Doradus Variables Research Articles

Nine Bright γ Doradus Variables Discovered with Ground-based Photometry

Abstract We have used precise photometric and high-dispersion spectroscopic observations to study nine candidate γ Doradus (γ Dor) stars, identified as optically variable comparison stars in our photometric studies of Sun-like stars. In this paper, we confirm these nine candidates as new γ Dor variables. All exhibit sinusoidal variability with amplitudes between 6 and 65 mmag in Johnson B and periods from 0.28 to 1.13 days. All lie in the same region of the H-R diagram as our previously confirmed γ Dor stars. Of the nine systems, one is a single-lined spectroscopic binary (SB1), two are double-lined spectroscopic binaries (SB2), and the remaining six are single stars. We present orbits for the three binary systems: HD 34415, HD 144839, and HD 182735. Their periods are 6400 days or 17.5 yr (adopted), 995 ± 7 days, and 1052.4 ± 0.2 days, respectively. We compare our photometric periods with those we derive for six of the nine stars observed with the Transiting Exoplanet Survey Satellite to strengthen the confirmation of these new γ Dor variables. Finally, because the distribution of γ Dor stars in the H-R diagram overlaps with the red edge of the δ Scuti (δ Sct) instability strip, we use our ground-based photometric results for the 73 γ Dor stars confirmed with our Automatic Photoelectric Telescopes over the past two decades to look at the incidence of hybrid δ Sct/γ Dor stars; these are highly valuable targets for asteroseismology.

Wind-envelope interaction as the origin of the slow cyclic brightness variations of luminous blue variables

Luminous blue variables (LBVs) are hot, very luminous massive stars displaying large quasi-periodic variations in brightness, radius, and photospheric temperature on timescales of years to decades. The physical origin of this variability, called S Doradus cycle after its prototype, has remained elusive. We study the feedback of stellar wind mass-loss on the envelope structure in stars near the Eddington limit. We calculated a time-dependent hydrodynamic stellar evolution, applying a stellar wind mass-loss prescription with a temperature dependence inspired by the predicted systematic increase in mass-loss rates below 25 kK. We find that when the wind mass-loss rate crosses a well-defined threshold, a discontinuous change in the wind base conditions leads to a restructuring of the stellar envelope. The induced drastic radius and temperature changes, which occur on the thermal timescale of the inflated envelope, in turn impose mass-loss variations that reverse the initial changes, leading to a cycle that lacks a stationary equilibrium configuration. Our proof-of-concept model broadly reproduces the typical observational phenomenology of the S Doradus variability. We identify three key physical ingredients that are required to trigger the instability: inflated envelopes in close proximity to the Eddington limit, a temperature range where decreasing opacities do not lead to an accelerating outflow, and a mass-loss rate that increases with decreasing temperature, crossing a critical threshold value within this temperature range. Our scenario and model provide testable predictions, and open the door for a consistent theoretical treatment of the LBV phase in stellar evolution, with consequences for their further evolution as single stars or in binary systems.

Inferring the internal structure of γ Doradus variables from Rossby modes

A unique type of oscillation modes has recently been identified in γ Doradus variables. These low-frequency modes are called Rossby modes (or r modes) because they consist of Rossby waves in each spherical layer. These waves are characterised by toroidal motions that are restored by the latitudinal variation in the Coriolis force. The horizontal oscillations are weakly coupled in the radial direction. We show that these modes can be used to probe the interior of the stars. The method of the ν − √Δν diagram, which has originally been developed to analyse another type of modes, Kelvin g-modes (or prograde sectoral g-modes), is extended to take Rossby modes into account. We first show based on a theoretical model and then on two stars, KIC 3240967 and KIC 12066947, that the method can be adapted to Rossby modes straightforwardly. In addition, we demonstrate that simultaneous analysis of Kelvin and Rossby modes results in (1) smaller uncertainties in the internal rotation rate and the characteristic period of gravity modes, and (2) a substantial reduction of the correlation between the estimates of the two parameters.

The Massive Binary System RY Sct – New Solution of the Light Curves

RY Sct is a unique massive binary star system in a short transitional evolutionary phase. It is an eclipsing binary where both stars have overfilled their Roche cavities, so that the system is in an “overcontact” state. It is surrounded by a young circumstellar nebula. The binary is regarded as a rare progenitor of a type WR+OB system. RY Sct is close to the Eddington luminosity limit, so that radiative pressure plays an important role in mass loss. At least one of the components of the close binary may be associated with a rare class of LBV stars or the so-called S Doradus variables. We have calculated the orbital elements using the old photometric data of Zakirov and Kumsiashvili and new spectral data, together with an analysis of HST observations. A model was examined according to which the components of RY Sct are in an overcontact state and fill their Roche cavities. The brighter primary component transfers mass to the more massive but less bright secondary. The binary system is surrounded by a common envelope. As initial parameters we, chose a mass ratio q = 4.42 and orbital period P = 11d.12471, and used an orbital inclination of i = 75.6°.

Pulsation and rotation in NGC 6811: the Kepler short-cadence stars

ABSTRACT We have analysed a selected sample of 36 Kepler short-cadence stars in the field of NGC 6811. The results reveal that all the targets are variable: two red giant stars with solar-like oscillations, 21 main-sequence pulsators (16 δ Scuti and five γ Doradus stars), and 13 rotating variables. Three new γ Doradus (γ Dor) variables (one is a hot γ Dor star) are detected in this work together with five new rotating variables. An in-depth frequency analysis of the δ Scuti (δ Sct) and γ Dor stars in the sample shows that the frequency spectra are very rich, in particular for the δ Sct-type variables. They present very dense frequency distributions and wide diversity in frequency patterns, even for stars being members of the cluster and with very similar location in the Hertzsprung–Russell (H–R) diagram. Rotational modulation is found for a high percentage of these main-sequence pulsating stars, which is an indication of stellar activity being common on the surfaces of these hot stars. In some cases, activity dominates the luminosity variations. Significant amplitude variability is also detected in at least some of the pulsation modes of highest amplitude in almost all the δ Sct stars. One of the δ Sct pulsators is a member of a binary system with the pulsating component tidally distorted. This system also shows strong stellar activity, including several flares that probably originate in the δ Sct component.

How common is LBV S Doradus variability at low metallicity?

It remains unclear whether massive star evolution is facilitated by mass loss through stellar winds only or whether episodic mass loss during an eruptive luminous blue variable (LBV) phase is also significant. LBVs exhibit unique photometric and spectroscopic variability (termed S Doradus variables). This may have tremendous implications for our understanding of the first stars, gravitational wave events, and supernovae. A key question here is whether all evolved massive stars passing through the blue supergiant phase are dormant S Doradus variables transforming during a brief period or whether LBVs are truly unique objects. By investigating the OGLE light curves of 64 B supergiants (Bsgs) in the Small Magellanic Cloud (SMC) on a timescale of three years with a cadence of one night, the incidence of S Doradus variables amongst the Bsgs population is investigated. From our sample, we find just one Bsg, AzV 261, that displays the photometric behaviour characteristic of S Doradus variables. We obtain and study a new VLT X-shooter spectrum of AzV 261 in order to investigate whether the object has changed its effective temperature over the last decade. We do not find any effective temperature variations indicating that the object is unlikely to be a LBV S Doradus variable. As there is only one previous bona fide S Doradus variable known to be present in the SMC (R 40), we find the maximum duration of the LBV phase in the SMC to be at most a few 103yr or more likely that canonical Bsgs, and S Doradus LBVs are intrinsically different objects. We discuss the implications for massive star evolution in low-metallicity environments, characteristic of the early Universe.

Pulsation, Rotation and Flares in A Stars

AbstractNew observations of Kepler δ Scuti stars show that our understanding of pulsation in these stars is incomplete. A large fraction of A and B stars exhibit rotational modulation in light, suggesting that spots exist in stars with radiative envelopes. Flares are seen in some A stars, as may be expected if starspots are present. Differential rotation shear increases from M to F but decreases for A stars; it reaches a maximum among the γ Doradus variables. Current views of stars with radiative envelopes may need to be reviewed in the light of these observations.

Convection and Overshoot in Models of γ Doradus and δ Scuti Stars

Abstract We investigate the pulsation properties of stellar models that are representative of δ Scuti and γ Doradus variables. We have calculated a grid of stellar models from 1.2 to 2.2 M ⊙, including the effects of both rotation and convective overshoot using MESA, and we investigate the pulsation properties of these models using GYRE. We discuss the observable patterns in the frequency spacing for p modes and the period spacings for g modes. Using the observable patterns in the g mode period spacings, it may be possible to observationally constrain the convective overshoot and rotation of a model. We also calculate the pulsation constant (Q) for all models in our grid and investigate the variation with convective overshoot and rotation. The variation in the Q values of the radial modes can be used to place constraints on the convective overshoot and rotation of stars in this region. As a test case, we apply this method to a sample of 22 High-Amplitude δ Scuti stars (HADS) and provide estimates for the convective overshoot of the sample.

High-resolution spectroscopy and abundance analysis of δ Scuti stars near the γ Doradus instability strip

$\delta$ Scuti stars are remarkable objects for asteroseismology. In spite of decades of investigations, there are still important questions about these pulsating stars to be answered, such as their positions in $\log$$T_{\rm eff}$ $-$ $\log g$ diagram, or the dependence of the pulsation modes on atmospheric parameters and rotation. Therefore, we performed a detailed spectroscopic study of $41$ $\delta$ Scuti stars. The selected objects are located near the $\gamma$ Doradus instability strip to make a reliable comparison between both types of variables. Spectral classification, stellar atmospheric parameters ($T_{\rm eff}$, $\log g$, $\xi$) and $v \sin i$ values were determined. The spectral types and luminosity classes of stars were found to be A1$-$F5 and III$-$V, respectively. The $T_{\rm eff}$ ranges from $6600$ to $9400$ K, whereas the obtained $\log g$ values are from $3.4$ to $4.3$. The $v \sin i$ values were found between $10$ and $222$ km s$^{-1}$. The derived chemical abundances of $\delta$ Scuti stars were compared to those of the non-pulsating stars and $\gamma$ Doradus variables. It turned out that both $\delta$ Scuti and $\gamma$ Doradus variables have similar abundance patterns, which are slightly different from the non-pulsating stars. These chemical differences can help us to understand why there are non-pulsating stars in classical instability strip. Effects of the obtained parameters on pulsation period and amplitude were examined. It appears that the pulsation period decreases with increasing $T_{\rm eff}$. No significant correlations were found between pulsation period, amplitude and $v \sin i$.

Spectroscopic survey of γ Doradus stars – I. Comprehensive atmospheric parameters and abundance analysis of γ Doradus stars

We present a spectroscopic survey of known and candidate $\gamma$\,Doradus stars. The high-resolution, high signal-to-noise spectra of 52 objects were collected by five different spectrographs. The spectral classification, atmospheric parameters (\teff, $\log g$, $\xi$), $v\sin i$ and chemical composition of the stars were derived. The stellar spectral and luminosity classes were found between G0-A7 and IV-V, respectively. The initial values for \teff\ and \logg\ were determined from the photometric indices and spectral energy distribution. Those parameters were improved by the analysis of hydrogen lines. The final values of \teff, \logg\ and $\xi$ were derived from the iron lines analysis. The \teff\ values were found between 6000\,K and 7900\,K, while \logg\,values range from 3.8 to 4.5\,dex. Chemical abundances and $v\sin i$ values were derived by the spectrum synthesis method. The $v\sin i$ values were found between 5 and 240\,km\,s$^{-1}$. The chemical abundance pattern of $\gamma$\,Doradus stars were compared with the pattern of non-pulsating stars. It turned out that there is no significant difference in abundance patterns between these two groups. Additionally, the relations between the atmospheric parameters and the pulsation quantities were checked. A strong correlation between the $v\sin i$ and the pulsation periods of $\gamma$\,Doradus variables was obtained. The accurate positions of the analysed stars in the H-R diagram have been shown. Most of our objects are located inside or close to the blue edge of the theoretical instability strip of $\gamma$\,Doradus.

The Occurrence of Non-pulsating Stars in the gamma Doradus/delta Scuti Pulsation Instability Region

As part of the NASA Kepler Guest Observer program, we requested and obtained long-cadence data on about 2700 faint (Kepler magnitude 14-16) stars with effective temperatures and surface gravities that lie near or within the pulsation instability region for main-sequence gamma Doradus and delta Scuti pulsating variables. These variables are of spectral type A-F with masses 1.4 to 2.5 solar masses. The delta Scuti stars pulsate in radial and non-radial acoustic modes, with periods of a few hours (frequencies ~10 cycles/day), while gamma Doradus variables pulsate in nonradial gravity modes with periods 0.3 to 3 days (frequencies ~1 cycle/day). Here we consider the light curves and Fourier transforms of 633 stars in an unbiased sample observed by Kepler in Quarters 6-13 (June 201 0-June 2012). We show the location of these stars in the log surface gravity-effective temperature diagram compared to the instability region limits established from ground-based observations, and taking into account uncertainties and biases in the Kepler Input Catalog TettValues. While hundreds of variables have been discovered in the Kepler data, about 60% of the stars in our sample do not show any frequencies between 0.2 and 24.4 cycles/day with amplitude above 20 parts per million. We find that six of these apparently constant stars lie within the instability region. We discuss some possible reasons that these stars do not show photometric variability in the Kepler data. We also comment on the 'nonconstant' stars, and find 26 variable star candidates, many of which also do not lie within the expected instability region.

The Occurrence of Non-pulsating Stars in the gamma Doradus/delta Scuti Pulsation Instability Region

As part of the NASA KeplerGuest Observer program, we requested and obtained long-cadence data on about 2700 faint (Kepler magnitude 14-16) stars with effective temperatures and surface gravities that lie near or within the pulsation instability region for main-sequence gamma Doradus and delta Scuti pulsating variables. These variables are of spectral type A-F with masses 1.4 to 2.5 solar masses. The delta Scuti stars pulsate in radial and non-radial acoustic modes, with periods of a few hours (frequencies ~10 cycles/day), while gamma Doradus variables pulsate in nonradial gravity modes with periods 0.3 to 3 days (frequencies ~1 cycle/day). Here we consider the light curves and Fourier transforms of 633 stars in an unbiased sample observed by Kepler in Quarters 6-13 (June 201 0-June 2012). We show the location of these stars in the log surface gravity-effective temperature diagram compared to the instability region limits established from ground-based observations, and taking into account uncertainties and biases in the KeplerInput Catalog Teff values. While hundreds of variables have been discovered in the Keplerdata, about 60% of the stars in our sample do not show any frequencies between 0.2 and 24.4 cycles/day with amplitude above 20 parts per million. We find that six of these apparently constant stars lie within the instability region. We discuss some possible reasons that these stars do not show photometric variability in the Keplerdata. We also comment on the ‘nonconstant’ stars, and find 26 variable star candidates, many of which also do not lie within the expected instability region.

A γ Doradus candidate in eclipsing binary BD And?

The BVR photometric light curves of the eclipsing binary BD And were obtained in 2008 and 2009. We estimated the mass ratio of the system as 0.97 and the photometric solutions were derived. The results show that BD And is a detached binary system, whose components have a little temperature difference of about 40K. By analyzing photometric available light minimum times, we also derived an update ephemeris and found for the first time a possible periodic oscillation with an amplitude of 0.011days and a period of 9.6years. The results indicate that the periodic oscillation could be caused by a third component physically attached to the eclipsing binary. After removing the light variations due to the eclipses and proximity effects, the light-curve distortions are further explained by the pulsation of the primary component with a dominant period of ∼1day. In accordance with the position of the primary component on the Hertzsprung–Russell diagram and its pulsation period, the primary component of BD And could be an excellent γ Doradus candidate. It is rarely phenomenon that a component of the eclipsing binary system is a γ Doradus variable.

A VOLUME-LIMITED PHOTOMETRIC SURVEY OF 114 γ DORADUS CANDIDATES

We have carried out a photometric survey of a complete, volume-limited sample of gamma Doradus candidates. The sample was extracted from the Hipparcos catalog and consists of 114 stars with colors and absolute magnitudes within the range of known gamma Doradus stars. We devoted one year of observing time with our T12 0.8 m automatic photometric telescope to acquire nightly observations of the complete sample of stars. From these survey observations, we identify 37 stars with intrinsic variability of 0.002 mag or more. Of these 37 variables, eight have already been confirmed as gamma Doradus stars in our earlier papers; we scheduled the remaining 29 variables on our T3 0.4 m automatic telescope to acquire more intensive observations over the next two years. We obtained complementary spectroscopic observations with the Kitt Peak coude feed telescope. Analysis of our new photometric and spectroscopic data reveals 15 new gamma Doradus variables (and confirms two others), eight new delta Scuti variables (and confirms one other), and three new variables with unresolved periodicity. Of the 114 gamma Doradus candidates in our volume-limited sample, we find 25 stars that are new or previously-known gamma Doradus variables. This results in an incidence of 22% for gamma Doradus variability among candidate field stars for this volume of the solar neighborhood. The corresponding space density of gamma Doradus stars is 0.094 stars per 10^3 pc^3 or 94 stars per 10^6 pc^3. We provide an updated list of 86 bright, confirmed, gamma Doradus field stars.

THE Hα VARIATIONS OF THE LUMINOUS BLUE VARIABLE P CYGNI: DISCRETE ABSORPTION COMPONENTS AND THE SHORT S DORADUS-PHASE

P Cygni is a prototype of the Luminous Blue Variables (or S Doradus variables), and the star displays photometric and emission line variability on a timescale of years (known as the "short S Doradus phase" variations). Here we present new high resolution H-alpha spectroscopy of P Cyg that we combine with earlier spectra and concurrent V-band photometry to document the emission and continuum flux variations over a 24 y time span. We show that the emission and continuum fluxes vary in concert on timescales of 1.6 y and longer, but differ on shorter timescales. The H-alpha profile shape also varies on the photometric timescales, and we describe the observed co-variations of the emission peak and absorption trough properties. We argue that the episodes of photometric and emission brightening are caused by increases in the size of the emission region that are related to variations in wind mass loss rate and outflow speed. We find evidence of blueward accelerating, Discrete Absorption Components (DACs) in the absorption trough of the H-alpha profile, and these features have slower accelerations and longer durations than those observed in other lines. The DAC strengths also appear to vary on the photometric timescales, and we suggest that the propagation of the DAC-related wind structures is closely related to changes in the overall wind mass loss rate and velocity.

Photometry and Spectroscopy of 11 γ Doradus Stars

We have used precise photometric and high-dispersion spectroscopic observations to study 11 γ Doradus stars, 10 of them newly confirmed. Only five of these 11 γ Doradus stars appear to be single; two are primaries of double-lined spectroscopic binaries, one is the secondary of a double-lined binary, two are primaries of visual binaries and, in the case of the double-lined binary (HD 86371), either or both components could be a pulsating γ Doradus star. We have determined a preliminary orbital period of 5.32 days for the double-lined binary HD 41547. Several of the stars show spectroscopic line-profile and low-amplitude radial velocity variability indicative of pulsation. All 11 stars are photometrically variable with amplitudes between 4 and 94 mmag in Johnson B and periods between 0.38 and 1.86 days. The 11 stars have between two and five independent periods. The variability at all periods approximates a sinusoid. We provide a new tabulation of all 66 γ Doradus stars confirmed to date and list some of their properties. All are dwarfs or subgiants and lie within a well-defined region of the H-R diagram that overlaps the cool edge of the δ Scuti instability strip. Four of the new γ Doradus variables from this paper also lie within the δ Scuti instability strip but do not exhibit the additional higher frequency variability typical of δ Scuti stars. Among the 66 confirmed γ Doradus variables, we find no correlation between the period of the strongest pulsation mode and the (B - V) color index, absolute magnitude, or luminosity.

Pulsation of the Lambda Bootis star HD 210111

CONTEXT. The Lambda Bootis stars are a small spectroscopic subgroup of Population I A-type stars and show significant underabundances of metals. Many are Delta Scuti pulsators. AIMS. HD 210111 was selected for a detailed multisite pulsation study to determine whether its pulsation properties differ from those of normal A stars. METHODS. 262 hours of high-precision photometry were obtained at the SAAO and SSO observatories. RESULTS. 13 statistically significant pulsation frequencies were detected with very small photometric amplitudes from 1 to 7 millimag in the visual. A comparison with earlier 1994 measurements indicates a small increase in amplitude. As a byproduct, one of the comparison stars, HD 210571, was discovered to be a millimag variable with a frequency of 1.235 c/d and is probably a new Gamma Doradus variable. The observed wide range of excited frequencies from 12 to 30 c/d in HD 210111 can be explained with both the single- and double-star hypothesis. HD 210111 is in a similar evolutionary status to FG Vir, which also shows a wide range of excited frequencies with a similar frequency spacing near 4 c/d. This is interpreted as successive radial orders of the excited nonradial modes. In the double-star hypothesis previously evoked for HD 210111, the low and the high frequencies originate in different stars: here HD 210111 would resemble Theta^2 Tau. CONCLUSIONS. The pulsation of the Lambda Bootis star HD 210111 does not differ from that of normal Delta Scuti stars.

Survey for Transiting Extrasolar Planets in Stellar Systems. IV. Variables in the Field of NGC 1245

The Survey for Transiting Extrasolar Planets in Stellar Systems (STEPSS) project is a search for planetary transits in open clusters. In this paper, we analyze the STEPSS observations of the open cluster NGC 1245 to determine the variable star content of the cluster. Out of 6787 stars observed with V < 22, of which ~870 are cluster members, we find 14 stars with clear intrinsic variability that are potential cluster members, and 29 clear variables that are not cluster members. None of these variables have been previously identified. We present light curves, finding charts, and stellar/photometric data on these variable objects. Several of the interacting binaries have estimated distances consistent with the cluster distance determined from isochrone fits to the color magnitude diagram. Four stars at the main sequence turnoff of the cluster have light curves consistent with gamma Doradus variability. If these gamma Doradus candidates are confirmed, they represent the oldest and coolest members of this class of variable discovered to date.

Eleven New γ Doradus Stars

We present new high-dispersion spectroscopic and precise photometric observations to identify 11 new γ Doradus variables. Seven of these new γ Doradus stars appear to be single, three are primaries of single-lined binaries, and one has two distant visual companions; none are double-lined or close visual binaries. Several of the stars show spectroscopic line-profile and low-amplitude radial velocity variability indicative of pulsation. All 11 stars are photometrically variable with amplitudes between 8 and 93 mmag in Johnson B and periods between 0.398 and 2.454 days. One star is monoperiodic; the rest have between two and five independent periods. The variability at all periods approximates a sinusoid, although three of the stars exhibit cycle-to-cycle variation in the level of maximum brightness, similar to the Blazhko effect observed in some RR Lyrae stars. We provide a new tabulation of all 54 γ Doradus stars confirmed to date and list some of their properties. All are dwarfs or subgiants and lie within a well-defined region of the H-R diagram that overlaps the cool edge of the δ Scuti instability strip. Four of the new γ Doradus variables from this paper also lie within the δ Scuti instability strip but do not exhibit the additional higher frequency variability typical of δ Scuti stars. The variability type of several of these stars given in the General Catalog of Variable Stars and in SIMBAD should now be revised.

A Dozen New Doradus Stars

We use new high-dispersion spectroscopic and precise photometric observations to identify 12 new γ Doradus stars. Two of the 12 systems are double-lined binaries that show obvious velocity variability. Five other stars have metallic lines with composite profiles characterized by a narrow feature near the center of each broad component. Spectrograms of the Hα line indicate that all five stars are binaries rather than shell stars. The remaining five stars in our sample are probably single. All 12 stars are photometrically variable with amplitudes between 6 and 87 mmag in Johnson B and periods between 0.3 and 1.2 days. Four stars are monoperiodic; the rest have between two and five independent periods. The variability at all periods approximates a sinusoid. Although many of the stars lie within the δ Scuti instability strip, none exhibit the higher frequency variability seen in δ Scuti stars. We have increased the sample of known γ Doradus stars by 40% and revised the positions of a number of variables in the H-R diagram by accounting for duplicity. Our list of 42 confirmed γ Doradus variables gives some of their properties. All are dwarfs or subgiants and lie within a well-defined region of the H-R diagram that overlaps the cool edge of the δ Scuti instability strip. We compare the observed location of the γ Doradus variables with a recently published theoretical γ Doradus instability strip and find good agreement.