Late-type Main-sequence Stars Research Articles

ChandraandXMM-Newtonobservations of the low-luminosity X-ray pulsators SAX J1324.4−6200 and SAX J1452.8−5949

We present results from our Chandra and XMM–Newton observations of two low-luminosity X-ray pulsators SAX J1324.4−6200 and SAX J1452.8−5949 which have spin periods of 172 and 437 s, respectively. The XMM–Newton spectra for both sources can be fitted well with a simple power-law model of photon index, Γ∼ 1.0. A blackbody model can equally well fit the spectra with a temperature, kT∼ 2 keV, for both sources. During our XMM–Newton observations, SAX J1324.4−6200 is detected with coherent X-ray pulsations at a period of 172.86 ± 0.02 s while no pulsations with a pulse fraction greater than 18 per cent (at 95 per cent confidence level) in 0.2–12 keV energy band are detected in SAX J1452.8−5949. The spin period of SAX J1324.4−6200 is found to be increasing on a time-scale of which would suggest that the accretor is a neutron star and not a white dwarf. Using subarcsec spatial resolution of the Chandra telescope, possible counterparts are seen for both sources in the near-infrared images obtained with the son of infrared spectrometer and array camera (SOFI) instrument on the New Technology Telescope. The X-ray and near-infrared properties of SAX J1324.4−6200 suggest it to be a persistent high-mass accreting X-ray pulsar at a distance ≤8 kpc. We identify the near-infrared counterpart of SAX J1452.8−5949 to be a late-type main-sequence star at a distance ≤10 kpc, thus ruling out SAX J1452.8−5949 to be a high-mass X-ray binary. However, with the present X-ray and near-infrared observations, we cannot make any further conclusive conclusion about the nature of SAX J1452.8−5949.

DETECTION OF A DEGENERATE COMPANION OF THE SX PHOENICIS STAR KZ HYDRAE BY STUDYING ITS LONG-TERM VARIABILITY

We present 109 new times of maximum light of the SX Phoenicis (SX Phe) star KZ Hydrae (KZ Hya) based mainly on our extensive photometric observations for two decades, leading to determination of a general ephemeris combined with the data in the literature. The O - C diagram reveals a continuously increasing period change combined with the light traveling time effect of the orbital motion of KZ Hya in a binary system with an orbital period of 26.8 +/- 0.2 years. The deduced mass of the companion cannot be smaller than 0.83 M circle dot. Since no sign of the companion has ever been observed spectroscopically and the observed color index b-y excludes the possibility of the companion being a late-type main-sequence or dwarf star, KZ Hya becomes the first SX Phe star for which a degenerate companion is detected. The pulsation properties are studied by analyzing the Fourier spectra of the observed light curves while the fundamental parameters are deduced with the simultaneous multicolor uvby beta data, showing this star to be a strong low-metallicity high-amplitude SX Phe-type variable pulsating in the fundamental radial mode. No signs of multiperiodicity or significant long-term changes in amplitude are detected in the pulsation of this variable.

Suzaku Detection of an Intense X-Ray Flare from an A-Type Star, HD161084

Abstract We report on a serendipitous detection of an intense X-ray flare from the Tycho reference source on HD161084 during a Suzaku observation of the galactic center region for $\sim$20ks. The X-ray Imaging Spectrometer recorded a flare from this A1-type dwarf or subgiant star with a flux of $\sim$1.4$\times$10$^{-12}$erg s$^{-1}$ cm$^{-2}$ (0.5-10keV) and a decay time scale of $\sim$0.5hr. The spectrum is hard with a prominent FeXXV K$\alpha$ emission line at 6.7keV, which is explained by a $\sim$5keV thin-thermal plasma model attenuated by a $\sim$1.4$\times$10$^{21}$cm$^{-2}$ extinction. The low extinction, which is consistent with the optical reddening, indicates that the source is a foreground star toward the galactic center region. Based on a spectroscopic parallax distance of $\sim$530pc, the peak X-ray luminosity amounts to $\sim$1$\times$10$^{32}$erg s$^{-1}$ (0.5-10keV). This is much larger than the X-ray luminosity of ordinary late-type main-sequence stars, and the X-ray emission is unattributable to a hidden late-type companion that comprises a wide binary system with the A star. We discuss possible nature of HD161084, and suggest that it is most likely an interacting binary with elevated magnetic activity in the companion, such as the Algol-type system. The flux detected by Suzaku during the burst is $\sim$100-times larger than the quiescent level measured using the archived XMM-Newton and Chandra data. The large flux amplification makes this star a unique example among sources of this class.

Ages for Illustrative Field Stars Using Gyrochronology: Viability, Limitations, and Errors

We here develop an improved way of using a rotating star as a clock, set it using the Sun, and demonstrate that it keeps time well. This technique, called gyrochronology, permits the derivation of ages for solar- and late-type main sequence stars using only their rotation periods and colors. The technique is clarified and developed here, and used to derive ages for illustrative groups of nearby, late-type field stars with measured rotation periods. We first demonstrate the reality of the interface sequence, the unifying feature of the rotational observations of cluster and field stars that makes the technique possible, and extends it beyond the proposal of Skumanich by specifying the mass dependence of rotation for these stars. We delineate which stars it cannot currently be used on. We then calibrate the age dependence using the Sun. The errors are propagated to understand their dependence on color and period. Representative age errors associated with the technique are estimated at ~15% (plus possible systematic errors) for late-F, G, K, & early-M stars. Ages derived via gyrochronology for the Mt. Wilson stars are shown to be in good agreement with chromospheric ages for all but the bluest stars, and probably superior. Gyro ages are then calculated for each of the active main sequence field stars studied by Strassmeier and collaborators where other ages are not available. These are shown to be mostly younger than 1Gyr, with a median age of 365Myr. The sample of single, late-type main sequence field stars assembled by Pizzolato and collaborators is then assessed, and shown to have gyro ages ranging from under 100Myr to several Gyr, and a median age of 1.2Gyr. Finally, we demonstrate that the individual components of the three wide binaries XiBooAB, 61CygAB, & AlphaCenAB yield substantially the same gyro ages.

Synthetic NLTE accretion disc spectra for the dwarf nova SS Cygni during an outburst cycle

Dwarf nova outbursts result from enhanced mass transport through the accretion disc of a cataclysmic variable system. We assess the question of whether these outbursts are caused by an enhanced mass transfer from the late-type main sequence star onto the white dwarf (so-called mass transfer instability model, MTI) or by a thermal instability in the accretion disc (disc instability model, DIM). We compute non-LTE models and spectra of accretion discs in quiescence and outburst and construct spectral time sequences for discs over a complete outburst cycle. We then compare our spectra to published optical spectroscopy of the dwarf nova SS Cygni. In particular, we investigate the hydrogen and helium line profiles that are turning from emission into absorption during the rise to outburst. The evolution of the hydrogen and helium line profiles during the rise to outburst and decline clearly favour the disc-instability model. Our spectral model sequences allow us to distinguish inside-out and outside-in moving heating waves in the disc of SS Cygni, which can be related to symmetric and asymmetric outburst light curves, respectively.

Color, Rotation, Age, and Chromospheric Activity Correlations in Late‐Type Main‐Sequence Stars

A study of the rotation-activity-age relationships in a large sample of single late-type main-sequence stars is carried out, based on rotation period data from the literature, on our measurements of the Mg II k line chromospheric flux in IUE high-resolution spectra, and on Hipparcos parallaxes. The analysis of the rotation period data of open clusters and field stars indicates that the rotational period decreases linearly with increasing mass in stars of the same age and evolves as a power of age as t0.45. This information has enabled us to disentangle the role of age in the chromospheric activity-rotation relationship, as well as to analyze the behavior of the Mg II emission flux in terms of activity evolutionary paths and of activity isochrones. It is shown that the Mg II emission maintains a saturated level in very young stars until a certain age that depends on stellar mass, while at later stages it decays at a faster rate for more massive stars. We have also determined the convection turnover time from observational data and analyzed the relationship between the Mg II emission flux and the Rossby number. We reach the conclusion that there is no clear advantage in using the Rossby number instead of stellar age in describing the chromospheric Mg II losses, at least with the convective turnover time values currently in use.

Lithium and Beryllium Depletions in Late-type Main-sequence Stars

Using a non-local stellar convection theory for chemically inhomogeneous stars, we have calculated the lithium and beryllium depletions in mainsequence models of 0.7-1.15 M ⊙ and have compared them with the observed depletions in open clusters of different ages. The results show that the general properties of lithium depletion in late type main-sequence stars with T eff < 5900K can be reproduced by convective overshooting mixing. We conclude that convective overshooting could be an important mechanism for the lithium depletion observed in late-type dwarfs.

The Na i D resonance lines in main-sequence late-type stars

We study the sodium D lines (D1: 5895.92 \AA; D2: 5889.95 \AA) in late-type dwarf stars. The stars have spectral types between F6 and M5.5 (B-V between 0.457 and 1.807) and metallicity between [Fe/H] = -0.82 and 0.6. We obtained medium resolution echelle spectra using the 2.15-m telescope at the argentinian observatory CASLEO. The observations have been performed periodically since 1999. The spectra were calibrated in wavelength and in flux. A definition of the pseudo-continuum level is found for all our observations. We also define a continuum level for calibration purposes. The equivalent width of the D lines is computed in detail for all our spectra and related to the colour index (B-V) of the stars. When possible, we perform a careful comparison with previous studies. Finally, we construct a spectral index (R_D') as the ratio between the flux in the D lines, and the bolometric flux. We find that, once corrected for the photospheric contribution, this index can be used as a chromospheric activity indicator in stars with a high level of activity. Additionally, we find that combining some of our results, we obtain a method to calibrate in flux stars of unknown colour.

The X-ray activity-rotation relation of T Tauri stars in Taurus-Auriga

Context. The Taurus-Auriga star-forming complex hosts the only population of T Tauri stars in which an anticorrelation of X-ray activity and rotation period has been observed. Aims. We aim to explain the origin of the X-ray activity‐rotation relation in Taurus-Auriga. We also aim to put the X-ray activity of these stars into the context of the activity of late-type mai n-sequence stars and T Tauri stars in the Orion Nebula Cluster. Methods. We have used XMM-Newton’s European Photon Imaging Cameras to perform the most sensi tive survey to date of Xray emission (0.3-10 keV) from young stars in Taurus-Auriga. We investigated the dependences of X-ray activity measures ‐ X-ray luminosity, LX, its ratio with the stellar luminosity, LX/L⋆, and the surface-averaged X-ray flux, FXS ‐ on rotation period and compared them with predictions based solely on the observed dependence of LX on a star’s L⋆ and whether it is accreting or not. We tested for differences in the distributions of LX/L⋆ of fast and slow rotators, accretors and non-accretors, and compared the dependence of LX/L⋆ on the ratio of the rotation period and the convective turnover timescale, the Rossby number, with that of late-type main-sequence stars. Results. We found significant anticorrelations of LX and FXS with rotation period, but these could be explained by the typically higher stellar luminosity and effective temperature of fast-rotators in Taurus-Auriga and a near-linear dependence of LX on L⋆. We found no evidence for a dependence of LX/L⋆ on rotation period, but for accretors to have lower LX/L⋆ than non-accretors at all rotation periods. The Rossby numbers of accretors and non-accretors were found to be the same as those of late-type main-sequence stars showing saturated X-ray emission. Conclusions. Non-accreting T Tauri stars show X-ray activity entirely consistent with the saturated activity of fast-rotating late -type main-sequence stars. Accreting T Tauri stars show lower X-ray activity, but this cannot be attributed to their slower ro tation.

Solar-Like Activity and Planetary Transits

Late-type MS stars display optical variability on different time scales due to the presence of photospheric brightness inhomogeneities produced by surface magnetic fields, analogous to cool spots and bright faculae observed in the Sun. We are developing methods to model the optical variability of main-sequence late-type stars to understand the impact of solar-like activity on the detection of planetary transits and to significantly improve the detection efficiency. Our techniques will also allow to map the longitude distribution of active regions on stars that harbor planets and to look for a possible connection between stellar activity and the presence of planets around a star.

A closer look at the X-ray transient XTE J1908+094: identification of two new near-infrared candidate counterparts

We had reported in Chaty, Mignani, Israel (2002) on the near-infrared (NIR) identification of a possible counterpart to the black hole candidate XTE J1908+094 obtained with the ESO/NTT. Here, we present new, follow-up, CFHT adaptive optics observations of the XTE J1908+094 field, which resolved the previously proposed counterpart in two objects separated by about 0.8. Assuming that both objects are potential candidate counterparts, we derive that the binary system is a low-mass system with a companion star which could be either an intermediate/late type (A-K) main sequence star at a distance of 3-10 kpc, or a late-type ($>$K) main sequence star at a distance of 1-3 kpc. However, we show that the brighter of the two objects (J ~ 20.1, H ~ 18.7, K' ~ 17.8) is more likely to be the real counterpart of the X-ray source. Its position is more compatible with our astrometric solution, and colours and magnitudes of the other object are not consistent with the lower limit of 3 kpc derived independently from the peak bolometric flux of XTE J1908+094. Further multi-wavelength observations of both candidate counterparts are crucial in order to solve the pending identification.

XMM-Newton spectroscopy of the metal depleted T Tauri star TWA 5

We present results of X-ray spectroscopy for TWA 5, a member of the young TW Hydrae association, observed with XMM-Newton. TWA~5 is a multiple system which shows Halpha emission, a signature typical of classical T Tauri stars, but no infrared excess. From the analysis of the RGS and EPIC spectra, we have derived the emission measure distribution vs. temperature of the X-ray emitting plasma, its abundances, and the electron density. The characteristic temperature and density of the plasma suggest a corona similar to that of weak-line T Tauri stars and active late-type main sequence stars. TWA 5 also shows a low iron abundance (~0.1 times the solar photospheric one) and a pattern of increasing abundances for elements with increasing first ionization potential reminiscent of the inverse FIP effect observed in highly active stars. The especially high ratio Ne/Fe~10 is similar to that of the classical T Tauri star TW Hya, where the accreting material has been held responsible for the X-ray emission. We discuss the possible role of an accretion process in this scenario. Since all T Tauri stars in the TW Hydrae association studied so far have very high Ne/Fe ratios, we also propose that environmental conditions may cause this effect.

The possibility of simulation of the chromospheric variations for main-sequence stars

We present a method of chromospheric flux simulation for 13 late-type main-sequence stars. These Sun-like stars have well-determined cyclic flux variations similar to 11 yr solar activity cycle. Our flux prediction is based on chromospheric HK emission time series measurements from Mount Wilson Observatory and comparable solar data. We show that solar three - component modeling explains well the stellar observations. We find that the 10 - 20% of K - stars disc surfaces are occupied by bright active regions.

WIYN Open Cluster Study. XVIII. The Tidal Circularization Cutoff Period of the Old Open Cluster NGC 188

Based on a sample of 22 main-sequence solar-type spectroscopic binaries, we find the tidal circularization cutoff in the open cluster NGC 188 (age 7 Gyr) to be well defined at a period of 15.0 days. When compared to shorter tidal circularization cutoff periods in the younger clusters M67 and the Hyades, this result establishes that tidal circularization processes are effective in solar-type main-sequence binaries for timescales longer than 1 Gyr. This result is in contrast to the theoretical prediction by Zahn & Bouchet that tidal circularization due to equilibrium tides is ineffective for late-type main-sequence stars, and that tidal circularization cutoff periods are instead set only by pre-main-sequence tidal circularization. The NGC 188 cutoff period is also longer than can be explained by present theories of tidal circularization due to dynamical tides.

FU Orionis: A Binary Star?

By using the ALFA adaptive optics system at the 3.6m telescope of the Calar Alto Observatory we detected a faint red star in the apparent vicinity of FU Ori, the prototype of the FUor outburst stars. Independent confirmation of the detection is obtained from archival PUEO/CFHT images. The separation between the companion candidate and FU Ori is 0.50 and their brightness contrast is around 4 magnitudes. We discuss the possible nature of the newly detected star based on near-infrared photometry and its proper motion relative to FU Ori. The photometric data are consistent with a nearby late-type main sequence star, a background giant star, and a pre-main sequence star. On the basis of the proper motion and the stellar surface density in the direction towards FU Ori, we argue that the probabilities of the first two options are very low.

The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs

We present the results of a new study on the relationship between coronal X-ray emission and stellar rotation in late-type main-sequence stars. We have selected a sample of 259 dwarfs in the range 0.5–2.0, including 110 field stars and 149 members of the Pleiades, Hyades, α Persei, IC 2602 and IC 2391 open clusters. All the stars have been observed with ROSAT, and most of them have photometrically-measured rotation periods available. Our results confirm that two emission regimes exist, one in which the rotation period is a good predictor of the total X-ray luminosity, and the other in which a constant saturated X-ray to bolometric luminosity ratio is attained; we present a quantitative estimate of the critical rotation periods below which stars of different masses (or spectral types) enter the saturated regime. In this work we have also empirically derived a characteristic time scale, , which we have used to investigate the relationship between the X-ray emission level and an X-ray-based Rossby number : we show that our empirical time scale resembles the theoretical convective turnover time for , but it also has the same functional dependence on as in the color range . Our results imply that – for non-saturated coronae – the Lx – Prot relation is equivalent to the vs. Re relation.

Acoustic and magnetic wave heating in stars

In the first paper of this series we developed a method to construct theoretical, time-dependent and two-component chromosphere models for late-type main sequence stars. The models consist of non-magnetic regions heated by acoustic waves and magnetic flux tube regions heated by magnetic tube waves. By specifying the magnetic filling factor, theoretical models of stellar atmospheres with different chromospheric activity can be calculated. Here, these models are used to simulate the emerging Ca II and Mg II chromospheric emission fluxes and compare them with observations. The comparison shows that the wave heating alone can explain most but not all of the observed range of chromospheric activity. In addition, the results obtained clearly imply that the base of stellar chromospheres is heated by acoustic waves, the heating of the middle and upper chromospheric layers is dominated by magnetic waves associated with magnetic flux tubes, and that other non-wave (e.g., reconnective) heating mechanisms are required to explain the structure of the highest layers of stellar chromospheres.

A ROSAT medium-sensitivity Galactic plane survey at 180 &lt; l &lt; 280

We have performed a moderately deep soft X-ray (0.1–2 keV) survey of the Galactic plane using pointed observations with the ROSAT Position Sensitive Proportional Counter (PSPC). The survey is more than an order of magnitude more sensitive than previous X-ray surveys near the Galactic plane. The data consist of nine fields each of ≈10 ks exposure, pointed at positions on or very close to the Galactic plane (ǀbǀ<0°.3) in the longitude range 180°≲l≲280°. This region has relatively low X-ray absorbing material out to distances of several hundred pc and presents fewer source-confusion problems than at other longitudes. The total sky area surveyed was 2.5 deg2; this yielded 93 sources, 89 of which were detected in the ‘hard’(0.4–2.0 keV) band. Nine sources were detected in both ‘soft’(0.1–0.4 keV) and hard bands. In the hard band, the survey coverage is ≳90 per cent for sources brighter than 0.002 count s−1(∼2×10−14 erg cm−2 s−1), but falls steeply below this value, with the weakest sources being ∼0.001 count s−1. The median limiting flux is ≈0.0013 count s−1(∼1.3×10−14erg cm−2 s−1). There are 64 sources with hard-band count rates > 0.002 count s−1. We present the catalogue of X-ray sources and the number–flux relations (log N–log S). Eighteen sources have possible identifications from the SIMBAD data base. We have searched the Tycho-2 and USNO-A2.0 catalogues to find all possible optical counterparts brighter than ∼ 19th magnitude, and attempt to classify these on the basis of log (fX/fopt) versus optical colour diagrams and near-infrared photometry from the 2MASS Second Incremental Data Release. Hence, we have found the majority of these sources to be consistent with being late-type main-sequence stars, as previous studies have proposed from incompletely identified surveys. Comparison of the measured number–flux relations with predictions of Galactic (stellar) and extragalactic populations supports the view that the population of young stars in the plane is denser than previously thought.

A new look at the relationship between activity, dynamo number and Rossby number in late-type stars

ABSTRA C T The correlation between stellar activity, as measured by the indicator DRHK, and the Rossby number Ro in late-type stars is revisited in light of recent developments in solar dynamo theory. Different stellar interior models, based on both mixing-length theory and the full spectrum of turbulence, are used in order to see to what extent the correlation of activity with Rossby number is model dependent, or otherwise can be considered universal. Although we find some modest model dependence, we find that the correlation of activity with Rossby number is significantly better than with rotation period alone for all the models we consider. Dynamo theory suggests that activity should scale with the dynamo number. A current model of the solar dynamo, the so-called interface dynamo, proposes that the amplification of the toroidal magnetic field by differential rotation (the v-effect) and the production of the poloidal magnetic field from toroidal by helical turbulence (the a-effect) take place in different, adjacent layers near the base of the convection zone. A new scale analysis based on the interface dynamo shows that the appropriate dynamo number does not depend on the Rossby number alone, but also depends on an additional dimensionless factor related to the differential rotation. This leads to a new interpretation of the correlation between activity and Rossby number, which in turn leads to some conclusions about the magnitude of differential rotation in the dynamo layers of late-type main-sequence stars.

Simultaneous ROSAT XRT and WFC observations of a sample of active dwarf stars

The X-ray observations of the ROSAT-PSPC All-Sky Survey have revealed bright and energetic coronae for a number of late-type main-sequence stars, many of them flare stars. We have detected 31 X-ray flares on 14 stars. A search for simultaneous X-ray and EUV (extreme ultraviolet) flares using ROSAT Wide Field Camera survey data revealed a large number of simultaneous flares. These results indicate that the heating mechanisms of the X-ray and EUV-emitting regions of the stellar coronae are similar. We find X-ray quiescent variability for nine of the 14 stars and simultaneous X-ray and EUV quiescent variability for seven of these nine stars. These results imply that the stellar coronae are in a continuous state of low-level activity. There are tight linear correlations of X-ray flare luminosity with the ‘quiescent’ X-ray as well as with the stellar bolometric luminosity. The similarity between the X-ray-to-EUV quiescent and flare luminosity ratios suggests that the two underlying spectra are also similar. Both are indeed consistent with the previously determined Einstein two-temperature models. We suggest that both the variability and spectral results could indicate that the quiescent emission is composed of a multitude of unresolved flares.