arXiv subjects

E. M. Green

Publications and source records attributed to E. M. Green.

At least 19 recordsLinked to original sources

arXiv2025

The Arizona-Montréal spectroscopic survey of hot subluminous stars

Hot subdwarf B (sdB) and O (sdO) type stars are evolved helium-burning objects that lost their hydrogen envelope before the helium flash when their progenitors were close to the tip of the red giant branch. They populate the extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram (HRD). Using the high-quality, homogeneous spectra of 336 hot subluminous star candidates from the Arizona-Montréal Spectroscopic Survey, we aim to improve our understanding of the atmospheric and stellar properties of hot subdwarf stars. We used large grids of model atmospheres to fit the observed spectra and derived their atmospheric parameters: effective temperature (Teff), surface gravity, and helium abundance. The model grids were further utilized to fit the spectral energy distribution of each star and the $Gaia$ parallax was used to compute the stellar parameters radius, luminosity, and mass. We detected helium stratification in six sdB stars with Teff around 30 kK, making them good candidates for also showing $^3$He enrichment in their atmospheres. The mass distributions of H-rich sdBs and sdOs are similar and centered around 0.47 $\text{M}_\odot$, consistent with the canonical formation scenario of helium ignition under degenerate conditions. Among the H-rich hot subdwarfs, we found no difference between the mass distributions of close binaries and apparently single stars. The He-sdOs have a significantly wider mass distribution than their H-rich counterparts, with an average mass of about 0.78 $\text{M}_\odot$. This strongly favors a merger origin for these He-rich objects. We identified a small number of candidate low-mass ($<$0.45$ \text{M}_\odot$) sdBs located below the EHB that might have originated from more massive progenitors. Finally, we identified more than 80 pulsating stars in our sample and found these to fall into well-defined $p$- and $g$-mode instability regions.

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arXiv2024

A 500 pc volume-limited sample of hot subluminous stars I. Space density, scale height, and population properties

We present the first volume-limited sample of spectroscopically confirmed hot subluminous stars out to 500 pc, defined using the accurate parallax measurements from the {\em Gaia} space mission data release 3 (DR3). The sample comprises a total of 397 members, with 305 ($\sim 77\%$) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 ($\sim58\%$) are hydrogen-rich sdBs, 65 are sdOBs ($\sim 21\%$), 32 are sdOs ($\sim 11\%$), and 30 are He-sdO/Bs ($\sim 10\%$). Among them, 48 ($\sim 16\%$) exhibit an infrared excess in their spectral energy distribution fits, suggesting a composite binary system. The hot subdwarf population is estimated to be 90\% complete, assuming that most missing systems are these composite binaries located within the main sequence (MS) in the \emph{Gaia} colour-magnitude diagram (CMD). The remaining sources in the sample include cataclysmic variables (CVs), blue horizontal branch stars (BHBs), hot white dwarfs (WDs), and MS stars. We derived the mid-plane density $ρ_{0}$ and scale height $\mathrm{h}_{z}$ for the non-composite hot subdwarf star population using a hyperbolic sechant profile (sech$^2$). The best-fit values are $ρ_{0}\,=\,5.17\pm 0.33 \times10^{-7}$ stars/pc$^{3}$ and $\mathrm{h}_{z} = 281 \pm 62$ pc. When accounting for the composite-colour hot subdwarfs and their estimated completeness, the mid-plane density increases to $ρ_{0}\,=\,6.15^ {+1.16}_{-0.53} \times10^{-7}$ stars/pc$^{3}$. This corrected space density is an order of magnitude lower than predicted by population synthesis studies, supporting previous observational estimates.

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arXiv2020

The EXOTIME project: Signals in the $ O-C $ diagrams of the rapidly pulsating subdwarfs DW Lyn, V1636 Ori, QQ Vir, and V541 Hya

We aim to investigate variations in the arrival time of coherent stellar pulsations due to the light-travel time effect to test for the presence of sub-stellar companions. Those companions are the key to one possible formation scenario of apparently single sub-dwarf B stars. We made use of an extensive set of ground-based observations of the four large amplitude p-mode pulsators DW Lyn, V1636 Ori, QQ Vir, and V541 Hya. Observations of the TESS space telescope are available on two of the targets. The timing method compares the phase of sinusoidal fits to the full multi-epoch light curves with phases from the fit of a number of subsets of the original time series. Observations of the TESS mission do not sample the pulsations well enough to be useful due to the (currently) fixed two-minute cadence. From the ground-based observations, we infer evolutionary parameters from the arrival times. The residual signals show many statistically significant periodic signals, but no clear evidence for changes in arrival time induced by sub-stellar companions. The signals can be explained partly by mode beating effects. We derive upper limits on companion masses set by the observational campaign.

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arXiv2019

TESS first look at evolved compact pulsators : Discovery and asteroseismic probing of the g-mode hot B subdwarf pulsator EC 21494-7018

We present the discovery and asteroseismic analysis of a new g-mode hot B subdwarf (sdB) pulsator, EC 21494-7018 (TIC 278659026), monitored in TESS first sector using 120-second cadence. The light curve analysis reveals that EC 21494-7018 is a sdB pulsator counting up to 20 frequencies associated with independent g-modes. The seismic analysis singles out an optimal model solution in full agreement with independent measurements provided by spectroscopy (atmospheric parameters derived from model atmospheres) and astrometry (distance evaluated from Gaia DR2 trigonometric parallax). Several key parameters of the star are derived. Its mass (0.391 +/- 0.009 Msun) is significantly lower than the typical mass of sdB stars, and suggests that its progenitor has not undergone the He-core flash, and therefore could originate from a massive (>2 Msun) red giant, an alternative channel for the formation of hot B subdwarfs. Other derived parameters include the H-rich envelope mass (0.0037 +/- 0.0010 Msun), radius (0.1694 +/- 0.0081 Rsun), and luminosity (8.2+/-1.1 Lsun). The optimal model fit has a double-layered He+H composition profile, which we interpret as an incomplete but ongoing process of gravitational settling of helium at the bottom of a thick H-rich envelope. Moreover, the derived properties of the core indicate that EC 21494-7018 has burnt ~43% (in mass) of its central helium and possesses a relatively large mixed core (Mcore = 0.198 +/- 0.010 Msun), in line with trends already uncovered from other g-mode sdB pulsators analysed with asteroseismology. Finally, we obtain for the first time an estimate of the amount of oxygen (in mass; X(O)core = 0.16 -0.05 +0.13) produced at this stage of evolution by an helium-burning core. This result, along with the core-size estimate, is an interesting constraint that may help to narrow down the still uncertain C12(alpha,gamma)O16 nuclear reaction rate.

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arXiv2019

Searches After Gravitational-waves Using ARizona Observatories (SAGUARO): System Overview and First Results from Advanced LIGO/Virgo's Third Observing Run

We present Searches After Gravitational-waves Using ARizona Observatories (SAGUARO), a comprehensive effort dedicated to the discovery and characterization of optical counterparts to gravitational wave (GW) events. SAGUARO utilizes ground-based facilities ranging from 1.5m to 10m in diameter, located primarily in the Northern Hemisphere. We provide an overview of SAGUARO's telescopic resources, pipeline for transient detection, and database for candidate visualization. We describe SAGUARO's discovery component, which utilizes the $5$~deg$^2$ field-of-view optical imager on the Mt. Lemmon 1.5m telescope, reaching limits of $\approx 21.3$~AB mag while rapidly tiling large areas. We also describe the follow-up component of SAGUARO, used for rapid vetting and monitoring of optical candidates. With the onset of Advanced LIGO/Virgo's third observing run, we present results from the first three SAGUARO searches following the GW events S190408an, S190425z and S190426c, which serve as a valuable proof-of-concept of SAGUARO. We triggered and searched 15, 60 and 60 deg$^{2}$ respectively, 17.6, 1.4 and 41.8 hrs after the initial GW alerts. We covered 7.8, 3.0 and 5.1\% of the total probability within the GW event localizations, reaching 3$σ$ limits of 19.8, 21.3 and 20.8 AB mag, respectively. Although no viable counterparts associated with these events were found, we recovered 6 known transients and ruled out 5 potential candidates. We also present Large Binocular Telescope spectroscopy of PS19eq/SN2019ebq, a promising kilonova candidate that was later determined to be a supernova. With the ability to tile large areas and conduct detailed follow-up, SAGUARO represents a significant addition to GW counterpart searches.

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arXiv2019

Discovery of a second pulsating intermediate helium-enriched sdOB star

We present the discovery of long-period, low-amplitude, g-mode pulsations in the intermediate He-rich hot subdwarf (sdOB) star Feige 46. Up until now only one other He-enriched sdOB star (LS IV -14116) was known to exhibit such pulsations. From our ground-based light curves of Feige 46, we extracted five independent periodicities ranging from 2294 s to 3400 s. We fitted our low-resolution, high signal-to-noise optical spectrum of the star with our grid of non-LTE model atmospheres and derived the following atmospheric parameters: Teff = 36120 $\pm$ 230 K, log g = 5.93 $\pm$ 0.04 and log N(He)/N(H) = -0.32 $\pm$ 0.03 (formal fitting errors only). These parameters are very similar to those of LS IV -14116 and place Feige 46 well outside of the instability strip where the hydrogen-rich g-mode sdB pulsators are found. We used the Gaia parallax and proper motion of Feige 46 to perform a kinematic analysis of this star and found that it likely belongs to the Galactic halo population. This is most certainly an intriguing and interesting result given that LS IV -14116 is also a halo object. The mechanism responsible for the pulsations in these two peculiar objects remains unclear but a possible scenario involves the $ε$-mechanism. Although they are the only two members in their class of variable stars, these pulsators appear to have more in common than just their pulsation properties.

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arXiv2017

Spectral analysis of four surprisingly similar hot hydrogen-rich subdwarf O stars

We perform a comprehensive spectroscopic analysis of four such bright sdO stars, namely Feige 34, Feige 67, AGK$+$81 266, and LS II$+$18 9, among which the first three are used as standard stars for flux calibration. We used non-local thermodynamic equilibrium model atmospheres in combination with high quality optical and UV spectra. Photometric data were also used to compute the spectroscopic distances of our stars and to characterize the companion responsible for the infrared excess of Feige 34. The four bright sdO stars have very similar atmospheric parameters with Teff between 60 000 and 63 000 K and log g in the range 5.9 to 6.1. This places these objects right on the theoretical post-EHB evolutionary tracks. The UV spectra are dominated by strong iron and nickel lines and suggest abundances that are enriched with respect to those of the Sun by factors of 25 and 60. On the other hand, the lighter elements, C, N, O, Mg, Si, P, and S, are depleted. The stars have very similar abundances, although AGK$+$81 266 shows differences in its light element abundances. For instance, the helium abundance of this object is 10 times lower than that observed in the other three stars. All our stars show UV spectral lines that require additional line broadening that is consistent with a rotational velocity of about 25 km s$^{-1}$. The infrared excess of Feige 34 is well reproduced by a M0 main-sequence companion and the surface area ratio of the two stars suggests that the system is a physical binary. However, the lack of radial velocity variations points towards a low inclination and/or long orbital period. Spectroscopic and Hipparcos distances are in good agreement for our three brightest stars.

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arXiv2016

The population of hot subdwarf stars studied with Gaia I. The catalogue of known hot subdwarf stars

In preparation for the upcoming all-sky data releases of the Gaia mission we compiled a catalogue of known hot subdwarf stars and candidates drawn from the literature and yet unpublished databases. The catalogue contains 5613 unique sources and provides multi-band photometry from the ultraviolet to the far infrared, ground based proper motions, classifications based on spectroscopy and colours, published atmospheric parameters, radial velocities and light curve variability information. Using several different techniques we removed outliers and misclassified objects. By matching this catalogue with astrometric and photometric data from the Gaia mission, we will develop selection criteria to construct a homogeneous, magnitude-limited all-sky catalogue of hot subdwarf stars based on Gaia data.

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arXiv2016

Multi-epoch Spectropolarimetry of SN 2011fe

We present multiple spectropolarimetric observations of the nearby Type Ia supernova (SN) 2011fe in M101, obtained before, during, and after the time of maximum apparent visual brightness. The excellent time coverage of our spectropolarimetry has allowed better monitoring of the evolution of polarization features than is typical, which has allowed us new insight into the nature of normal SNe Ia. SN 2011fe exhibits time-dependent polarization in both the continuum and strong absorption lines. At early epochs, red wavelengths exhibit a degree of continuum polarization of up to 0.4\%, likely indicative of a mild asymmetry in the electron-scattering photosphere. This behavior is more common in sub-luminous SNe Ia than in normal events, such as SN2011fe. The degree of polarization across a collection of absorption lines varies dramatically from epoch to epoch. During the earliest epoch a $λ$4600-5000 Å complex of absorption lines shows enhanced polarization at a different position angle than the continuum. We explore the origin of these features, presenting a few possible interpretations, without arriving at a single favored ion. During two epochs near maximum, the dominant polarization feature is associated with the Si{\sc ii} $λ$6355 Å absorption line. This is common for SNeIa, but for SN2011fe the polarization of this feature increases after maximum light, whereas for other SNeIa, that polarization feature was strongest before maximum light.

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arXiv2016

Spectral analysis of the sdO standard star Feige 34

We present our current work on the spectral analysis of the hot sdO star Feige 34. We combine high S/N optical spectra and fully-blanketed non-LTE model atmospheres to derive its fundamental parameters (Teff, log g) and helium abundance. Our best fits indicate Teff =63 000 K, log g=6.0 and log N(He)/N(H)=-1.8. We also use available ultraviolet spectra (IUE and FUSE) to measure metal abundances. We find the star to be enriched in iron and nickel by a factor of ten with respect to the solar values, while lighter elements have subsolar abundances. The FUSE spectrum suggests that the spectral lines could be broadened by rotation.

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arXiv2015

A NLTE analysis of the hot subdwarf O star BD+28 4211. II. The optical spectrum

We present the second part of our detailed analysis of the hot sdO and spectroscopic standard star BD+28 4211, in which we focus on the optical spectrum. In the first part of our study, we determined the abundances of some 11 metals detected in the atmosphere of BD+28 4211 using UV spectra of the star and corroborated the fundamental parameters estimated in past studies (Teff $\sim$ 82,000 K, log g $\sim$ 6.2, and solar N(He)/N(H)). In this work, we aim at rederiving these secured parameters on the sole basis of high-quality optical spectra. A first grid of non-LTE line-blanketed model atmospheres, including metals with the abundances derived from the UV spectrum, does not give satisfactory results when we apply a standard simultaneous fitting procedure to the observed H and He lines of our optical spectra. The line profiles are not finely reproduced and the resulting effective temperatures, in particular, are too low by $\sim$10,000 K. We next investigate the probable cause of this failure, that is, the importance of missing opacity sources on the atmospheric stratification. We compare line profiles computed from models with artificially boosted metallicities, from solar abundances to 15$\times$ these values. We find that the structural effects saturate for a metallicity of $\sim$10x solar, and use this to compute a second full grid of models and synthetic spectra. This metal-enriched grid allows us to achieve significantly improved spectral fits with models having the expected parameters. Our test case thus reveals that there is still a need for models with enhanced metallicity for better estimating the atmospheric parameters of objects such as hot subdwarfs and hot white dwarfs if only optical spectra are available.

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arXiv2015

High-speed Photometric Observations of ZZ Ceti White Dwarf Candidates

We present high-speed photometric observations of ZZ Ceti white dwarf candidates drawn from the spectroscopic survey of bright DA stars from the Villanova White Dwarf Catalog by Gianninas et al., and from the recent spectroscopic survey of white dwarfs within 40 parsecs of the Sun by Limoges et al. We report the discovery of six new ZZ Ceti pulsators from these surveys, and several photometrically constant DA white dwarfs, which we then use to refine the location of the ZZ Ceti instability strip.

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arXiv2014

Improved determination of the atmospheric parameters of the pulsating sdB star Feige 48

As part of a multifaceted effort to exploit better the asteroseismological potential of the pulsating sdB star Feige 48, we present an improved spectroscopic analysis of that star based on new grids of NLTE, fully line-blanketed model atmospheres. To that end, we gathered four high S/N time-averaged optical spectra of varying spectral resolution from 1.0 Å to 8.7 Å, and we made use of the results of four independent studies to fix the abundances of the most important metals in the atmosphere of Feige 48. The mean atmospheric parameters we obtained from our four spectra of Feige 48 are : Teff= 29,850 $\pm$ 60 K, log $g$ = 5.46 $\pm$ 0.01, and log N(He)/N(H) = $-$2.88 $\pm$ 0.02. We also modeled for the first time the He II line at 1640 Å from the STIS archive spectrum of the star and we found with this line an effective temperature and a surface gravity that match well the values obtained with the optical data. With some fine tuning of the abundances of the metals visible in the optical domain we were able to achieve a very good agreement between our best available spectrum and our best-fitting synthetic one. Our derived atmospheric parameters for Feige 48 are in rather good agreement with previous estimates based on less sophisticated models. This underlines the relatively small effects of the NLTE approach combined with line blanketing in the atmosphere of this particular star, implying that the current estimates of the atmospheric parameters of Feige 48 are reliable and secure.

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arXiv2013

The orbits of subdwarf-B + main-sequence binaries. II. Three eccentric systems; BD+29 3070, BD +34 1543 and Feige 87

The predicted orbital-period distribution of the subdwarf-B (sdB) population is bi-modal with a peak at short (< 10 days) and long (> 250 days) periods. Observationally, many short-period sdB systems are known, but the predicted long period peak is missing as orbits have only been determined for a few long-period systems. As these predictions are based on poorly understood binary-interaction processes, it is of prime importance to confront the predictions with reliable observational data. We therefore initiated a monitoring program to find and characterize long-period sdB stars. In this paper we aim to determine the orbital parameters of the three long-period sdB+MS binaries BD+29 3070, BD+34 1543 and Feige 87, to constrain their absolute dimensions and the physical parameters of the components. High-resolution spectroscopic time series were obtained with HERMES at the Mercator telescope on La Palma, and analyzed to determine the radial velocities of both the sdB and MS components. Photometry from the literature was used to construct the spectral-energy distribution (SED) of the binaries. Atmosphere models were used to fit these SEDs and to determine the surface gravities and temperatures of both components of all systems. Spectral analysis was used to check the results of the SEDs. An orbital period of 1283 +- 63 d, a mass ratio of q = 0.39 +- 0.04 and a significant non-zero eccentricity of e = 0.15 +- 0.01 were found for BD+29 3070. For BD+34 1543 we determined P = 972 +- 2 d, q = 0.57 +- 0.01 and again a clear non-zero eccentricity of e = 0.16 +- 0.01. Last, for Feige 87 we found P = 936 +- 2 d, q = 0.55 +- 0.01 and e = 0.11 +- 0.01. BD+29 3070, BD+34 1543 and Feige 87 are long period sdB + MS binaries on clearly eccentric orbits. These results are in conflict with the predictions of stable Roche-lobe overflow models.

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arXiv2013

Photometric Survey to Search for Field sdO Pulsators

We present the results of a campaign to search for subdwarf O (sdO) star pulsators among bright field stars. The motivation for this project is the recent discovery by Randall et al. (2011), of four rapidly pulsating sdO stars in the globular cluster Omega Cen, with Teff near 50,000 K, 5.4 < log g < 6.0, and hydrogen-rich atmospheres. The only previously known sdO pulsator is significantly hotter at 68,500 K and log g = 6.1. All of the sdO pulsators identified so far are fainter than 17.4 in the V band and, thus, are poor candidates for an in-depth follow-up with asteroseismology. We therefore obtained high S/N light curves and spectroscopy for a number of field sdO stars to attempt to discover bright counterparts to these stars, particularly the Omega Cen pulsators. Our primary sample consisted of 19 sdO stars with hydrogen-rich atmospheres, log N(He)/N(H) < -1.0, effective temperatures in the range 40,000 K < Teff < 67,000 K, and surface gravities 5.3 < log g < 6.1. We also observed 17 additional helium-rich sdO stars with log N(He)/N(H) > -0.1 and similar temperatures and gravities. To date, we have found no detectable pulsations at amplitudes above 0.08% (4 times the mean noise level) in any of the 36 field sdO stars that we observed. The presence of pulsations in Omega Cen sdO stars and their apparent absence in seemingly comparable field sdO stars is perplexing. While very suggestive, the significance of this result is difficult to assess more completely right now due to remaining uncertainties about the temperature width and purity of the Omega Cen instability strip and the existence of any sdO pulsators with weaker amplitudes than the current detection limit in globular clusters.

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arXiv2013

Updates on the Pulsating SdB Star Feige 48 through Spectroscopy

As part of a concentrated effort to understand and exploit better the seismic properties of Feige 48, we reestimated its atmospheric parameters using, for the first time, a self-consistent approach based on the detailed metal abundances that have been determined in previous studies of both its UV and optical spectrum. We computed a small 3D grid of 150 fully line-blanketed, NLTE model atmospheres incorporating the 8 most abundant metals found in the atmosphere of that star. We next compared quantitatively the synthetic spectra that we calculated from these models with three observed spectra having different resolutions (1 A, 6 A, and 8.7 A) and wavelength ranges. Our findings are Teff = 29,890 K, log g = 5.46, and log N(He)/N(H) = -2.88, with very small formal fitting errors. These estimates are in good agreement with those of previous studies, thus showing that the presence of metals do not affect significantly the bulk atmospheric properties of Feige 48. We also modeled the He II line at 1640 A from the STIS spectrum of the star and we found with this line an effective temperature and a surface gravity that match well the values obtained with the optical data. We also present a radial velocity curve based on several years of observing Feige 48 at the MMT. This curve indicates a precise orbital period of 8.25 h. Coupled to our discovery of a weak but very real periodicity of 8.25 h in the light curve of Feige 48, that was gathered during a recent extensive photometric campaign at the Mount Bigelow 1.55 m Kuiper Telescope, we conclude that there is a reflection effect in the system, and that the unseen companion is a low mass MS star and not a white dwarf as has been previously suggested.

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arXiv2013

Observational Asteroseismology of Hot Subdwarf Stars with the Mont4K/Kuiper Combination at the Steward Observatory Mount Bigelow Station

In the last few years, we have carried out several extensive observational campaigns on pulsating hot subdwarf stars using the Mont4K CCD camera attached to the 1.55 m Kuiper Telescope on Mount Bigelow. The Mont4K is a joint partnership between the University of Arizona and Universite de Montreal. It was designed and built at Steward Observatory. Using the Mont4K/Kuiper combination, we have so far, and among others, gathered high-sensitivity broadband light curves for PG 1219+534, PB8783, HS 0702+6043, and Feige 48. We report very briefly on some of the most interesting observational results that came out of these campaigns.

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arXiv2012

The orbits of subdwarf B + main-sequence binaries. I: The sdB+G0 system PG 1104+243

The predicted orbital period histogram of an sdB population is bimodal with a peak at short (< 10 days) and long (> 250 days) periods. Observationally, there are many short-period sdB systems known, but only very few long-period sdB binaries are identified. As these predictions are based on poorly understood binary interaction processes, it is of prime importance to confront the predictions to observational data. In this contribution we aim to determine the absolute dimensions of the long-period sdB+MS binary system PG1104+243. High-resolution spectroscopy time-series were obtained with HERMES at the Mercator telescope at La Palma, and analyzed to obtain radial velocities of both components. Photometry from the literature was used to construct the spectral energy distribution (SED) of the binary. Atmosphere models were used to fit this SED and determine the surface gravity and temperature of both components. The gravitational redshift provided an independent confirmation of the surface gravity of the sdB component. An orbital period of 753 +- 3 d and a mass ratio of q = 0.637 +- 0.015 were found from the RV-curves. The sdB component has an effective temperature of Teff = 33500 +- 1200 K and a surface gravity of logg = 5.84 +- 0.08 dex, while the cool companion is found to be a G-type star with Teff = 5930 +- 160 K and logg = 4.29 +- 0.05 dex. Assuming a canonical mass of Msdb = 0.47 Msun, the MS component has a mass of 0.74 +- 0.07 Msun, and its Teff corresponds to what is expected for a terminal age main-sequence star with sub-solar metalicity. PG1104+243 is the first long-period sdB binary in which accurate physical parameters of both components could be determined, and the first sdB binary in which the gravitational redshift is measured. Furthermore, PG1104+243 is the first sdB+MS system that shows consistent evidence for being formed through stable Roche-lobe overflow.

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