A Survey for Pulsating Hot B Subdwarfs in the Northern Hemisphere
TL;DR: In this paper, the authors present the final results of a survey to search for pulsating hot B subdwarf (sdB) stars, which yielded a yield of 5.4%.
Abstract: We present the final results of a survey to search for pulsating hot B subdwarf (sdB) stars. We picked our candidates on the basis of their values of Teff and log g, as estimated from available quantitative optical spectroscopy. With the help of a three-channel photometer, we gathered "white-light" light curves using a uniform sampling time of 10 s for a total of 74 stars. Our survey had a yield of ~5.4%, leading to the discovery of detectable luminosity variations in four objects: PG 1047+003, KPD 2109+4401, KPD 1930+2752, and PG 0014+067. For the light curves of the other 70 stars, we derive upper limits on the amplitudes of possible periodic signals (in the period window 20-1000 s) that range from ~0.05% of the mean brightness of the star at an apparent magnitude of y ~ 12 to ~0.20% at y ~ 15. We use our survey results as well as those of others to discuss the location and extent of the empirical instability "strip" for pulsating sdB stars in the log g-Teff diagram. We also address the question of the coexistence of pulsators and nonvariable stars in the same region of that diagram.
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TL;DR: In this article, the authors discovered a new class of low-amplitude, multimode sdB pulsators with periods of the order of an hour, implying that they are due to gravity modes rather than pressure modes.
Abstract: During the course of an ongoing CCD monitoring program to investigate low-level light variations in subdwarf B (sdB) stars, we have serendipitously discovered a new class of low-amplitude, multimode sdB pulsators with periods of the order of an hour. These periods are more than a factor of 10 longer than those of previously known multimode sdB pulsators (EC 14026 stars), implying that they are due to gravity modes rather than pressure modes. The longer period pulsators are found only among cooler sdB stars, where they are surprisingly common. The iron opacity instability that drives the short-period EC 14026 stars is effective only in hot sdB stars, leaving the driving mechanism for the deeper gravity modes in cool sdB stars currently unknown. We present the first observational results for our newly identified sdB variables and discuss possible implications.
242 citations
TL;DR: In this article, the authors present the results of a stability survey carried out for a sequence of representative models of subdwarf B stars spanning the range of effective temperature 22; 000 KTeff � 38;000 K.
Abstract: We present the results of a stability survey carried out for a sequence of representative models of subdwarf B stars spanning the range of effective temperature 22; 000 KTeff � 38; 000 K. We show that long-period, high-order g-modes are excited in the cooler models through the same � -mechanism that successfully explains the presence of short-period, low-order p-modes in the hotter EC 14026 pulsators. This is analogous to the case of theCep/slowly pulsating B stars on the main sequence. We stress that radiative levitation is needed to boost the iron abundance in the driving region for both types of pulsating subdwarf B stars. And indeed, we find that pulsation modes cannot be excited in B subdwarf models if the metallicity is assumed to be uniform and solar. On the basis of our current models, we propose that the pulsation modes detected in long- period pulsating subdwarf B stars have values of the degree index l ¼ 3 and/or 4, not the canonical values l ¼ 1; 2, a suggestion that is, in principle, testable through multicolor photometry or time-resolved spectros- copy. In this way, we are able to explain quite well, at least at the qualitative level, the main observed characteristics of these pulsators. On the first account, the excited high-order g-modes with l ¼ 3 and 4 in our models have periods that overlap with the range of quasi-periods observed in these stars. On the second account, if the observable modes in these pulsators have indeed such '' high '' values of l as we suggest, we find a natural explanation for the fact that their amplitudes are distinctly and systematically smaller than the amplitudes observed in EC 14026 stars. Finally, our results are also consistent with the observed fact that the long-period pulsators appear systematically cooler than the short-period EC 14026 stars. We point out, however, that our analysis suggests effective temperatures for the long-period B subdwarf pulsators that are somewhat lower than current spectroscopic estimates. The solution to this problem may come from future improvements in the models, the establishment of an effective temperature scale for subdwarf B stars that is free of systematic effects, or both. Subject headings: stars: horizontal-branch — stars: interiors — stars: oscillations — subdwarfs
160 citations
Additional excerpts
...5 of Billères et al. 2002)....
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TL;DR: In this paper, a model atmosphere analysis of the time averaged optical spectrum of PG 1219+534 obtained at the new Multiple Mirror Telescope (MMT) leads to estimates of Teff = 33 600 ± 370 K and log g = 5.810 ± 0.046.
Abstract: Over the last several years, we have embarked on a long term effort to exploit the strong potential that hot B subdwarf (sdB) pulsators have to offer in terms of asteroseismology. This effort is multifaceted as it involves, on the observational front, the acquisition of high sensitivity photometric data supplemented by accurate spectroscopic measurements, and, on the theoret- ical and modeling fronts, the development of appropriate numerical tools dedicated to the asteroseismological interpretation of the seismic observations. In this paper, we report on the observations and thorough analysis of the rapidly pulsating sdB star (or EC 14026 star) PG 1219+534. Our model atmosphere analysis of the time averaged optical spectrum of PG 1219+534 obtained at the new Multiple Mirror Telescope (MMT) leads to estimates of Teff = 33 600 ± 370 K and log g = 5.810 ± 0.046 (with log N(He)/N(H) = −1.49 ± 0.08), in good agreement with previous spectroscopic measurements of its atmospheric parameters. This places PG 1219+534 right in the middle of the EC 14026 instability region in the log g − Teff plane. A standard Fourier analysis of our high signal-to-noise ratio Canada-France-Hawaii Telescope (CFHT) light curves reveals the presence of nine distinct harmonic oscillations with periods in the range 122−172 s, a significant improvement over the original detection of only four periods by Koen et al. (1999, MNRAS, 305, 28). On this basis, we have carried out a detailed asteroseismic analysis of PG 1219+534 using the well-known forward method and assuming that the observed modes have � ≤ 3. Our analysis leads ob- jectively to the identification of the (k, � ) indices of the nine periods observed in the star PG 1219+534, and to the determination of its structural parameters. The periods all correspond to low-order acoustic modes with adjacent values of k and with � = 0, 1, 2, and 3. They define a band of unstable modes, in close agreement with nonadiabatic pulsation theory. Furthermore, the average dispersion between the nine observed periods and the periods of the corresponding nine theoretical modes of the optimal model is only ∼0.6%, comparable to the results of a similar analysis carried out by Brassard et al. (2001) on the rapid sdB pulsator PG 0014+067. On the basis of our combined spectroscopic and asteroseismic analysis, the inferred global structural parameters of PG 1219+534 are Teff = 33 600 ± 370 K, log g = 5.8071 ± 0.0057, log Menv/M∗ = −4.254 ± 0.147, M∗ = 0.457 ± 0.012 M� , R/R� = 0.1397 ± 0.0028, and L/L� = 22.01 ± 1.85. Combined with detailed model atmosphere calculations, we estimate, in addition, that this star has an absolute visual magnitude MV = 4.62 ± 0.06 and is located at a distance d = 531 ± 23 pc (using V = 13.24 ± 0.20). Finally, if we interpret the absence of fine structure (frequency multiplets) as indicative of a slow rotation rate of that star, we further find that PG 1219+534 rotates with a period longer than 3.4 days, and has a maximum rotational broadening velocity of V sin i < 2. 1k m s −1 .
93 citations
Cites background from "A Survey for Pulsating Hot B Subdwa..."
...Various surveys by the SAAO group, by the Montréal group (see Billères et al. 2002), by a Norwegian-German-Italian team (see, e.g., Silvotti et al. 2002), and by other teams (see the reviews of Charpinet 2001 and Kilkenny 2002) rapidly led to additional discoveries, bringing the total of known EC…...
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TL;DR: In this article, a search for pulsating subdwarf B stars was conducted with the Nordic Optical Telescope on La Palma over 59 nights between 1999 and 2009, and twenty new short period pulsators were found.
Abstract: Context. A search programme for pulsating subdwarf B stars was conducted with the Nordic Optical Telescope on La Palma over 59 nights between 1999 and 2009. Aims. The purpose of the programme was to significantly extend the number of rapidly pulsating sdB stars to better understand the properties of this new group of variable compact stars. Methods. Candidates were selected initially from the HS and HE surveys, but were supplemented with additional objects from other surveys. Short sequences of time-series photometry were made on the candidates to determine the presence of rapid pulsations. Results. In total twenty new pulsators were found in this survey, most of which have already been published and some extensively studied. We present four new short period pulsators, bringing the total of such pulsators up to 49. We also give limits on pulsation amplitudes for 285 objects with no obvious periodic variations, summarise the results of the survey, and provide improved physical parameters on the composite pulsators for which only preliminary estimates were published earlier.
80 citations
TL;DR: In this article, a search for pulsating subdwarf B stars was conducted with the Nordic Optical Telescope on La Palma over 59 nights between 1999 and 2009, and twenty new pulsators were found, most of which have already been published and some extensively studied.
Abstract: A search programme for pulsating subdwarf B stars was conducted with the Nordic Optical Telescope on La Palma over 59 nights between 1999 and 2009. The purpose of the programme was to significantly extend the number of rapidly pulsating sdB stars to better understand the properties of this new group of variable compact stars. Candidates were selected initially from the HS and HE surveys, but were supplemented with additional objects from other surveys. Short sequences of time-series photometry were made on the candidates to determine the presence of rapid pulsations. In total twenty new pulsators were found in this survey, most of which have already been published and some extensively studied. We present four new short period pulsators, bringing the total of such pulsators up to 49. We also give limits on pulsation amplitudes for 285 objects with no obvious periodic variations, summarise the results of the survey, and provide improved physical parameters on the composite pulsators for which only preliminary estimates were published earlier.
74 citations
Cites background or result from "A Survey for Pulsating Hot B Subdwa..."
...After a limit on pulsation of 0.08% was presented by Billères et al. (2002), no further attempts were made to observe this target....
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...6a we show the location in the (Teff , log g) plane of the pulsators from our sample (blue) together with pulsators from the literature (red), as listed in Table 9, with symbol size proportional to Amax....
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...Supplementary data: a1 Allard et al. (1994), b1 Baran & Fox Machado (2010), c1 Charpinet et al. (2005), g1 Geier et al. (2007), h1 Heber et al. (2000), k1 Kilkenny et al. (2002), k2 Kilkenny et al. (2003), l1 Lisker et al. (2005), o1 Oreiro et al. (2005), o2 O’Toole et al. (2004), r1 Reed et al. (2006), r2 Reed et al. (2007a), r3 Reed et al. (2007b), r4 Reed et al. (2010), r5 Randall et al. (2007), r6 Randall et al. (2009b), s1 Silvotti et al. (2002a, 2007), s2 Silvotti et al. (2006), s3 Schuh et al. (2006), t1 Telting & Østensen (2004), v1 Vučković et al. (2006), v2 Vučković et al. (2007), z1 Zhou et al. (2006), ‡ Our determination of Teff , log g and log y. time and across the instability region to improve, as most (Walker et al. 2003) and CoRoT (Michel et al. 2008) have done for brighter pulsating stars....
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...The long period g-mode sdBVs also pul- mpg gives the photographic magnitude from the HS survey plates, ‘Class’ our spectroscopic classification, followed by the effective temperature (Teff), surface gravity (log g), and atmospheric helium abundance (log y= log (NHe/NH)), from our model fits....
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...We found no pulsators in this sample, but it has been heavily exploited by other groups; Billères et al. (2002) surveyed 14 of them and found one: UYSex....
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References
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TL;DR: In this paper, the authors present le catalogue de Stromgren et al. which donne les positions avec une precision de 8'', les magnitudes photographiques B a 0,29 mag pres, les types spectraux, des references and les couleurs de Stemgren, and 1715 constituent un echantillon statistiquement complet couvrant 10714 degres carres de 266 champs du Palomar
Abstract: On presente le catalogue qui donne les positions avec une precision de 8'', les magnitudes photographiques B a 0,29 mag pres, les types spectraux, des references et les couleurs de Stromgren. Parmi les 1874 objets du catalogue, 1715 constituent un echantillon statistiquement complet couvrant 10714 degres carres de 266 champs du Palomar
673 citations
376 citations
TL;DR: In this paper, high signal-to-noise ratio optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet-excess survey is analyzed with a new grid of model atmospheres and synthetic spectra.
Abstract: High signal-to-noise ratio optical spectrophotometry of a sample of field subluminous B stars drawn largely from the Palomar Green ultraviolet-excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from a detailed analysis of hydrogen and helium absorption line profiles. The derived temperatures and gravities place the subluminous B stars in the theoroetical Hertzsprung-Russell (H-R) diagram along and bounded below by theoretical sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are composed of helium-burning cores of approximately 0.5 solar mass overlain by very thin layers of hydrogen (approximately less than 0.02 solar mass). Various scenarios for their past evolutionary history are examined in the context of their probable future evolution into white dwarfs of lower than average mass.
326 citations
TL;DR: In this article, it was shown that pulsation modes are driven through an opacity bump due to a local enhancement of the iron abundance in the envelopes of sdB stars, which provides a natural explanation for the instabilities found in the newly discovered class of pulsating sdB star.
Abstract: We present new calculations that strongly reinforce the idea—originally proposed by Charpinet et al.—that pulsation modes are driven through an opacity bump due to a local enhancement of the iron abundance in the envelopes of sdB stars. Our improved models incorporate nonuniform iron abundance distributions obtained through the condition of diffusive equilibrium between gravitational settling and radiative levitation. They also include special Rosseland opacity tables that take into account the large variations of the iron abundance about the cosmic value that are predicted by equilibrium radiative levitation theory. For representative models with M = 0.48 M☉ and log g = 5.8, we find strong instabilities for low-order radial and nonradial (p and f) pulsation modes in the range 36,500 K Teff 29,000 K. The four pulsating sdB stars currently known all have effective temperatures in that range. In addition, one of our models with Teff = 34,000 K has a band of unstable modes with periods in the range 116-195 s, in excellent agreement with those of the known pulsators. We therefore claim that our proposed iron bump mechanism provides a natural explanation for the instabilities found in the newly discovered class of pulsating sdB stars.
291 citations
TL;DR: In this article, a survey of the pulsation properties of hot B subdwarfs is presented, where all pulsation modes with l = 0, 1, 2, and 3 in the 80-1500 s period window are considered.
Abstract: We present key sample results of a systematic survey of the pulsation properties of models of hot B subdwarfs. We use equilibrium structures taken from detailed evolutionary sequences of solar metallicity (Z = 0.02) supplemented by grids of static envelope models of various metallicities (Z = 0.02, 0.04, 0.06, 0.08, and 0.10). We consider all pulsation modes with l = 0, 1, 2, and 3 in the 80-1500 s period window, the interval currently most suitable for fast photometric detection techniques. We establish that significant driving is often present in hot B subdwarfs and is due to an opacity bump associated with heavy-element ionization. We find that models with Z ≥ 0.04 show low radial order unstable modes; both radial and nonradial (p, f, and g) pulsations are excited. The unstable models have Teff 30,000 K and log g 5.7, depending somewhat on the metallicity. We emphasize that metal enrichment need only occur locally in the driving region. On this basis, combined with the accepted view that local enrichments and depletions of metals are commonplace in the envelopes of hot B subdwarfs, we predict that some of these stars should show luminosity variations resulting from pulsational instabilities.
238 citations