Chandra observation of the IRAS$\,$20126+4104 core region. In the inner $6^{\prime\prime}$ two X-ray sources were detected, which are coincident with the radio jet source I20S and the variable radio source I20Var. No X-ray emission was detected from the nearby massive protostar I20N. The spectra of both detected sources are hard and highly absorbed, with no emission below $3\,$keV. For I20S, the measured $0.5-8\,$keV count rate was $4.3\,$cts$\,$ks$^{-1}$. The X-ray spectrum was fit with an absorbed 1T APEC model with an energy of kT$\,=10\,$keV and an absorbing column of N$_H = 1.2\times 10^{23}\,$cm$^{-2}$. An unabsorbed X-ray luminosity of about $1.4\times 10^{32}\,$erg$\,$s$^{-1}$ was estimated. The spectrum shows broad line emission between 6.4 and 6.7\, keV, indicative of emission from both neutral and highly ionized iron. The X-ray lightcurve indicates that I20S is marginally variable; however, no flare emission was observed. The variable radio source I20Var was detected with a count rate of $0.9\,$cts$\,$ks$^{-1}$ but there was no evidence of X-ray variability. The best fit spectral model is a 1T APEC model with an absorbing hydrogen column of N$_H = 1.1\times 10^{23}\,$cm$^{-2}$ and a plasma energy of kT = 6.0$\,$keV. The unabsorbed X-ray luminosity is about $3\times 10^{31}\,$erg$\,$s$^{-1}$.

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arXiv2010

A Catalog of CH_3OH 7_0-6_1 A^+ Maser Sources in Massive Star-Forming Regions. II. Masers in NGC 6334F, G8.67-0.36, and M17

We present Very Large Array observations of the 7_0-6_1 A^+ methanol maser transition at 44 GHz towards NGC 6334F, G8.67-0.36, and M17. These arcsecond resolution observations complete a previous, larger VLA survey of this maser transition in high-mass star-forming regions reported by Kurtz et al. We confirm the presence of 44 GHz methanol maser emission in all three sources, detecting eight distinct maser components in NGC 6334F, twelve components in G8.67-0.36 and one in M17.

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arXiv2010

The Rare 23.1-GHz Methanol Masers in NGC 7538 IRS 1

We present high angular resolution (FWHM_beam < 0.2") observations of the 23.1-GHz methanol (CH_3OH) transition toward the massive-star forming region NGC 7538 IRS 1. The two velocity components previously reported by Wilson et al. are resolved into distinct spatial features with brightness temperatures (T_B) greater than 10^4 K, proving their maser nature. Thus, NGC 7538 IRS 1 is the third region confirmed to show methanol maser emission at this frequency. The brighter 23.1-GHz spot coincides in position with a rare formaldehyde (H_2CO) maser, and marginally with a 22.2-GHz water (H_2O) maser, for which we report archival observations. The weaker CH_3OH spot coincides with an H_2O maser. The ratio of T_B for the 23.1-GHz masers to that of the well-known 12.2-GHz CH_3OH masers in this region roughly agrees with model predictions. However, the 23.1-GHz spots are offset in position from the CH_3OH masers at other frequencies. This is difficult to interpret in terms of models that assume that all the masers arise from the same clumps, but it may result from turbulent conditions within the gas or rapid variations in the background radiation field.

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arXiv2010

High Angular Resolution Observations of Four Candidate BLAST High-Mass Starless Cores

We discuss high-angular resolution observations of ammonia toward four candidate high-mass starless cores (HMSCs). The cores were identified by the Balloon-borne Large Aperture Submillimeter Telescope (BLAST) during its 2005 survey of the Vulpecula region where 60 compact sources were detected simultaneously at 250, 350, and 500 micron. Four of these cores, with no IRAS-PSC or MSX counterparts, were observed with the NRAO Very Large Array (VLA) in the NH3(1,1) and (2,2) spectral lines. Our observations indicate that the four cores are cold (Tk <~ 14K) and show a filamentary and/or clumpy structure. They also show a significant velocity substructure within ~1km/s. The four BLAST cores appear to be colder and more quiescent than other previously observed HMSC candidates, suggesting an earlier stage of evolution.

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arXiv2009

Radio Continuum and Methanol Observations of DR21(OH)

We report high sensitivity sub-arcsecond angular resolution observations of the massive star forming region DR21(OH) at 3.6, 1.3, and 0.7 cm obtained with the Very Large Array. In addition, we conducted observations of CH3OH 44 GHz masers. We detected more than 30 new maser components in the DR21(OH) region. Most of the masers appear to trace a sequence of bow-shocks in a bipolar outflow. The cm continuum observations reveal a cluster of radio sources; the strongest emission is found toward the molecular core MM1. The radio sources in MM1 are located about 5" north of the symmetry center of the CH3OH outflow, and therefore, they are unlikely to be associated with the outflow. Instead, the driving source of the outflow is likely located in the MM2 core. Although based on circumstantial evidence, the radio continuum from MM1 appears to trace free-free emission from shock-ionized gas in a jet. The orientation of the putative jet in MM1 is approximately parallel to the CH3OH outflow and almost perpendicular to the large scale molecular filament that connects DR21 and DR21(OH). This suggests that the (accretion) disks associated with the outflows/jets in the DR21 - DR21(OH) region have symmetry axes mostly perpendicular to the filament.

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arXiv2007

HI and OH Absorption toward NGC6240

VLA observations of large-scale HI and OH absorption in the merging galaxy of NGC6240 are presented with 1 arcsec resolution. HI absorption is found across large areas of the extended radio continuum structure with a strong concentration towards the double-nucleus. The OH absorption is confined to the nuclear region. The HI and OH observations identify fractions of the gas disks of the two galaxies and confirm the presence of central gas accumulation between the nuclei. The data clearly identify the nucleus of the southern galaxy as the origin of the symmetric superwind outflow and also reveal blue-shifted components resulting from a nuclear starburst. Various absorption components are associated with large-scale dynamics of the system including a foreground dust lane crossing the radio structure in the northwest region.

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arXiv2004

The G9.62+0.19-F Hot Molecular Core - The infrared view on very young massive stars

(abridged) We present the results of an extensive infrared study of the massive star-forming region G9.62+0.19. The data cover information from broad- and narrow-band filters in the wavelength range from 1 to 19 micrometer and are obtained with ESO's infrared cameras ISAAC and TIMMI2 and with SpectroCam-10 (Mt. Palomar). The high sensitivity and resolution provided by these facilities revealed intriguing new details of this star-forming region and especially about the embedded hot molecular core (HMC) - component F. We analyse the newly found infrared sub-structure of four objects in this HMC region. While one of these objects (F2) is probably a foreground field star, the nature of the brightest object in the near-infrared there (F1) remains somewhat enigmatic. Our new astrometry proves that this object is not coincident with the peak of the molecular line emission of the HMC, but displaced by 1.7 arcsecs (nearly 10000 AU on a linear scale). We estimate this object to be an additional embedded object with a dense dust shell. Very near the HMC location we find L' band emission which strongly rises in flux towards longer wavelengths. We presume that this emission (F4) arises from the envelope of the HMC which is known to be associated with a molecular outflow roughly aligned along the line of sight. Thus, the clearing effect of this outflow causes strong deviations from spherical symmetry which might allow infrared emission from the HMC to escape through the outflow cavities. This presents the first direct detection of an HMC at a wavelength as short as 3.8 micron. At 11.7 and 18.75 micron, the HMC counterpart F4 ultimately proves to be the most luminous IR source within the G9.62+0.19-F region.

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arXiv2003

The Grandeur of Massive Star Formation Revealed with ISAAC

We present selected results from our ongoing investigation of high-mass star-forming regions which are based on infrared observations with ESO's ISAAC camera at the 8.2-m ANTU VLT telescope. Although these young stellar objects comprise a high degree of complexity, our data enable us to disentangle these crowded regions. By means of broad- and narrow-band imaging between 1 - 5 micron we performed a thorough characterisation of the embedded population. Special emphasis was put on the importance of an accurate astrometry which has a major impact on the interpretation of the data. In the case of G9.62+0.19, we clarified the true nature of the infrared emission in the immediate vicinity of an hot molecular core (HMC). We unveil the counterpart of this HMC in the thermal infrared. For GGD27 IRS2, we found thermal emission at 3.8 and 4.7 micron caused by a deeply embedded object that is powering a large radio jet and has counterparts in our mid-infrared and VLA 7-mm data. The presented results mark a further step on the way to disclose the mechanisms of massive star formation. They demonstrate the value of sensitive infrared imaging with the current generation of IR cameras on 8-m-class telescopes.

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