arXiv subjects

K. J. Bell

Publications and source records attributed to K. J. Bell.

9 recordsLinked to original sources

arXiv2020

Mode identification in three pulsating hot subdwarfs observed with TESS satellite

We report on the detection of pulsations of three pulsating subdwarf B stars observed by the TESS satellite and our results of mode identification in these stars based on an asymptotic period relation. SB 459 (TIC 067584818), SB 815 (TIC 169285097) and PG 0342+026 (TIC 457168745) have been monitored during single sectors resulting in 27 days coverage. These datasets allowed for detecting, in each star, a few tens of frequencies, which we interpreted as stellar oscillations. We found no multiplets, though we partially constrained mode geometry by means of period spacing, which recently became a key tool in analyses of pulsating subdwarf B stars. Standard routine that we have used allowed us to select candidates for trapped modes that surely bear signatures of non-uniform chemical profile inside the stars. We have also done statistical analysis using collected spectroscopic and asteroseismic data of previously known subdwarf B stars along with our three stars. Making use of high precision trigonometric parallaxes from the Gaia mission and spectral energy distributions we converted atmospheric parameters to stellar ones. Radii, masses and luminosities are close to their canonical values for extreme horizontal branch stars. In particular, the stellar masses are close to the canonical one of 0.47 M$_\odot$ for all three stars but uncertainties on the mass are large. The results of the analyses presented here will provide important constrains for asteroseismic modelling.

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arXiv2020

A White Dwarf with Transiting Circumstellar Material Far Outside the Roche Limit

We report the discovery of a white dwarf exhibiting deep, irregularly shaped transits, indicative of circumstellar planetary debris. Using Zwicky Transient Facility DR2 photometry of ZTF$\,$J013906.17+524536.89 and follow-up observations from the Las Cumbres Observatory, we identify multiple transit events that recur every ${\approx}\,107.2\,$d, much longer than the $4.5{-}4.9\,$h orbital periods observed in WD$\,$1145+017, the only other white dwarf known with transiting planetary debris. The transits vary in both depth and duration, lasting $15{-}25\,$d and reaching $20{-}45\,\%$ dips in flux. Optical spectra reveal strong Balmer lines, identifying the white dwarf as a DA with $T_{\mathrm{eff}}=10{,}530\pm140\,\mathrm{K}$ and $\log(g)=7.86\pm0.06$. A $\mathrm{Ca\,II\,K}$ absorption feature is present in all spectra both in and out of transit. Spectra obtained during one night at roughly $15\,\%$ transit depth show increased $\mathrm{Ca\,II\,K}$ absorption with a model atmospheric fit suggesting $[\mathrm{Ca/H}]=-4.6\pm0.3$, whereas spectra taken on three nights out of transit have $[\mathrm{Ca/H}]$ of -5.5, -5.3, and -4.9 with similar uncertainties. While the $\mathrm{Ca\,II\,K}$ line strength varies by only 2-sigma, we consider a predominantly interstellar origin for Ca absorption unlikely. We suggest a larger column density of circumstellar metallic gas along the line of site or increased accretion of material onto the white dwarf's surface are responsible for the Ca absorption, but further spectroscopic studies are required. In addition, high-speed time series photometry out of transit reveals variability with periods of 900 and 1030$\,$s, consistent with ZZ Ceti pulsations.

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arXiv2020

TESS first look at evolved compact pulsators: Known ZZ Ceti stars of the southern ecliptic hemisphere as seen by TESS

Context. We present our findings on 18 formerly known ZZ Ceti stars observed by the TESS space telescope in 120s cadence mode during the survey observation of the southern ecliptic hemisphere. Aims. We focus on the frequency analysis of the space-based observations, comparing the results with the findings of the previous ground-based measurements. The frequencies detected by the TESS observations can serve as inputs for future asteroseismic analyses. Methods. We performed standard pre-whitening of the data sets to derive the possible pulsation frequencies of the different targets. In some cases, we fitted Lorentzians to the frequency groups that emerged as the results of short-term amplitude/phase variations that occurred during the TESS observations. Results. We detected more than 40 pulsation frequencies in seven ZZ Ceti stars observed in the 120s cadence by TESS, with better than 0.1 microHz precision. We found that HE 0532-5605 may be a new outbursting ZZ Ceti. Ten targets do not show any significant pulsation frequencies in their Fourier transforms, due to a combination of their intrinsic faintness and/or crowding on the large TESS pixels. We also detected possible amplitude/phase variations during the TESS observations in some cases. Such behaviour in these targets was not previously identified from ground-based observations.

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arXiv2020

Limits on Mode Coherence in Pulsating DA White Dwarfs Due to a Non-static Convection Zone

The standard theory of pulsations deals with the frequencies and growth rates of infinitesimal perturbations in a stellar model. Modes which are calculated to be linearly driven should increase their amplitudes exponentially with time; the fact that nearly constant amplitudes are usually observed is evidence that nonlinear mechanisms inhibit the growth of finite amplitude pulsations. Models predict that the mass of convection zones in pulsating hydrogen-atmosphere (DAV) white dwarfs is very sensitive to temperature (i.e., $M_{\rm CZ} \propto T_{\rm eff}^{-90}$), leading to the possibility that even low-amplitude pulsators may experience significant nonlinear effects. In particular, the outer turning point of finite-amplitude g-mode pulsations can vary with the local surface temperature, producing a reflected wave that is out of phase with what is required for a standing wave. This can lead to a lack of coherence of the mode and a reduction in its global amplitude. In this paper we show that: (1) whether a mode is calculated to propagate to the base of the convection zone is an accurate predictor of its width in the Fourier spectrum, (2) the phase shifts produced by reflection from the outer turning point are large enough to produce significant damping, and (3) amplitudes and periods are predicted to increase from the blue edge to the middle of the instability strip, and subsequently decrease as the red edge is approached. This amplitude decrease is in agreement with the observational data while the period decrease has not yet been systematically studied.

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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

Transition from spot to faculae domination -- An alternate explanation for the dearth of intermediate \textit{Kepler} rotation periods

The study of stellar activity cycles is crucial to understand the underlying dynamo and how it causes activity signatures such as dark spots and bright faculae. We study the appearance of activity signatures in contemporaneous photometric and chromospheric time series. Lomb-Scargle periodograms are used to search for cycle periods present in both time series. To emphasize the signature of the activity cycle we account for rotation-induced scatter in both data sets by fitting a quasi-periodic Gaussian process model to each observing season. After subtracting the rotational variability, cycle amplitudes and the phase difference between the two time series are obtained by fitting both time series simultaneously using the same cycle period. We find cycle periods in 27 of the 30 stars in our sample. The phase difference between the two time series reveals that the variability in fast rotating active stars is usually in anti-phase, while the variability of slowly rotating inactive stars is in phase. The photometric cycle amplitudes are on average six times larger for the active stars. The phase and amplitude information demonstrates that active stars are dominated by dark spots, whereas less active stars are dominated by bright faculae. We find the transition from spot to faculae domination at the Vaughan-Preston gap, and around a Rossby number equal to one. We conclude that faculae are the dominant ingredient of stellar activity cycles at ages >2.55 Gyr. The data further suggest that the Vaughan-Preston gap can not explain the previously detected dearth of Kepler rotation periods between 15-25 days. Nevertheless, our results led us to propose an explanation for the rotation period dearth to be due to the non-detection of periodicity caused by the cancellation of dark spots and bright faculae at 800 Myr.

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arXiv2018

Constraining Low-Mass White Dwarf Binaries from Ellipsoidal Variations

Stars are stretched by tidal interactions in tight binaries, and changes to their projected areas introduce photometric variations twice per orbit. Hermes et al. (2014, ApJ, 792, 39) utilized measurements of these ellipsoidal variations to constrain the radii of low-mass white dwarfs in eight single-lined spectroscopic binaries. We refine this method here, using Monte Carlo simulations to improve constraints on many orbital and stellar properties of binary systems that exhibit ellipsoidal variations. We analyze the recently discovered tidally distorted white dwarf binary system SDSS J1054-2121 in detail, and also revisit the Hermes et al. (2014) sample. Disagreements in some cases between the observations, ellipsoidal variation model, and Gaia radius constraints suggest that extrinsic errors are present, likely in the surface gravities determined through model atmosphere fits to stellar spectra.

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arXiv2017

Two white dwarfs in ultrashort binaries with detached, eclipsing, likely substellar companions detected by K2

Using data from the extended Kepler mission in K2 Campaign 10 we identify two eclipsing binaries containing white dwarfs with cool companions that have extremely short orbital periods of only 71.2 min (SDSS J1205-0242, a.k.a. EPIC 201283111) and 72.5 min (SDSS J1231+0041, a.k.a. EPIC 248368963). Despite their short periods, both systems are detached with small, low-mass companions, in one case a brown dwarf, and the other case either a brown dwarf or a low-mass star. We present follow-up photometry and spectroscopy of both binaries, as well as phase-resolved spectroscopy of the brighter system, and use these data to place preliminary estimates on the physical and binary parameters. SDSS J1205-0242 is composed of a $0.39\pm0.02$M$_\odot$ helium-core white dwarf which is totally eclipsed by a $0.049\pm0.006$M$_\odot$ ($51\pm6$M$_J$) brown dwarf companion, while SDSS J1231+0041 is composed of a $0.56\pm0.07$M$_\odot$ white dwarf which is partially eclipsed by a companion of mass $\lesssim 0.095$M$_\odot$. In the case of SDSS J1205-0242 we look at the combined constraints from common-envelope evolution and brown dwarf models; the system is compatible with similar constraints from other post common-envelope binaries given the current parameter uncertainties, but has potential for future refinement.

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arXiv2016

SDSS J1152+0248: An eclipsing double white dwarf from the Kepler K2 campaign

We report the discovery of the sixth known eclipsing double white dwarf (WD) system, SDSS J1152+0248, with a 2.3968 +/- 0.0003 h orbital period, in data from the Kepler Mission's K2 continuation. Analysing and modelling the K2 data together with ground-based fast photometry, spectroscopy, and radial-velocity measurements, we determine that the primary is a DA-type WD with mass M1 = 0.47 +/- 0.11 Msun, radius R1 = 0.0197 +/- 0.0035 Rsun, and cooling age t1 = 52 +/- 36 Myr. No lines are detected, to within our sensitivity, from the secondary WD, but it is likely also of type DA. Its central surface brightness, as measured from the secondary eclipse, is 0.31 of the primary's surface brightness. Its mass, radius, and cooling age, respectively, are M2 = 0.44 +/- 0.09 Msun, R2 = 0.0223 +0.0064 -0.0050 Rsun, and t2 = 230 +/- 100 Myr. SDSS J1152+0248 is a near twin of the double-lined eclipsing WD system CSS 41177.

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