A theoretical framework for BL Her stars – I. Effect of metallicity and convection parameters on period–luminosity and period–radius relations (original) (raw)
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Non-linear radial pulsation models for the early-type helium stars V652?Her and BX?Cir
Astronomy and Astrophysics, 2002
We report new non-linear pulsation models of the helium stars V652 Her and BX Cir. Linear theory has previously shown their pulsations to be due to iron-group bump instability. Recent high-resolution spectroscopic observations have provided high-precision measurements of their radial velocity curves and of their radii. Their masses remain less well determined. A hydrodynamic code including recent OPAL opacity data has been used to construct the models. These are compared with the observational data. In particular, we attempt to reproduce accurately the observed radial velocity and luminosity curves. The results impose additional constraints on those stellar dimensions, including mass, which remain poorly determined by observation. Final results show a model for V652 Her which reproduce the observed velocity and luminosity curves with 0.7 M and 23 400 K. For BX Cir, the mass must lie between 0.50 and 0.38 M if the temperature is in the range 22 400-24 000 K. However, the luminosity of the models is smaller than that measured directly by a factor of two.
A pulsational approach to the luminosity of horizontal branch stellar structures
Monthly Notices of the Royal Astronomical Society, 1999
We discuss an alternative approach to constrain the absolute bolometric luminosity of Zero Age Horizontal Branch (ZAHB) structures by using the observational pulsational properties of ab type RR Lyrae stars and theoretical expectations concerning both the relation connecting the pulsational properties of these variables to their evolutionary ones, as luminosity, mass and effective temperature and, also the location in the H-R diagram for the fundamental pulsators instability strip boundaries. Since the main goal of this work is to obtain an evaluation of the ZAHB bolometric luminosity as much as possible independent on stellar evolution theory, we have minimized the use of evolutionary prescriptions, being the only adopted evolutionary input the allowed mass range for fundamental pulsators. Nevertheless, the effects on our final results related to the use of these evolutionary prescriptions have been carefully checked.
A pulsational approach to near-infrared and visual magnitudes of RR Lyr stars
Monthly Notices of the Royal Astronomical Society, 2003
In this paper we present an improved theoretical scenario concerning near infrared and visual magnitudes of RR Lyrae variables, as based on up-to-date pulsating models. New relations connecting V and K absolute magnitudes with periods, mass, luminosity and metal content are discussed separately for fundamental and first overtone pulsators. We also show that the V −K colors are predicted to supply tight constraints on the pulsator intrinsic luminosity. On this basis, we revisit the case of the prototype variable RR Lyr, showing that the parallax inferred by this new pulsational approach appears in close agreement with HST absolute parallax. Moreover, available K and V measurements for field and cluster RR Lyrae variables with known reddening and metal content are used to derive a relation connecting the K absolute magnitude to period and metallicity (M K-[Fe/H]-logP) as well as a new calibration of the M V-[Fe/H] relation. The comparison between theoretical prescriptions and observations suggests that RR Lyrae stars in the field and in Galactic Globular Clusters (GGCs) should have quite similar evolutionary histories. The comparison between theory and observations also discloses a general agreement that supports the reliability of current pulsational scenario. On the contrary, current empirical absolute magnitudes based on the Baade-Wesselink (BW) method suggest relations with a zero-point that is fainter than predicted by pulsation models, together with a milder metallicity dependence. However, preliminary results based on a new calibration of the BW method provided by Cacciari et al. (2000) for RR Cet and SW And appear in a much better agreement with the pulsational predictions.
FUSEDetermination of Abundances in Long‐Period Pulsating V1093 Her (PG 1716+426) Stars1
The Astrophysical Journal, 2008
We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of five V1093 Her (PG 1716+426) stars, the subgroup of hydrogen-rich subdwarf B stars that exhibit very low amplitude, long-period luminosity variations. Our primary aim is to investigate whether these stars display abundances which differ from those observed in the nonvariable sdB stars and also in the shorter period V361 Hya variables. For the light elements and for those beyond the iron peak, our abundances are consistent with the trends observed in earlier studies. For the important iron peak elements Ti, V, Cr, Mn, Fe, Co, and Ni, which are thought to be directly linked to the driving mechanism in both long-period and short-period variables, the abundances determined in the V1093 Her stars appear very homogeneous and exhibit mild enrichments (by factors of 2Y5 for Cr, Mn, Fe, and Co) over the solar value. However, these abundances do not differ appreciably from those measured in a sample consisting of constant stars and one short-period pulsator. The implications of these findings for current models which involve both diffusion processes and stellar winds to account for the driving of nonradial pulsations in sdB stars are discussed.
On a New Theoretical Framework for RR Lyrae Stars. I. The Metallicity Dependence
The Astrophysical Journal, 2015
We present new nonlinear, time-dependent convective hydrodynamical models of RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio and a broad range in metal abundances (Z = 0.0001-0.02). The stellar masses and luminosities adopted to construct the pulsation models were fixed according to detailed central He burning Horizontal Branch evolutionary models. The pulsation models cover a broad range in stellar luminosity and effective temperatures and the modal stability is investigated for both fundamental and first overtones. We predict the topology of the instability strip as a function of the metal content and new analytical relations for the edges of the instability strip in the observational plane. Moreover, a new analytical relation to constrain the pulsation mass of double pulsators as a function of the period ratio and the metal content is provided. We derive new Period-Radius-Metallicity relations for fundamental and first-overtone pulsators. They agree quite well with similar empirical and theoretical relations in the literature. From the predicted bolometric light curves, transformed into optical (U BVRI) and near-infrared (JHK) bands, we compute the intensity-averaged mean magnitudes along the entire pulsation cycle and, in turn, new and homogenous metaldependent (RI JHK) Period-Luminosity relations. Moreover, we compute new dual and triple band optical, optical-NIR and NIR Period-Wesenheit-Metallicity relations. Interestingly, we find that the optical Period-W(V, B − V) is independent of the metal content and that the accuracy of individual distances is a balance between the adopted diagnostics and the precision of photometric and spectroscopic datasets.
Monthly Notices of the Royal Astronomical Society, 2014
Low amplitude is the defining characteristic of solar-like oscillations. The space projects Kepler and CoRoT give us a great opportunity to successfully detect such oscillations in numerous targets. Achievements of asteroseismology depend on new discoveries of connections between the oscillation frequencies and stellar properties. In the previous studies, the frequency of the maximum amplitude and the large separation between frequencies were used for this purpose. In the present study, we confirm that the large separation between the frequencies has two minima at two different frequency values. These are the signatures of the He II ionization zone, and as such have very strong diagnostic potential. We relate these minima to fundamental stellar properties such as mass, radius, luminosity, age and mass of convective zone. For mass, the relation is simply based on the ratio of the frequency of minimum ∆ν to the frequency of maximum amplitude. These frequency comparisons can be very precisely computed, and thus the mass and radius of a solar-like oscillating star can be determined to high precision. We also develop a new asteroseismic diagram which predicts structural and evolutionary properties of stars with such data. We derive expressions for mass, radius, effective temperature, luminosity and age in terms of purely asteroseismic quantities. For solar-like oscillating stars, we now will have five very important asteroseismic tools (two frequencies of minimum ∆ν, the frequency of maximum amplitude, and the large and small separations between the oscillation frequencies) to decipher properties of stellar interior astrophysics.
Astronomy and Astrophysics
Aims. We study the impact of a subsolar metallicity on various properties of non-rotating and rotating stars, such as surface velocities and abundances, lifetimes, evolutionary tracks, and evolutionary scenarios. Methods. We provide a grid of single star models covering a mass range of 0.8 to 120 M ⊙ with an initial metallicity Z = 0.002 with and without rotation. We discuss the impact of a change in the metallicity by comparing the current tracks with models computed with exactly the same physical ingredients but with a metallicity Z = 0.014 (solar). Results. We show that the width of the main-sequence (MS) band in the upper part of the Hertzsprung-Russell diagram (HRD), for luminosity above log L/L ⊙ > 5.5, is very sensitive to rotational mixing. Strong mixing significantly reduces the MS width.
The Potential of Asteroseismology for Hot, Subdwarf B Stars: A New Class of Pulsating Stars
Astrophysical Journal, 1996
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 needs only occur locally in the driving region. On this basis, combined with the accepted view that local enrichments and depletions of metals are common place in the envelopes of hot B subdwarfs, we predict that some of these stars should show luminosity variations resulting from pulsational instabilities.
Non-adiabatic linear pulsation models for low-mass helium stars
Monthly Notices of the Royal Astronomical Society, 1999
Non-adiabatic linear pulsation models have been calculated for low-mass stars with effective temperatures between 16 000 and 35 000 K, and with surface gravities in the range 3 , log g , 5X The radial pulsation models assume a homogeneous stellar envelope which is deficient in hydrogen and display the well-known Z-bump instability to radial pulsations. The aim of this paper has been to explore the behaviour of the Z-bump instability as a function of mass and composition around a reference model with M 0X5 M (Y X 0X00Y Z 0X02. It is shown that the Z-bump instability persists to low masses (M , 0X4 M () but is suppressed either by a reduction in metallicity Z or by a selective enhancement of the carbon abundance. An unexpected result is the discovery that Z-bump instability persists at hydrogen abundances X. 0X3, although the position of the red edge is sensitive to X. We have found that non-radial pulsations are also excited in the same instability region as radial pulsations. The implications of these results for individual low-mass helium stars are discussed. It is concluded that Z-bump driven pulsations (radial and/or non-radial) may be excited in some helium-rich subdwarf B stars, representing a possible major extension to the class of variable stars represented by the prototype V652 Her.
Period-Luminosity-Metal Abundance Relations for Population II Variable Stars
International Astronomical Union Colloquium
New period-luminosity-metallicity (P-L-[Fe/H]) relationships for Pop. II Cepheids, RR Lyrae stars, anomalous Cepheids and SX Phe (variable-blue straggler) stars are presented. These were computed by fitting regression lines to observed pulsation periods and mean B, V, K magnitudes for over 1200 stars in ∼40 stellar systems. The stars were assumed to be pulsating in either the fundamental (F) or first-overtone (H) modes (excluding double-mode and other multi-periodic variables). Eight P-L-[Fe/H] relationships (one for each of the two pulsation modes for the four kinds of stars) were simultaneously fitted for each filter. After accounting for the metal abundance differences, the slopes of the P-L relations were tested for departures from equality. The results are consistent with the assumption that, for each kind of star, the relations for the F and H stars are vertically offset, with a family of lines corresponding to the different metallicities. In the case of the globular cluster C...