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S. Kurtz

Publications and source records attributed to S. Kurtz.

At least 19 recordsLinked to original sources
arXiv2026

Physical characterization and modeling of candidate Hyper-Compact HII Regions

Hypercompact HII regions (HC) are regions of ionized gas associated with the early stages of high-mass star formation. With the aim of better understanding their characteristics, we studied five candidate HC HII regions. Here, we present observations with the Jansky Very Large Array (VLA) at 2 and 6 cm, with angular resolutions in the range of $\sim$1 -- 3\arcsec and report the images of the detected sources and the measured parameters. In addition, we explore several possible scenarios, considering the regions as both uniform and non-uniform spheres, and as winds, both spherical and collimated. In most cases, the sources were unresolved, but by applying the models, we estimate that their sizes vary in a range of 0.3 to 3.7 mpc while their electron densities are in the range of $1.3 \times 10^{5}$ to $2.4 \times 10^{6}$ cm$^{-3}$, indicating that most sources are consistent with small, weak UC HII regions, although a few remain viable candidates for HC HII regions, with G40.28$-$0.22 as the strongest case. We do not rule out the possibility that some sources are jets or stellar winds.

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arXiv2026

Multi-frequency mapping of the S255IR region at a wavelength of 1~mm

The results of interferometric observations of the star-forming region S255IR in the frequency range 210--250 GHz are presented. The observations were carried out with the antenna array SMA (Hawaii, USA). Fifty-three molecules were detected, including complex organic molecules (COMs) such as CH$_3$CHO, CH$_3$CN, CH$_3$CH$_2$CN, and many others. Typical rotational temperatures in the hot core SMA1 fall in the range 100--200 K. Optical depths in the lines of methanol and some other molecules in the cores SMA1 and SMA2 were estimated. In SMA1, the optical depth of one of the strongest methanol lines, $5_{-1}-4_{-1}E$, proved to be $23.8 \pm 1.5$. Based on this value, one can assume that the lines of other oxygen-containing COMs, such as CH$_3$OCHO, CH$_3$OCH$_3$, CH$_3$CH$_2$OH, which are typically much less abundant in hot cores than methanol, are optically thin in SMA1. Most of the detected molecules can be roughly divided into two groups. The molecules of the first group emit exclusively toward the hot core SMA1, while some or all lines of the molecules of the second group, in addition to SMA1, can be seen toward a ring-like structure to the west of SMA1. This structure is most likely associated with the walls of a cavity formed by high-velocity outflows driven by young stellar objects (YSOs) in molecular cores SMA1, SMA2, and possibly SMA3. The gas temperature and density in the cavity walls were estimated using methanol lines. The temperature was found to be about 50--60 K, and the density about $10^7-10^8$ cm$^{-3}$. The column density of methanol near the brightness peaks in the lines of this molecule is about $5\times 10^{15}$~cm$^{-2}$. The column densities of other COMs in the ring-like structure will be determined in future studies with increased sensitivity achieved by spectral line stacking.

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arXiv2023

Excited Hydroxyl Outflow in the High-Mass Star-Forming Region G34.26+0.15

G34.26+0.15 is a region of high-mass star formation that contains a broad range of young stellar objects in different stages of evolution, including a hot molecular core, hyper-compact HII regions and a prototypical cometary ultra-compact HII region. Previous high-sensitivity single dish observations by our group resulted in the detection of broad 6035 MHz OH absorption in this region; the line showed a significant blue-shifted asymmetry indicative of molecular gas expansion. We present high-sensitivity Karl G. Jansky Very Large Array (VLA) observations of the 6035 MHz OH line conducted to image the absorption and investigate its origin with respect to the different star formation sites in the region. In addition, we report detection of 6030 MHz OH absorption with the VLA and further observations of 4.7 GHz and 6.0 GHz OH lines obtained with the Arecibo Telescope. The 6030 MHz OH line shows a very similar absorption profile as the 6035 MHz OH line. We found that the 6035 MHz OH line absorption region is spatially unresolved at $\sim 2$" scales, and it is coincident with one of the bright ionized cores of the cometary HII region that shows broad radio recombination line emission. We discuss a scenario where the OH absorption is tracing the remnants of a pole-on molecular outflow that is being ionized inside-out by the ultra-compact HII region.

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arXiv2023

Broadband VLA Spectral Line Survey of a Sample of Ionized Jet Candidates

The study of the interaction between ionized jets, molecular outflows and their environments is critical to understanding high-mass star formation, especially because jets and outflows are thought to be key in the transfer of angular momentum outwards from accretion disks. We report a low-spectral resolution VLA survey for hydrogen radio recombination lines, OH, NH$_3$, and CH$_3$OH lines toward a sample of 58 high-mass star forming regions that contain numerous ionized jet candidates. The observations are from a survey designed to detect radio continuum; the novel aspect of this work is to search for spectral lines in broadband VLA data (we provide the script developed in this work to facilitate exploration of other datasets). We report detection of 25$\,$GHz CH$_3$OH transitions toward ten sources; five of them also show NH$_3$ emission. We found that most of the sources detected in CH$_3$OH and NH$_3$ have been classified as ionized jets or jet candidates and that the emission lines are coincident with, or very near ($\lesssim 0.1$ pc) these sources, hence, these molecular lines could be used as probes of the environment near the launching site of jets/outflows. No radio recombination lines were detected, but we found that the RMS noise of stacked spectra decreases following the radiometer equation. Therefore, detecting radio recombination lines in a sample of brighter free-free continuum sources should be possible. This work demonstrates the potential of broadband VLA continuum observations as low-resolution spectral line scans.

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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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arXiv2021

Discovery of a Highly Collimated Flow from the High-Mass Protostar ISOSS J23053+5953 SMM2

We present Very Large Array C, X, and Q-band continuum observations, as well as 1.3 mm continuum and CO(2-1) observations with the Submillimeter Array toward the high-mass protostellar candidate ISOSS J23053+5953 SMM2. Compact cm continuum emission was detected near the center of the SMM2 core with a spectral index of 0.24 between 6 and 3.6 cm, and a radio luminosity of 1.3 mJy kpc$^2$. The 1.3 mm thermal dust emission indicates a mass of the SMM2 core of 45.8 Msun. The CO(2-1) observations reveal a large, massive molecular outflow centered on the SMM2 core. This fast outflow ($>$ 50 km/s from the cloud systemic velocity) is highly collimated, with a broader, lower-velocity component. The large values for outflow mass (45.2 Msun), and momentum rate (6 x 10$^{-3}$ Msun km/s/yr) derived from the CO emission are consistent with those of flows driven by high-mass YSOs. The dynamical timescale of the flow is between 1.5 - 7.2 x 10$^4$ yr. We also found from the C18O to thermal dust emission ratio that CO is depleted by a factor of about 20, possibly due to freeze out of CO molecules on dust grains. Our data are consistent with previous findings that ISOSS J23053+5953 SMM2 is an emerging high-mass protostar in an early phase of evolution, with an ionized jet, and a fast, highly collimated, and massive outflow.

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arXiv2021

Thermal Formaldehyde Emission in NGC7538 IRS1

Spectral lines from formaldehyde (H2CO) molecules at cm wavelengths are typically detected in absorption and trace a broad range of environments, from diffuse gas to giant molecular clouds. In contrast, thermal emission of formaldehyde lines at cm wavelengths is rare. In previous observations with the 100m Robert C. Byrd Green Bank Telescope (GBT), we detected 2 cm formaldehyde emission toward NGC7538 IRS1 - a high-mass protostellar object in a prominent star-forming region of our Galaxy. We present further GBT observations of the 2 cm and 1 cm H2CO lines to investigate the nature of the 2 cm H2CO emission. We conducted observations to constrain the angular size of the 2 cm emission region based on a East-West and North-South cross-scan map. Gaussian fits of the spatial distribution in the East-West direction show a deconvolved size (at half maximum) of the 2 cm emission of 50" +/- 8". The 1 cm H2CO observations revealed emission superimposed on a weak absorption feature. A non-LTE radiative transfer analysis shows that the H2CO emission is consistent with quasi-thermal radiation from dense gas (~10^5 to 10^6 cm^-3). We also report detection of 4 transitions of CH3OH (12.2, 26.8, 28.3, 28.9 GHz), the (8,8) transition of NH3 (26.5 GHz), and a cross-scan map of the 13 GHz SO line that shows extended emission (> 50").

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arXiv2021

A Galactic survey of radio jets from massive protostars

In conjunction with a previous southern-hemisphere work, we present the largest radio survey of jets from massive protostars to date with high-resolution, ($\sim 0.04^{\prime\prime}$) VLA observations towards two subsamples of massive star-forming regions of different evolutionary statuses: 48 infrared-bright, massive YSOs and 8 IRDCs containing 16 luminous (${\rm L_{bol}}>10^3\,{\rm L_\odot}$) cores. For $94\%$ of the MYSO sample we detect thermal radio ($α\geq -0.1$ whereby $S_ν\propto ν^α$) sources coincident with the protostar, of which $84\%$ (13 jets and 25 candidates) are jet-like. Radio luminosity is found to scale with ${\rm L_{bol}}$ similarly to the low-mass case supporting a common mechanism for jet production across all masses. Associated radio lobes tracing shocks are seen towards $52\%$ of jet-like objects and are preferentially detected towards jets of higher radio and bolometric luminosities, resulting from our sensitivity limitations. We find jet mass loss rate scales with bolometric luminosity as $\dot{m}_{\rm jet} \propto {\rm L_{bol}}^{0.9\pm0.2}$, thereby discarding radiative, line-driving mechanisms as the dominant jet-launching process. Calculated momenta show that the majority of jets are mechanically capable of driving the massive, molecular outflow phenomena since $p_{\rm jet}>p_{\rm outflow}$. Finally, from their physical extent we show that the radio emission can not originate from small, optically-thick \textsc{Hii} regions. Towards the IRDC cores, we observe increasing incidence rates/radio fluxes with age using the proxy of increasing luminosity-to-mass $\left( \frac{L}{M} \right)$ and decreasing infrared flux ratios $\left(\frac{S_{70 \rm μm}}{S_{24 \rm μm}}\right)$. Cores with $\frac{L}{M}<40\,{\rm L_{sol}}{\rm M_{sol}}^{-1}$ are not detected above ($5.8{\rm GHz}$) radio luminosities of $\sim1{\rm mJy\,kpc}^2$.

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arXiv2020

High-Sensitivity Observations of Molecular Lines with the Arecibo Telescope

We report on one of the highest sensitivity surveys for molecular lines in the frequency range 6.0 to 7.4 GHz conducted to date. The observations were done with the 305m Arecibo Telescope toward a sample of twelve intermediate/high-mass star forming regions. We searched for a large number of transitions of different molecules, including CH3OH and OH. The low RMS noise of our data (~5 mJy for most sources and transitions) allowed detection of spectral features that have not been seen in previous lower sensitivity observations of the sources, such as detection of excited OH and 6.7 GHz CH3OH absorption. A review of 6.7 GHz CH3OH detections indicates an association between absorption and radio continuum sources in high-mass star forming regions, although selection biases in targeted projects and low sensitivity of blind surveys imply incompleteness. Absorption of excited OH transitions was also detected toward three sources. In particular, we confirm a broad 6.035 GHz OH absorption feature in G34.26+0.15 characterized by an asymmetric blue-shifted wing indicative of expansion, perhaps a large scale outflow in this HII region.

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arXiv2019

VLBI observations of the G25.65+1.05 water maser superburst

This paper reports observations of a 22 GHz water maser `superburst' in the G25.65+1.05 massive star forming region, conducted in response to an alert from the Maser Monitoring Organisation (M2O). Very long baseline interferometry (VLBI) observations using the European VLBI Network (EVN) recorded a maser flux density of $1.2 \times 10^{4}$ Jy. The superburst was investigated in the spectral, structural and temporal domains and its cause was determined to be an increase in maser path length generated by the superposition of multiple maser emitting regions aligning in the line of sight to the observer. This conclusion was based on the location of the bursting maser in the context of the star forming region, its complex structure, and its rapid onset and decay.

astro-ph.SR↗
arXiv2019

Detection of 4765 MHz OH Emission in a Pre-Planetary Nebula -- CRL 618

Jets and outflows are ubiquitous phenomena in astrophysics, found in our Galaxy in diverse environments, from the formation of stars to late-type stellar objects. We present observations conducted with the 305m Arecibo Telescope of the pre-planetary nebula CRL 618 (Westbrook Nebula) - a well studied late-type star that has developed bipolar jets. The observations resulted in the first detection of 4765 MHz OH in a late-type stellar object. The line was narrow (FWHM ~ 0.6 km/s) and ~40 km/s blueshifted with respect to the systemic velocity, which suggests association with the expanding jets/bullets in CRL 618. We also report non-detection at Arecibo of any other OH transition between 1 and 9 GHz. The non-detections were obtained during the observations in 2008, when the 4765 MHz OH line was first discovered, and also in 2015 when the 4765 MHz OH line was not detected. Our data indicate that the 4765 MHz OH line was a variable maser. Modeling of the 4765 MHz OH detection and non-detection of the other transitions is consistent with the physical conditions expected in CRL 618. The 4765 MHz OH maser could originate from dissociation of H2O by shocks after sublimation of icy objects in this dying carbon-rich stellar system, although other alternatives such as OH in an oxygen-rich circumstellar region associated with a binary companion are also possible.

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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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arXiv2018

The Coordinated Radio and Infrared Survey for High-mass Star Formation. IV: A new radio selected sample of compact Galactic Planetary Nebulae

We present a new radio-selected sample of PNe from the CORNISH survey. This is a radio continuum survey of the inner Galactic plane covering Galactic longitude, $10^\circ 1000 K) and located closer than 7 kpc. Within this sample is a water-maser PN with a spectral index of $-0.55\pm 0.08$, which indicates non-thermal radio emission. Such a radio-selected sample, unaffected by extinction, will be particularly useful to compare with population synthesis models and should contribute to the understanding of the formation and evolution of PNe.

astro-ph.SR↗
arXiv2018

The Coordinated Radio and Infrared Survey for High-Mass Star Formation III. A catalogue of northern ultra-compact H II regions

A catalogue of 239 ultra-compact HII regions (UCHIIs) found in the CORNISH survey at 5 GHz and 1.5" resolution in the region $10^{\circ} < l < 65^{\circ}, ~|b| < 1^{\circ}$ is presented. This is the largest complete and well-selected sample of UCHIIs to date and provides the opportunity to explore the global and individual properties of this key state in massive star formation at multiple wavelengths. The nature of the candidates was validated, based on observational properties and calculated spectral indices, and the analysis is presented in this work. The physical sizes, luminosities and other physical properties were computed by utilising literature distances or calculating the distances whenever a value was not available. The near- and mid-infrared extended source fluxes were measured and the extinctions towards the UCHIIs were computed. The new results were combined with available data at longer wavelengths and the spectral energy distributions (SEDs) were reconstructed for 177 UCHIIs. The bolometric luminosities obtained from SED fitting are presented. By comparing the radio flux densities to previous observational epochs, we find about 5% of the sources appear to be time variable. This first high-resolution area survey of the Galactic plane shows that the total number of UCHIIs in the Galaxy is ~ 750 - a factor of 3-4 fewer than found in previous large area radio surveys. It will form the basis for future tests of models of massive star formation.

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arXiv2018

Class I methanol masers in low-mass star formation regions

We present a review of the properties of Class I methanol masers detected in low-mass star forming regions (LMSFRs). These masers, henceforth called LMMIs, are associated with postshock gas in the lobes of chemically active outflows in LMSFRs NGC1333, NGC2023, HH25, and L1157. LMMIs share the main properties with powerful masers in regions of massive star formation and are a low-luminosity edge of the total Class I maser population. However, the exploration of just these objects may push forward the exploration of Class I masers, since many LMSFRs are located only 200--300 pc from the Sun, making it possible to study associated objects in detail. EVLA observations with a $0.2"$ spatial resolution show that the maser images consist of unresolved or barely resolved spots with brightness temperatures up to $5\times 10^5$ K. The results are "marginally" consistent with the turbulent model of maser emission.

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arXiv2017

Analytical Methods for Measuring the Parameters of Interstellar Gas Using the Data of Methanol Observations

We analyze methanol excitation in the absence of external radiation and consider LTE methods for probing interstellar gas. We show that rotation diagrams correctly estimate the gas kinetic temperature only if they are built from lines with the upper levels located in the same K-ladders, such as the J_0-J_{-1}E lines at 157~GHz, the J_1-J_0E lines at 165~GHz or the J_2-J_1E lines at 25~GHz. The gas density should be no less than 10^7~cm^{-3}. Rotation diagrams built from lines with different K values of the upper levels (2_K-1_K at 96~GHz, 3_K-2_K at 145~GHz, or 5_K-4_K at 241~GHz) significantly underestimate the temperature but allow a density estimation. In addition, the diagrams based on the 2_K-1_K lines make possible methanol column density estimates within a factor of about 2--5. We suggest that rotation diagrams should be used in the following manner. First, one should build two rotation diagrams, one from the lines at 96, 145, or 241~GHz, and another from the lines at 157, 165, or 25~GHz. The former diagram is used to estimate the gas density. If the density is about 10^7~cm^{-3} or higher, the latter diagram reproduces the temperature fairly well. If the density is around 10^6~cm^{-3}, the temperature obtained from the latter diagram should be multiplied by a factor of 1.5--2. If the density is about 10^5~cm^{-3} or lower, then the latter diagram yields a temperature that is lower than the kinetic temperature by a factor of three or larger and should be used only as a lower limit on the kinetic temperature. Errors of methanol column density determined from the integrated intensity of a single line may be larger than an order of magnitude even when the gas temperature is well-known. However, if the J_0-(J-1)_0E lines, as well as the J_1-(J-1)_1A^{+} or A^{-} lines are used, the relative error of the column density proves to be no larger than several units.

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arXiv2017

Early Science with the Large Millimetre Telescope: Fragmentation of molecular clumps in the Galaxy

Sensitive, imaging observations of the 1.1 mm dust continuum emission from a 1 deg^2 area collected with the AzTEC bolometer camera on the Large Millimeter Telescope are presented. A catalog of 1545 compact sources is constructed based on a Wiener-optimization filter. These sources are linked to larger clump structures identified in the Bolocam Galactic Plane Survey. Hydrogen column densities are calculated for all sources and mass and mean volume densities are derived for the subset of sources for which kinematic distances can be assigned. The AzTEC sources are localized, high density peaks within the massive clumps of molecular clouds and comprise 5-15% of the clump mass. We examine the role of the gravitational instability in generating these fragments by comparing the mass of embedded AzTEC sources to the Jeans' mass of the parent BGPS object. For sources with distances less than 6 kpc the fragment masses are comparable to the clump Jeans' mass, despite having isothermal Mach numbers between 1.6 and 7.2. AzTEC sources linked to ultra-compact HII regions have mass surface densities greater than the critical value implied by the mass-size relationship of infrared dark clouds with high mass star formation while AzTEC sources associated with Class II methanol masers have mass surface densities greater than 0.7 g cm^{-2} that approaches the proposed threshold required to form massive stars.

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arXiv2017

Long-term Variability of H$_2$CO Masers in Star-forming Regions

We present results of a multi-epoch monitoring program on variability of 6$\,$cm formaldehyde (H$_2$CO) masers in the massive star forming region NGC$\,$7538$\,$IRS$\,$1 from 2008 to 2015 conducted with the GBT, WSRT, and VLA. We found that the similar variability behaviors of the two formaldehyde maser velocity components in NGC$\,$7538$\,$IRS$\,$1 (which was pointed out by Araya and collaborators in 2007) have continued. The possibility that the variability is caused by changes in the maser amplification path in regions with similar morphology and kinematics is discussed. We also observed 12.2$\,$GHz methanol and 22.2$\,$GHz water masers toward NGC$\,$7538$\,$IRS$\,$1. The brightest maser components of CH$_3$OH and H$_2$O species show a decrease in flux density as a function of time. The brightest H$_2$CO maser component also shows a decrease in flux density and has a similar LSR velocity to the brightest H$_2$O and 12.2$\,$GHz CH$_3$OH masers. The line parameters of radio recombination lines and the 20.17 and 20.97$\,$GHz CH$_3$OH transitions in NGC$\,$7538$\,$IRS$\,$1 are also reported. In addition, we observed five other 6$\,$cm formaldehyde maser regions. We found no evidence of significant variability of the 6$\,$cm masers in these regions with respect to previous observations, the only possible exception being the maser in G29.96$-$0.02. All six sources were also observed in the H$_2^{13}$CO isotopologue transition of the 6$\,$cm H$_2$CO line; H$_2^{13}$CO absorption was detected in five of the sources. Estimated column density ratios [H$_2^{12}$CO]/[H$_2^{13}$CO] are reported.

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