arXiv subjects

R. H. Østensen

Publications and source records attributed to R. H. Østensen.

At least 19 recordsLinked to original sources

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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arXiv2024

Short-period pulsating hot-subdwarf stars observed by TESS II. Northern ecliptic hemisphere

We present results of a continuation of our Transiting Exoplanet Survey Satellite (TESS) search for short-period pulsations in compact stellar objects observed during Years 2 and 4 of the TESS mission that targeted the northern ecliptic hemisphere. For many of the targets, we exploit unpublished spectroscopic data to confirm or determine the object's spectral classification. From the TESS photometry, we identify 50 short-period hot-subdwarf pulsators, including 35 sdB and 15 sdOB stars. The sample contains 26 pulsators not known before the TESS mission. Nine stars show signals at both low and high frequencies, and are therefore ``hybrid'' pulsators. For each pulsator, we report the list of prewhitened frequencies and we show amplitude spectra calculated from the TESS data. We attempt to identify possible multiplets caused by stellar rotation, and we report five candidates with rotation periods between 11 and 46d. Having the search for p-mode pulsating hot subdwarfs in TESS Sectors 1 - 60 done, we discuss the completeness of the study, as well as instability strip and the evolutionary status of the stars we found. We also compare the distribution of pulsation periods as a function of effective temperature and surface gravity with theoretical predictions.

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arXiv2021

Space observations of AA Doradus provide consistent mass determinations. New HW-Vir systems observed with TESS

We present an overview of eclipsing systems of the HW-Virginis type, based on space observations from the TESS Mission. We perform a detailed analysis of the properties of AA Dor, which was monitored for almost a full year. This excellent time-series dataset permitted us to search for both stellar pulsations and eclipse timing variations. In addition, we used the high-precision trigonometric parallax from Gaia Early Data Release 3 to make an independent determination of the fundamental stellar parameters. No convincing pulsations were detected down to a limit of 76 parts per million, however we detected one peak with false alarm probability of 0.2%. 20 sec cadences being collected during Year 3 should confirm or reject our detection. From eclipse timing measurements we were able to confirm that the orbital period is stable, with an upper limit to any period change of 5.75 $\cdot$ 10$^{-13}$ s/s. The apparent offset of the secondary eclipse is consistent with the predicted Rømer delay when the primary mass is that of a canonical extended horizontal branch star. Using parallax and a spectral energy distribution corroborates that the mass of the primary in AA Dor is canonical, and its radius and luminosity is consistent with an evolutionary state beyond core helium burning. The mass of the secondary is found to be at the limit of hydrogen burning.

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arXiv2021

A transit survey to search for planets around hot subdwarfs: I. methods and performance tests on light curves from Kepler, K2, TESS, and CHEOPS

Context. Hot subdwarfs experienced strong mass loss on the Red Giant Branch (RGB) and are now hot and small He-burning objects. Aims. In this project we aim to perform a transit survey in all available light curves of hot subdwarfs from space-based telescopes (Kepler, K2, TESS, and CHEOPS), with our custom-made pipeline SHERLOCK, in order to determine the occurrence rate of planets around these stars, as a function of orbital period and planetary radius. Methods. In this first paper, we perform injection-and-recovery tests of synthetic transits for a selection of representative Kepler, K2 and TESS light curves, to determine which transiting bodies, in terms of object radius and orbital period, we will be able to detect with our tools. We also provide such estimates for CHEOPS data, which we analyze with the pycheops package. Results. Transiting objects with a radius $\lesssim$ 1.0 $R_{\Earth}$ can be detected in most of Kepler, K2 and CHEOPS targets for the shortest orbital periods (1 d and below), reaching values as small as $\sim$0.3 $R_{\Earth}$ in the best cases. Reaching sub-Earth-sized bodies is achieved only for the brightest TESS targets, and the ones observed during a significant number of sectors. We also give a series of representative results for farther and bigger planets, for which the performances strongly depend on the target magnitude, the length and the quality of the data. Conclusions. The TESS sample will provide the most important statistics for the global aim of measuring the planet occurrence rate around hot subdwarfs. The Kepler, K2 and CHEOPS data will allow us to search for planetary remnants, i.e. very close and small (possibly disintegrating) objects, which would have partly survived the engulfment in their red giant host.

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arXiv2020

EPIC 216747137: a new HW Vir eclipsing binary with a massive sdOB primary and a low-mass M-dwarf companion

EPIC 216747137 is a new HW~Virginis system discovered by the Kepler spacecraft during its K2 "second life". Like the other HW Vir systems, EPIC 216747137 is a post-common-envelope eclipsing binary consisting of a hot subluminous star and a cool low-mass companion. The short orbital period of 3.87 hours produces a strong reflection effect from the secondary (~9% in the R band). Together with AA Dor and V1828 Aql, EPIC 216747137 belongs to a small subgroup of HW Vir systems with a hot evolved sdOB primary. We find the following atmospheric parameters for the hot component: Teff=40400$\pm$1000 K, logg=5.56$\pm$0.06, log(N(He)/N(H))=$-$2.59$\pm$0.05. The sdOB rotational velocity vsini=51$\pm$10 km/s implies that the stellar rotation is slower than the orbital revolution and the system is not synchronized. When we combine photometric and spectroscopic results with the Gaia parallax, the best solution for the system corresponds to a primary with a mass of about 0.62 Msun close to, and likely beyond, the central helium exhaustion, while the cool M-dwarf companion has a mass of about 0.11 Msun.

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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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arXiv2018

KIC 10001893: A pulsating sdB star with multiple trapped modes

KIC 10001893 is a V1093 Her type pulsating subdwarf-B star, which was observed extensively by the Kepler spacecraft. It was a part of the survey searching for compact pulsators in the Kepler field of view. An amplitude spectrum of the star demonstrates a rich content of g-modes between 102 and 496 μHz as well as a few p-modes above 2000 μHz. In total, we found 110 frequencies. The g-mode region contains 104 frequencies, while the p-mode region contains just six, altogether indicating the hybrid nature of KIC 10001893. The main goal of our analysis was to identify the detected modes and to find some features, which will significantly help modeling of the star. We found no multiplets, which points at a pole-on orientation, however, we defined modal degrees and relative radial orders using asymptotic period spacing. Eventually, we assigned 32 dipole l = 1 and 18 quadrupole l = 2 modes. The most remarkable feature we found are trapped modes, which are clearly seen in a reduce period diagram. It is the first time that three trapped modes are detected in one pulsating sdB star. Since the more trapped modes we find, the better sounding of the stellar interior we can do, this feature provides important constraints on the physical structure of the star. Mode trapping is likely caused by the He-H transition region and therefore it provides crucial constraints for making realistic theoretical models of hot subdwarfs.

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arXiv2018

Unravelling the baffling mystery of the ultrahot wind phenomenon in white dwarfs

The presence of ultra-high excitation (UHE) absorption lines (e.g., O VIII) in the optical spectra of several of the hottest white dwarfs poses a decades-long mystery and is something that has never been observed in any other astrophysical object. The occurrence of such features requires a dense environment with temperatures near $10^6$K, by far exceeding the stellar effective temperature. Here we report the discovery of a new hot wind white dwarf, GALEXJ014636.8+323615. Astonishingly, we found for the first time rapid changes of the equivalent widths of the UHE features, which are correlated to the rotational period of the star ($P=0.242035$d). We explain this with the presence of a wind-fed circumstellar magnetosphere in which magnetically confined wind shocks heat up the material to the high temperatures required for the creation of the UHE lines. The photometric and spectroscopic variability of GALEXJ014636.8+323615 can then be understood as consequence of the obliquity of the magnetic axis with respect to the rotation axis of the white dwarf. This is the first time a wind-fed circumstellar magnetosphere around an apparently isolated white dwarf has been discovered and finally offers a plausible explanation of the ultra hot wind phenomenon.

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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

Discovery of a variable lead-rich hot subdwarf: UVO 0825+15

UVO0825+15 is a hot bright helium-rich subdwarf which lies in {\it K2} Field 5 and in a sample of intermediate helium-rich subdwarfs observed with {\it Subaru/HDS}. The {\it K2} light curve shows low-amplitude variations, whilst the {\it Subaru} spectrum shows Pb{\sc iv} absorption lines, indicative of a very high lead overabundance. UVO0825+15 also has a high proper motion with kinematics typical for a thick disk star. Analyses of ultraviolet and intermediate dispersion optical spectra rule out a short-period binary companion, and provide fundamental atmospheric parameters of $T_{\rm off}=38\,900\pm270$\,K, $\log g/{\rm cm\,s^{-2}}=5.97\pm0.11$, $\log n_{\rm He}/n_{\rm H}=-0.57\pm0.01$, $E_{B-V}\approx0.03$, and angular radius $θ= 1.062\pm0.006\times10^{-11}$ radians (formal errors). The high-resolution spectrum shows that carbon is $>2$ dex subsolar, iron is approximately solar and all other elements heavier than argon are at least 2 -- 4 dex overabundant, including germanium, yttrium and lead. Approximately 150 lines in the blue-optical spectrum remain unidentified. The chemical structure of the photosphere is presumed to be determined by radiatively-dominated diffusion. The {\it K2} light curve shows a dominant period around 10.8\,h, with a variable amplitude, its first harmonic, and another period at 13.3\,h. The preferred explanation is multi-periodic non-radial oscillation due to g-modes with very high radial order, although this presents difficulties for pulsation theory. Alternative explanations fail for lack of radial-velocity evidence. UVO0825+15 represents the fourth member of a group of hot subdwarfs having helium-enriched photospheres and 3--4 dex overabundances of trans-iron elements, and is the first lead-rich subdwarf to show evidence of pulsations.

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arXiv2016

RV variable, hot post-AGB stars from the MUCHFUSS project - Classification, atmospheric parameters, formation scenarios

In the course of the MUCHFUSS project we have recently discovered four radial velocity (RV) variable, hot (Teff $\approx$ 80,000 - 110,000 K) post-asymptotic giant branch (AGB) stars. Among them, we found the first known RV variable O(He) star, the only second known RV variable PG 1159 close binary candidate, as well as the first two naked (i.e., without planetary nebula (PN)) H-rich post-AGB stars of spectral type O(H) that show significant RV variations. We present a non-LTE spectral analysis of these stars along with one further O(H)-type star whose RV variations were found to be not significant. We also report the discovery of an far-infrared excess in the case of the PG 1159 star. None of the stars in our sample displays nebular emission lines, which can be explained well in terms of a very late thermal pulse evolution in the case of the PG 1159 star. The "missing" PNe around the O(H)-type stars seem strange, since we find that several central stars of PNe have much longer post-AGB times. Besides the non-ejection of a PN, the occurrence of a late thermal pulse, or the re-accretion of the PN in the previous post-AGB evolution offer possible explanations for those stars not harbouring a PN (anymore). In case of the O(He) star J0757 we speculate that it might have been previously part of a compact He transferring binary system. In this scenario, the mass transfer must have stopped after a certain time, leaving behind a low mass close companion that could be responsible for the extreme RV shift of 107.0 $\pm$ 22.0 km/s measured within only 31 min.

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arXiv2015

The population of white dwarf binaries with hot subdwarf companions

Hot subdwarfs (sdBs) are core helium-burning stars, which lost almost their entire hydrogen envelope in the red-giant phase. Since a high fraction of those stars are in close binary systems, common envelope ejection is an important formation channel. We identified a total population of 51 close sdB+WD binaries based on time-resolved spectroscopy and multi-band photometry, derive the WD mass distribution and constrain the future evolution of these systems. Most WDs in those binaries have masses significantly below the average mass of single WDs and a high fraction of them might therefore have helium cores. We found 12 systems that will merge in less than a Hubble time and evolve to become either massive C/O WDs, AM\,CVn systems, RCrB stars or even explode as supernovae type Ia.

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arXiv2015

Hot subdwarf binaries from the MUCHFUSS project - Analysis of 12 new systems and a study of the short-period binary population

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions like massive white dwarfs (M>1.0 M$_\odot$), neutron stars, or stellar-mass black holes. We present orbital and atmospheric parameters and put constraints on the nature of the companions of 12 close hot subdwarf B star (sdB) binaries found in the course of the MUCHFUSS project. The systems show periods between 0.14 and 7.4 days. Three systems most likely have white dwarf companions. SDSS J083006.17+475150.3 is likely to be a rare example of a low-mass helium-core white dwarf. SDSS J095101.28+034757.0 shows an excess in the infrared that probably originates from a third companion in a wide orbit. SDSS J113241.58-063652.8 is the first helium deficient sdO star with a confirmed close companion. This study brings to 142 the number of sdB binaries with orbital periods of less than 30 days and with measured mass functions. We present an analysis of the minimum companion mass distribution and show that it is bimodal. One peak around 0.1 M$_\odot$ corresponds to the low-mass main sequence and substellar companions. The other peak around 0.4 M$_\odot$ corresponds to the white dwarf companions. The derived masses for the white dwarf companions are significantly lower than the average mass for single carbon-oxygen white dwarfs. In a T$_{\rm eff}$-log(g) diagram of sdB+dM companions, we find signs that the sdB components are more massive than the rest of the sample. The full sample was compared to the known population of extremely low-mass white dwarf binaries as well as short-period white dwarfs with main sequence companions. Both samples show a significantly different companion mass distribution. We calculate merger timescales and timescales when the companion will fill its Roche Lobe and the system evolves into a cataclysmic variable.

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arXiv2014

The blue-edge problem of the V1093 Her instability strip revisited using evolutionary models with atomic diffusion

We have computed a new grid of evolutionary subdwarf B star (sdB) models from the start of central He burning, taking into account atomic diffusion due to radiative levitation, gravitational settling, concentration diffusion, and thermal diffusion. We have computed the non-adiabatic pulsation properties of the models and present the predicted p-mode and g-mode instability strips. In previous studies of the sdB instability strips, artificial abundance enhancements of Fe and Ni were introduced in the pulsation driving layers. In our models, the abundance enhancements of Fe and Ni occur naturally, eradicating the need to use artificial enhancements. We find that the abundance increases of Fe and Ni were previously underestimated and show that the instability strip predicted by our simulations solves the so-called blue edge problem of the subdwarf B star g-mode instability strip. The hottest known g-mode pulsator, KIC 10139564, now resides well within the instability strip {even when only modes with low spherical degrees (l<=2) are considered.

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arXiv2014

Asteroseismology revealing trapped modes in KIC 10553698A

The subdwarf-B pulsator, KIC 10553698A, is one of 16 such objects observed with one-minute sampling for most of the duration of the Kepler Mission. Like most of these stars, it displays a rich g-mode pulsation spectrum with several clear multiplets that maintain regular frequency splitting. We identify these pulsation modes as components of rotationally split multiplets in a star rotating with a period of about 41 d. From 162 clearly significant periodicities, we are able to identify 156 as likely components of l=1 or l=2 multiplets. For the first time we are able to detect l=1 modes that interpose in the asymptotic period sequences and that provide a clear indication of mode trapping in a stratified envelope, as predicted by theoretical models. A clear signal is also present in the Kepler photometry at 3.387 d. Spectroscopic observations reveal a radial-velocity amplitude of 64.8 km/s. We find that the radial-velocity variations and the photometric signal have phase and amplitude that are perfectly consistent with a Doppler-beaming effect and conclude that the unseen companion, KIC 10553698B, must be a white dwarf most likely with a mass close to 0.6 solar masses.

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arXiv2013

Pulsating red giant stars in eccentric binary systems discovered from Kepler space-based photometry

The unparalleled photometric data obtained by NASA's Kepler space telescope led to an improved understanding of red giant stars and binary stars. Seismology allows us to constrain the properties of red giants. In addition to eclipsing binaries, eccentric non-eclipsing binaries, exhibiting ellipsoidal modulations, have been detected with Kepler. We aim to study the properties of eccentric binary systems containing a red giant star and derive the parameters of the primary giant component. We apply asteroseismic techniques to determine masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques are applied to extract the parameters of the system. The effects of stellar on the binary system are studied. The paper presents the asteroseismic analysis of 18 pulsating red giants in eccentric binary systems, for which masses and radii were constrained. The orbital periods of these systems range from 20 to 440days. From radial velocity measurements we find eccentricities between e=0.2 to 0.76. As a case study we present a detailed analysis of KIC5006817. From seismology we constrain the rotational period of the envelope to be at least 165 d, roughly twice the orbital period. The stellar core rotates 13 times faster than the surface. From the spectrum and radial velocities we expect that the Doppler beaming signal should have a maximum amplitude of 300ppm in the light curve. Through binary modelling, we determine the mass of the secondary component to be 0.29$\pm$0.03\,$M_\odot$. For KIC5006817 we exclude pseudo-synchronous rotation of the red giant with the orbit. The comparison of the results from seismology and modelling of the light curve shows a possible alignment of the rotational and orbital axis at the 2$σ$ level. Red giant eccentric systems could be progenitors of cataclysmic variables and hot subdwarf B stars.

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arXiv2013

Binaries discovered by the MUCHFUSS project; FBS 0117+396: An sdB+dM binary with a pulsating primary

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (muchfuss) aims to discover subdwarf-B stars with massive compact companions such as overmassive white dwarfs (M > 1.0 M_sun), neutron stars or black holes. From the 127 subdwarfs with substantial radial-velocity variations discovered in the initial survey, a number of interesting objects have been selected for extensive follow-up. After an initial photometry run with BUSCA revealed that FBS 0117+396 is photometrically variable both on long and short timescales, we chose it as an auxiliary target during a 6-night multi-color photometry run with Ultracam. Spectroscopy was obtained at a number of observatories in order to determine the binary period and obtain a radial-velocity amplitude. After establishing an orbital period of P = 0.252 d, and removing the signal associated with the irradiated hemisphere of the M-dwarf companion, we were able to detect ten pulsation periods in the Fourier spectrum of the light curve. Two pulsation modes are found to have short periods of 337 and 379 s, and at least eight modes are found with periods between 45 minutes and 2.5 hours. This establishes that FBS 0117+396 is an sdB+dM reflection binary, in which the primary is a hybrid pulsator, and the first one found with this particular melange of flavours.

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