Search

We develop a robust Machine Learning classifier model utilizing the eXtreme-Gradient Boosting (XGB) algorithm for improved classification of Galactic Wolf-Rayet (WR) stars based on Infrared (IR) colors and positional attributes. For our study, we choose an extensive dataset of 6555 stellar objects (from 2MASS and AllWISE data releases) lying in the Milky Way (MW) with available photometric magnitudes of different types including WR stars. Our XGB classifier model can accurately (with an 86\% detection rate) identify a sufficient number of WR stars against a large sample of non-WR sources. The XGB model outperforms other ensemble classifier models such as the Random Forest. Also, using the XGB algorithm, we develop a WR sub-type classifier model that can differentiate the WR subtypes from the non-WR sources with a high model accuracy ($>60\%$). Further, we apply both XGB-based models to a selection of 6457 stellar objects with unknown object types, detecting 58 new WR star candidates and predicting sub-types for 10 of them. The identified WR sources are mainly located in the Local spiral arm of the MW and mostly lie in the solar neighborhood., Comment: 19 pages, Accepted to APJ

We introduce a novel method that utilizes the longitude-velocity (l-v) envelope to constrain the Milky Way (MW) bar potential. Previous work (Habing 2016) used the l-v diagram to explain the distribution of the observed high-velocity stars. We successfully reproduce their results, but find that their method is limited to only one single type of periodic orbits. In contrast, we propose that the l-v envelope provides much more comprehensive constraints. We compare the properties of test particles in the Portai et al. (2017) MW potential model (P17) with the observed SiO maser stars from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey. We find that the l-v envelope generated by the bar potential demonstrates reasonable agreement with the observational data, albeit with slight discrepancies near the Galactic center. The inconsistencies suggest that the P17 potential yields a lower central rotation curve, a slightly larger quadrupole strength, or a possibly underestimated pattern speed. We also adopt an updated version of the P17 potential with a modified central mass component (CMC) proposed by Hunter et al. (2024) (H24). The fitting of the l-v envelope suggests that the H24 potential does not completely address the existing challenges and may hint at a possible underestimation of the central bar mass. Our study demonstrates that the l-v envelope can be used as a valuable tool for constraining the Galactic potential and provides insights into the Milky Way bar potential., Comment: 20 pages, 17 figures, ApJ accepted

We present a method to estimate distances to Asymptotic Giant Branch (AGB) stars in the Galaxy, using spectral energy distributions (SEDs) in the near- and mid-infrared. By assuming that a given set of source properties (initial mass, stellar temperature, composition, and evolutionary stage) will provide a typical SED shape and brightness, sources are color-matched to a distance-calibrated template and thereafter scaled to extract the distance. The method is tested by comparing the distances obtained to those estimated from Very Long Baseline Interferometry or Gaia parallax measurements, yielding a strong correlation in both cases. Additional templates are formed by constructing a source sample likely to be close to the Galactic center, and thus with a common, typical distance for calibration of the templates. These first results provide statistical distance estimates to a set of almost 15,000 Milky Way AGB stars belonging to the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey, with typical distance errors of $\pm 35$%. With these statistical distances a map of the intermediate-age population of stars traced by AGBs is formed, and a clear bar structure can be discerned, consistent with the previously reported inclination angle of 30$^\circ$ to the GC-Sun direction vector. These results motivate deeper studies of the AGB population to tease out the intermediate-age stellar distribution throughout the Galaxy, as well as determining statistical properties of the AGB population luminosity and mass-loss rate distributions., Comment: Accepted to APJ

Radio campaigns using maser stellar beacons have provided crucial information to characterize Galactic stellar populations. Currently, the Bulge Asymmetries and Dynamical Evolution (BAaDE) project is surveying infrared (IR) color-selected targets for SiO masers. This provides a sample of evolved stars that can be used to study the inner, optically obscured Galaxy using line of sight velocities and possibly VLBI proper motions. In order to use the BAaDE sample for kinematic studies, the stellar population should be characterized. In this study, the BAaDE targets have been cross-matched with IR (2MASS) and optical Gaia samples. By exploring the synergies of this cross-match together with Gaia parallaxes and extinction maps, the local ($d < 2$ kpc) AGB stars can be characterized. We have defined a \textit{BAaDE-Gaia} sample of 20,111 sources resulting from cross-matching BAaDE targets with IR and optical surveys. From this sample, a~{\local} sample of 1,812 evolved stars with accurate parallax measurements, confirmed evolved stellar evolution stage, and within 2 kpc distance around the Sun was selected, for which absolute (bolometric) magnitudes are estimated. The evolved stellar population with Gaia counterparts that are variable seems to be predominantly associated with AGB stars with moderate luminosity ($1,500^{+3,000}_{-500} \ L_\odot$) and periods between 250 and 1,250 days., Comment: Accepted for publication in ApJ

We present 43 GHz VLA spectra for 51 AGB sources with the goal of verifying an infrared (IR) color cut intended to separate Carbon-rich (C) and Oxygen-rich (O) AGB sources throughout the Galaxy. The color cut is a simple line in the $[K_s]-[A]$ versus $[A]-[E]$ color-color diagram based on 2MASS and MSX photometry, and was originally derived from SiO detection rates in the Bulge Asymmetries and Dynamical Evolution (BAaDE) sample. The division is fully supported by the spectra presented here, which show that SiO maser detections lie on the O-rich side, and SiO nondetections and a single HC$_3$N detection are found on the C-rich side of the division. We further compare the color cut with classifications of the sources based on Low-Resolution Spectra (LRS) from the Infrared Astronomical Satellite (IRAS), and find good agreement, verifying that the division is a reliable and efficient method for differentiating O- and C-rich AGB sources. These observations also demonstrate that single lines detected in the BAaDE survey around 42.9 GHz are almost certainly the $^{29}$SiO v=0 line. SiO maser sources where this rare isotopologue transition is brighter than the dominant $^{28}$SiO lines have not been reported before, and our observations show that these sources can reverse their behavior such that the typical ratios of $^{28}$SiO and $^{29}$SiO are restored within a few years., Comment: 18 pages, 6 figures accepted to ApJ

Detections of SiO masers from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey more tightly define the region where Oxygen-rich (O) Asymptotic Giant Branch (AGB) stars reside in multiple infrared (IR) color-color diagrams. Using MSX and 2MASS data along with radio spectra from the BAaDE survey we find that three main populations were observed in the BAaDE survey: O-rich AGB stars of which about $73\%$ host SiO masers, Carbon-rich (C) AGB stars which do not host these masers, and a small contaminating set of possible Young Stellar Objects (YSOs). The distinction between YSOs and AGB stars can be drawn using only MSX data, specifically the $[D]-[E]$ color, while the shorter wavelengths provided by 2MASS are necessary to divide potential C- and O-rich AGB stars. Divisions similar to these have been seen in multiple earlier IR-studies, but BAaDE currently provides a sample of $\sim$ 15000 sources which far exceeds previous studies in sample size, and therefore provides much more distinct divisions. With these IR distinctions in place, we discuss the sources which are exceptions in either their molecular detections or IR colors, as well as the distribution of the three populations in Galactic coordinates., Comment: accepted for publication in ApJ

We report on the first 1,432 sources observed using the Atacama Large Millimeter/submillimeter Array (ALMA), from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey, which aims to obtain tens of thousands of line-of-sight velocities from SiO masers in Asymptotic Giant Branch (AGB) stars in the Milky Way. A 71% detection rate of 86 GHz SiO masers is obtained from the infrared color-selected sample, and increases to 80% when considering the likely oxygen-rich stars using Midcourse Space Experiment (MSX) colors isolated in a region where [D]-[E] <= 1.38. Based on Galactic distributions, the presence of extended CS emission, and likely kinematic associations, the population of sources with [D]-[E] > 1.38 probably consists of young stellar objects, or alternatively, planetary nebulae. For the SiO detections we examined whether individual SiO transitions provide comparable stellar line-of-sight velocities, and found that any SiO transition is suitable for determining a stellar AGB line-of-sight velocity. Finally, we discuss the relative SiO detection rates and line strengths in the context of current pumping models., Comment: 25 pages, 16 figures

With the publication of Gaia DR2, 1.3 billion stars now have public parallax and proper motion measurements. In this contribution, we compare the results for sources that have both optical and radio measurements, focusing on circumstellar masers. For these large, variable and bright AGB stars, the VLBI astrometry results can be more robust. Moreover, there are a number of applications where VLBI astrometry provides unique data for studying stellar populations and Galactic structure. The BeSSel project not only provides parallax and proper motions at much larger distances than Gaia can reach, but it also uniquely samples the spiral arms of the Galaxy. The evolved stars in the BAaDE sample can potentially constrain the dynamics and stellar content of the inner bulge and bar of the Milky Way, not reachable in the optical., Comment: 6 pages, accepted for the Proceedings of the 14th European VLBI Network Symposium & Users Meeting (typo in Authors' name, reference mixed up)

Observations of dust-enshrouded evolved stars selected from infrared catalogs requiring high positional accuracy, like infrared spectroscopy or long baseline radio interferometric observations, often require preparational observational steps determining a position with an accuracy much better than 1". Using phase-referencing observations with the Very Large Array at its highest resolution, we have compared the positions of SiO 43 GHz masers in evolved stars, assumed to originate in their infrared detected circumstellar shells, with the positions listed in the MSX, WISE, 2MASS, and Gaia catalogs. Starting from an MSX position it is, in general, simple to match 2MASS and WISE counterparts. However, in order to obtain a Gaia match to the MSX source it is required to use a two-step approach due to the large number of nearby candidates and low initial positional accuracy of the MSX data. We show that the closest comparable position to the SiO maser in our limited sample never is the MSX position. When a plausible source with a characteristic signature of an evolved star with a circumstellar shell can be found in the area, the best indicator of the maser position is provided by the Gaia position, with the 2MASS position being second-best. Typical positional offsets from all catalogs to the SiO masers are reported., Comment: 8 pages, accepted ApJ

We observed the 43 GHz v=1, 2, and 3 and 86 GHz v=1 SiO maser transitions quasi-simultaneously for a Mira-variable-dominated sample of over 80 sources from the Bulge Asymmetries and Dynamical Evolution (BAaDE) project, using ATCA, and statistically compared the relative line strengths. On average, the 43 GHz v=1 line is brighter than the 86 GHz v=1 line by a factor of 1.36+/-0.15. As a result, an 86 GHz v=1 observed sample can be observed to 85.9% of the distance of a 43 GHz v=1 observed sample using the same sensitivity. We discuss what impact this may have on the BAaDE Galactic plane survey using the VLA and ALMA. Despite fewer v=3 detections, specific trends are discerned or strengthened when the 43 GHz v=3 line is detected. In particular the 43 and 86 GHz v=1 lines are on average equal for sources with no detectable 43 GHz v=3 emission, but the 43 GHz v=1 line strength is on average about twice as bright as the 86 GHz v=1 line for sources with detectable 43 GHz v=3 emission. Some weak correlations are found between line strengths and Midcourse Space Experiment flux densities and colors, which are tightened when considering only sources with detectable 43 GHz v=3 emission. We discuss these trends in the context of a radiative pumping model to highlight how the 43 GHz v=3 line, when coupled with the v=1 and v=2 lines, can further our understanding of variable conditions like density in the circumstellar envelopes.

We report on a large 44 GHz ($7_0-6_1$ A$^+$) methanol (CH$_3$OH) maser survey of the Galactic Center (GC). The Karl G. Jansky Very Large Array was used to search for CH$_3$OH maser emission covering a large fraction of the region around Sgr A. In 25 pointings, over 300 CH$_3$OH maser sources ($>10\sigma$) were detected. The majority of the maser sources have a single peak emission spectrum with line of sight velocities that range from about $-$13 km\,s$^{-1}$ to 72 km\,s$^{-1}$. Most maser sources were found to have velocities around 35$-$55 km\,s$^{-1}$, closely following velocities of neighboring interacting molecular clouds. The full width half maximum of each individual spectral feature is very narrow ($\sim$0.85 km\,s$^{-1}$ on average). In the north, where Sgr A East is known to be interacting with the 50 km\,s$^{-1}$ molecular cloud, more than 100 44 GHz CH$_3$OH masers were detected. In addition, three other distinct concentrations of masers were found, which appear to be located closer to the interior of the interacting molecular clouds. Possibly a subset of masers are associated with star formation, although conclusive evidence is lacking.

We report on Green Bank Telescope 23.87 GHz NH$_3$(3,3), emission observations in five supernova remnants interacting with molecular clouds (G1.4$-$0.1, IC443, W44, W51C, and G5.7$-$0.0). The observations show a clumpy gas density distribution, and in most cases the narrow line widths of $\sim3-4$\,km\,s$^{-1}$ are suggestive of maser emission. Very Large Array observations reveal 36~GHz and/or 44~GHz CH$_3$OH, maser emission in a majority (72\%) of the NH$_3$, peak positions towards three of these SNRs. This good positional correlation is in agreement with the high densities required for the excitation of each line. Through these observations we have shown that CH$_3$OH, and NH$_3$, maser emission can be used as indicators of high density clumps of gas shocked by supernova remnants, and provide density estimates thereof. Modeling of the optical depth of the NH$_3$(3,3) emission is compared to that of CH$_3$OH, constraining the densities of the clumps to a typical density of the order of $10^5$~cm$^{-3}$ for cospatial masers. Regions of gas with this density are found to exist in the post-shocked gas quite close to the SNR shock front, and may be associated with sites where cosmic rays produce gamma-ray emission via neutral pion decay.

We present results from calculations of the physical conditions necessary for the occurrence of 36.169 ($4_{-1}-3_{0}\, E$), 44.070 ($7_{0}-6_{1}\,A^+$), 84.521 ($5_{-1}-4_{0}\,E$), and 95.169 ($8_{0}-7_{1}\,A^+$) GHz methanol (CH$_3$OH) maser emission lines near supernova remnants (SNRs), using the MOLPOP-CEP program. The calculations show that given a sufficient methanol abundance, methanol maser emission arises over a wide range of densities and temperatures, with optimal conditions at $n\sim 10^4-10^6$ cm$^{-3}$ and $T>60$ K. The 36~GHz and 44~GHz transitions display more significant maser optical depths compared to the 84~GHz and 95~GHz transitions over the majority of physical conditions. It is also shown that line ratios are an important and applicable probe of the gas conditions. The line ratio changes are largely a result of the $E$-type transitions becoming quenched faster at increasing densities. The modeling results are discussed using recent observations of CH$_3$OH and hydroxyl (OH) masers near the supernova remnants G1.4$-$0.1, W28, and Sgr A East.

We perform calorimetry on the bright gamma ray burst (GRB) 030329 by fitting simultaneously the broadband radio afterglow and the observed afterglow image size to a semi-analytic magnetohydrodynamical (MHD) and afterglow emission model. Our semi-analytic method is valid in both the relativistic and non-relativistic regimes, and incorporates a model of the interstellar scintillation that substantially effects the broadband afterglow below 10 GHz. The model is fitted to archival measurements of the afterglow flux from 1 day to 8.3 years after the burst. Values for the initial burst parameters are determined and the nature of the circumburst medium is explored. Additionally, direct measurements of the lateral expansion rate of the radio afterglow image size allow us to estimate the initial Lorentz factor of the jet., Comment: 8 pages, 4 figures. Accepted for publication by Astrophysical Journal

We present VLBI and archival Karl G. Jansky Very Large Array (VLA) and Westerbork Synthesis Radio Telescope (WSRT) observations of the radio afterglow from the gamma-ray burst (GRB) of 2003 March 29 (GRB 030329) taken between 672 and 2032 days after the burst. The EVLA and WSRT data suggest a simple power law decay in the flux at 5 GHz, with no clear signature of any rebrightening from the counter jet. We report an unresolved source at day 2032 of size $1.18\pm0.13$ mas, which we use in conjunction with the expansion rate of the burst to argue for the presence of a uniform, ISM-like circumburst medium. We develop a semi-analytic method to model gamma-ray burst afterglows, and apply it to the 5 GHz light curve to perform burst calorimetry. A limit of $< 0.067$ mas yr$^{-1}$ is placed on the proper motion, supporting the standard afterglow model for gamma-ray bursts., Comment: 24 pages, 5 figures

It is now generally accepted that long-duration gamma ray bursts (GRBs) are due to the collapse of massive rotating stars. The precise collapse process itself, however, is not yet fully understood. Strong winds, outbursts, and intense ionizing UV radiation from single stars or strongly interacting binaries are expected to destroy the molecular cloud cores that give birth to them and create highly complex circumburst environments for the explosion. Such environments might imprint features on GRB light curves that uniquely identify the nature of the progenitor and its collapse. We have performed numerical simulations of realistic environments for a variety of long-duration GRB progenitors with ZEUS-MP, and have developed an analytical method for calculating GRB light curves in these profiles. Though a full, three-dimensional, relativistic magnetohydrodynamical computational model is required to precisely describe the light curve from a GRB in complex environments, our method can provide a qualitative understanding of these phenomena. We find that, in the context of the standard afterglow model, massive shells around GRBs produce strong signatures in their light curves, and that this can distinguish them from those occurring in uniform media or steady winds. These features can constrain the mass of the shell and the properties of the wind before and after the ejection. Moreover, the interaction of the GRB with the circumburst shell is seen to produce features that are consistent with observed X-ray flares that are often attributed to delayed energy injection by the central engine. Our algorithm for computing light curves is also applicable to GRBs in a variety of environments such as those in high-redshift cosmological halos or protogalaxies, both of which will soon be targets of future surveys such as JANUS or Lobster., Comment: 12 pages, 5 figures, Accepted by ApJ

Class I methanol masers are believed to be produced in the shock-excited environment around star-forming regions. Many authors have argued that the appearance of various subsets of class I masers may be indicative of specific evolutionary stages of star formation or excitation conditions. Until recently, however, no major interferometer was capable of imaging the important 36 GHz transition. We report on Expanded Very Large Array observations of the 36 GHz methanol masers and Submillimeter Array observations of the 229 GHz methanol masers in DR21(OH), DR21N, and DR21W. The distribution of 36 GHz masers in the outflow of DR21(OH) is similar to that of the other class I methanol transitions, with numerous multitransition spatial overlaps. At the site of the main continuum source in DR21(OH), class I masers at 36 and 229 GHz are found in virtual overlap with class II 6.7 GHz masers. To the south of the outflow, the 36 GHz masers are scattered over a large region but usually do not appear coincident with 44 GHz masers. In DR21W we detect an "S-curve" signature in Stokes V that implies a large value of the magnetic field strength if interpreted as due to Zeeman splitting, suggesting either that class I masers may exist at higher densities than previously believed or that the direct Zeeman interpretation of S-curve Stokes V profiles in class I masers may be incorrect. We find a diverse variety of different maser phenomena in these sources, suggestive of differing physical conditions among them., Comment: 8 pages, accepted for publication in ApJ

The Long Wavelength Array (LWA) will be a new multi-purpose radio telescope operating in the frequency range 10-88 MHz. Upon completion, LWA will consist of 53 phased array "stations" distributed over a region about 400 km in diameter in the state of New Mexico. Each station will consist of 256 pairs of dipole-type antennas whose signals are formed into beams, with outputs transported to a central location for high-resolution aperture synthesis imaging. The resulting image sensitivity is estimated to be a few mJy (5 sigma, 8 MHz, 2 polarizations, 1 hr, zenith) in 20-80 MHz; with resolution and field of view of (8", 8 deg) and (2",2 deg) at 20 MHz and 80 MHz, respectively. All 256 dipole antennas are in place for the first station of the LWA (called LWA-1), and commissioning activities are well underway. The station is located near the core of the EVLA, and is expected to be fully operational in early 2011., Comment: 10 pages, ISKAF2010 Science Meeting, June 2010, Assen, The Netherlands

The collisionally pumped, ground-state 1720 MHz maser line of OH is widely recognized as a tracer for shocked regions and observed in star forming regions and supernova remnants. Whereas some lines of excited states of OH have been detected and studied in star forming regions, the subject of excited-state OH in supernova remnants -- where high collision rates are to be expected -- is only recently being addressed. Modeling of collisional excitation of OH demonstrates that 1720, 4765 and 6049 MHz masers can occur under similar conditions in regions of shocked gas. In particular, the 6049 and 4765 MHz masers become more significant at increased OH column densities where the 1720 MHz masers begin to be quenched. In supernova remnants, the detection of excited-state OH line maser emission could therefore serve as a probe of regions of higher column densities. Using the Very Large Array, we searched for excited-state OH in the 4.7, 7.8, 8.2 and 23.8 GHz lines in four well studied supernova remnants with strong 1720 MHz maser emission (SgrAEast, W28, W44 and IC443). No detections were made, at typical detection limits of around 10 mJy/beam. The search for the 6 GHz lines were done using Effelsberg since the VLA receivers did not cover those frequencies, and are reported on in an accompanying letter (Fish, Sjouwerman & Pihlstrom 2007). We also cross-correlated the positions of known supernova remnants with the positions of 1612 MHz maser emission obtained from blind surveys. No probable associations were found, perhaps except in the SgrAEast region. The lack of detections of excited-state OH indicates that the OH column densities suffice for 1720 MHz inversion but not for inversion of excited-state transitions, consistent with the expected results for C-type shocks., Comment: Accepted to ApJ, 7 pages including 1 table and 4 figures

While masers in the 1720 MHz transition of OH are detected toward many supernova remnants (SNRs), no other OH transition is seen as a maser in SNRs. We present a search for masers at 6049 MHz, which has recently been predicted to produce masers by pure collisional excitation at conditions similar to that required for 1720 MHz masing. The Effelsberg 100 m telescope was used to observe the excited-state 6016, 6030, 6035, and 6049 MHz lines of OH toward selected SNRs, most of which have previously-detected bright 1720 MHz masers. No excited-state masers are found toward SNRs, consistent with previous observations of the 6049 MHz and other excited-state transitions. We do not see clear evidence of absorption toward SNR target positions, although we do see evidence of absorption in the molecular cloud at +50 km/s near Sgr A East. Weak absorption is detected at 6016 MHz toward W3(OH), while stronger, narrower emission is seen at 6049 MHz, suggesting that the 6049 MHz emission is a low-gain maser. We conclude that conditions in SNRs are not conducive to excited-state maser emission, especially in excited-state satellite lines., Comment: 4 pages using emulateapj.cls including 2 tables and 1 figure, accepted to ApJL

Excited-state OH maser emission has previously been reported in the circumstellar envelopes of only two evolved stars: the Mira star AU Geminorum and the hypergiant NML Cygni. We present Very Large Array (VLA) observations of the 1665, 1667, and excited-state 4750 MHz mainline OH transitions in AU Gem and Expanded Very Large Array (EVLA) observations of the excited-state 6030 and 6035 MHz OH mainline transitions in NML Cyg. We detect masers in both mainline transitions in AU Gem but no excited-state emission in either star. We conclude that the excited-state OH emission in AU Gem is either a transient phenomenon (such as for NML Cyg outlined below), or possibly an artifact in the data, and that the excited state OH emission in NML Cyg was generated by an episode of enhanced shock between the stellar mass-loss and an outflow of the Cyg OB2 association. With these single exceptions, it therefore appears that excited-state OH emission indeed should not be predicted nor observable in evolved stars as part of their normal structure or evolution., Comment: ApJ Letter, accepted, 4 pages, 2 figures

Recent observational and theoretical advances have called into question traditional OH maser pumping models in evolved (OH/IR) stars. The detection of excited-state OH lines would provide additional constraints to discriminate amongst these theoretical models. In this Letter, we report on VLA observations of the 4750 MHz and 4765 MHz lines of OH toward 45 sources, mostly evolved stars. We detect 4765 MHz emission in the star forming regions Mon R2 and LDN 1084, but we do not detect excited-state emission in any evolved stars. The flux density and velocity of the 4765 MHz detection in Mon R2 suggests that a new flaring event has begun., Comment: 4 pages, to appear in ApJL