- ID:
- ivo://CDS.VizieR/J/ApJ/864/57
- Title:
- Abundances of field & GC RR Lyrae. I. NGC3201
- Short Name:
- J/ApJ/864/57
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a detailed spectroscopic analysis of horizontal branch stars in the globular cluster NGC 3201. We collected optical (4580-5330{AA}), high-resolution (~34000), high signal-to-noise ratio (~200) spectra for 11 RR Lyrae stars and one red horizontal branch star with the multifiber spectrograph M2FS with the 6.5m Magellan telescope at the Las Campanas Observatory. From measured equivalent widths, we derived atmospheric parameters and abundance ratios for {alpha} (Mg, Ca and Ti), iron-peak (Sc, Cr, Ni and Zn), and s-process (Y) elements. We found that NGC 3201 is a homogeneous, monometallic ([Fe/H]=-1.47+/-0.04), {alpha}-enhanced ([{alpha}/Fe]=0.37+/-0.04) cluster. The relative abundances of the iron-peak and s-process elements were found to be consistent with solar values. In comparison with other large stellar samples, NGC 3201 RR Lyraes have similar chemical enrichment histories as do those of other old (t>=10Gyr) Halo components (globular clusters; red giants; blue and red horizontal branch stars; and RR Lyraes). We also provided a new average radial velocity estimate for NGC 3201 by using a template velocity curve to overcome the limit of single-epoch measurements of variable stars: Vrad=494+/-2km/s ({sigma}=8km/s).
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/782/59
- Title:
- Abundances of 8 RR Lyrae subclass C variable stars
- Short Name:
- J/ApJ/782/59
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a detailed chemical abundance study of eight RR Lyrae variable stars of subclass c (RRc). The target RRc stars chosen for study exhibit "Blazhko-effect" period and amplitude modulations to their pulsational cycles. Data for this study were gathered with the echelle spectrograph of the 100 inch du Pont telescope at Las Campanas Observatory. Spectra were obtained throughout each star's pulsation cycle. Atmospheric parameters--effective temperature, surface gravity, microturbulent velocity, and metallicity--were derived at multiple phase points. We found metallicities and element abundance ratios to be constant within observational uncertainties over the pulsational cycles of all stars. Moreover, the {alpha}-element and Fe-group abundance ratios with respect to iron are consistent with other horizontal-branch members (RRab, blue and red non-variables). Finally, we have used the [Fe/H] values of these eight RRc stars to anchor the metallicity estimates of a large-sample RRc snapshot spectroscopic study being conducted with the same telescope and instrument combination employed here.
- ID:
- ivo://CDS.VizieR/J/A+A/426/1007
- Title:
- Abundances of 8 single-lined active binaries
- Short Name:
- J/A+A/426/1007
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report the determination from high-resolution spectra of the atmospheric parameters and abundances of 13 chemical species (among which lithium) in 8 single-lined active binaries. These data are combined with our previous results for 6 other RS CVn systems to examine a possible relationship between the photospheric abundance patterns and the stellar activity level. The stars analyzed are generally found to exhibit peculiar abundance ratios compared to inactive, galactic disk stars of similar metallicities. We argue that this behaviour is unlikely an artefact of errors in the determination of the atmospheric parameters or non-standard mixing processes along the red giant branch, but diagnoses instead the combined action of various physical processes related to activity. The most promising candidates are cool spot groups covering a very substantial fraction of the stellar photosphere or NLTE effects arising from nonthermal excitation. However, we cannot exclude the possibility that more general shortcomings in our understanding of K-type stars (e.g. inadequacies in the atmospheric models) also play a significant role. Lastly, we call attention to the unreliability of the (V-R) and (V-I) colour indices as temperature indicators in chromospherically active stars.
- ID:
- ivo://CDS.VizieR/J/PAZh/37/781
- Title:
- Abundances of 7 southern-hemisphere Cepheids
- Short Name:
- J/PAZh/37/781
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- For seven faint southern Cepheids (WW Car, SX Car, UZ Car, UY Car, GX Car, HW Car, YZ Car), we have determined their atmospheric parameters and chemical composition for the first time based on ten high-resolution (R=50000) spectra taken with the 1.5-m Hexapod telescope at the Joint Observatory of the Northern Catholic University (Antofagasta, Chile) and the Ruhr University (Bochum, Germany). Six objects from the list demonstrate atmospheric parameters and chemical composition typical of Cepheids that have passed through the first dredge-up phase, while WW Car is probably an anomalous Cepheid. According to our preliminary estimates, it has an overabundance of CNO, a deficit of sodium and aluminium, and a slight deficit of magnesium, with iron and other elements being underabundant relative to the Sun.
- ID:
- ivo://CDS.VizieR/J/A+A/566/A118
- Title:
- A comprehensive view of Virgo stellar stream
- Short Name:
- J/A+A/566/A118
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- To explore the complex halo substructure that has been reported in the direction of the Virgo constellation, radial velocities and metallicities have been measured for 82 RR Lyrae stars (RRLS) that were identified by the QUEST survey. These stars are distributed over 90 sq. deg. of the sky, and lie from 4 to 23kpc from the Sun. Using an algorithm for finding groups in phase space and modeling the smooth halo component in the region, we identified the 5 most significant RRLS groups, some of which were previously known or suspected. We have examined the SEKBO and the Catalina catalog of RRLS (with available spectroscopic measurements), as well as the bright QUEST RRLS sample, the catalog of Red Giant stars from the Spaghetti survey, and three recent catalogs of blue horizontal branch (BHB) stars, for stars that may be related to the QUEST RRLS groups.
- ID:
- ivo://CDS.VizieR/J/AJ/127/2738
- Title:
- ACS VI photometry of M31 halo RR Lyrea
- Short Name:
- J/AJ/127/2738
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a complete census of RR Lyrae stars in a halo field of the Andromeda galaxy. These deep observations, taken as part of a program to measure the star formation history in the halo, spanned a period of 41 days with sampling on a variety of timescales, enabling the identification of short- and long-period variables. Although the long-period variables cannot be fully characterized within the time span of this program, the enormous advance in sensitivity provided by the Advanced Camera for Surveys on the Hubble Space Telescope allows accurate characterization of the RR Lyrae population in this field. We find 29 RRab stars with a mean period of 0.594 days, 25 RRc stars with a mean period of 0.316 days, and one RRd star with a fundamental period of 0.473 days and a first-overtone period of 0.353 days. These 55 RR Lyrae stars imply a specific frequency S_RR_~5.6, which is large given the high mean metallicity of the halo, but not surprising given that these stars arise from the old, metal-poor tail of the distribution.
- ID:
- ivo://CDS.VizieR/J/A+A/583/A65
- Title:
- Active Kepler stars differential rotation
- Short Name:
- J/A+A/583/A65
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In addition to the discovery of hundreds of exoplanets, the high-precision photometry from the CoRoT and Kepler satellites has led to measurements of surface rotation periods for tens of thousands of stars, which can potentially be used to infer stellar ages via gyrochronology. Our main goal is to derive ages of thousands of field stars using consistent rotation period measurements derived by different methods. Multiple rotation periods are interpreted as surface differential rotation (DR). We study the dependence of DR with rotation period and effective temperature. We reanalyze a previously studied sample of 24,124 Kepler stars using different approaches based on the Lomb- Scargle periodogram. Each quarter (Q1-Q14) is treated individually using a prewhitening approach. Additionally, the full time series and their different segments are analyzed.
- ID:
- ivo://CDS.VizieR/J/A+A/603/A52
- Title:
- Activity cycles in 3203 Kepler stars
- Short Name:
- J/A+A/603/A52
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In recent years it has been claimed that the length of stellar activity cycles is determined by the stellar rotation rate. It has been observed that the cycle period increases with rotation period along two distinct sequences, known as the active and inactive sequences. In this picture the Sun occupies a solitary position between the two sequences. Whether the Sun might undergo a transitional evolutionary stage is currently under debate. Our goal is to measure cyclic variations of the stellar light curve amplitude and the rotation period using four years of Kepler data. Periodic changes in the light curve amplitude or the stellar rotation period are associated with an underlying activity cycle. Using a recent sample of active stars we compute the rotation period and the variability amplitude for each individual Kepler quarter and search for periodic variations of both time series. To test for periodicity in each stellar time series we consider Lomb-Scargle periodograms and use a selection based on a false alarm probability (FAP). We detect amplitude periodicities in 3203 stars between 0.5-6 years covering rotation periods between 1-40 days. Given our sample size of 23,601 stars and our selection criteria that the FAP is less than 5%, this number is almost three times higher than that expected from pure noise. We do not detect periodicities in the rotation period beyond those expected from noise. Our measurements reveal that the cycle period shows a weak dependence on rotation rate, slightly increasing for longer rotation periods. We further show that the shape of the variability deviates from a pure sine curve, consistent with observations of the solar cycle. The cycle shape does not show a statistically significant dependence on effective temperature. We detect activity cycles in more than 13% of our final sample with a FAP of 5% (calculated by randomly shuffling the measured 90-day variability measurements for each star). Our measurements do not support the existence of distinct sequences in the Prot-Pcyc plane, although there is some evidence for the inactive sequence for rotation periods between 5-25 days. Unfortunately,the total observing time is too short to draw sound conclusions on activity cycles with similar lengths to that of the solar cycle.
- ID:
- ivo://CDS.VizieR/J/MNRAS/431/2240
- Title:
- Activity in A-type stars from Kepler
- Short Name:
- J/MNRAS/431/2240
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Two years of Kepler data are used to investigate low-frequency variations in A-type stars. In about 875 (40%) A-type stars, the periodogram shows a simple peak and its harmonic. If we assume that the photometric period is the period of rotation, we can derive the equatorial rotational velocity from a suitable radius estimate. It turns out that the distribution of equatorial velocities derived in this way is similar to the distribution of equatorial velocities of A-type main-sequence stars in the general field derived from spectroscopic line broadening, verifying our initial assumption. We suggest that the light variation is due to rotational modulation caused by starspots or some other corotating structure. In many stars the rotation peak in the periodogram has a characteristic shape which is not understood. The light amplitudes are highly variable. We deduce from the amplitude distribution that the sizes of starspots in A-type stars are similar to the largest sunspots. From the widths of the peaks in the periodogram we deduce that differential rotation in these stars is similar to that in the Sun. We find that the period-colour relationship used for gyrochronology in late-type stars extends to early F-type and probably late A-type stars as well. Flares in A-type stars have been recently detected. We add 13 additional A-type flare stars to this sample, which means that about 1.5% of A-type stars in the Kepler field show flares. We conclude that A-type stars are active and, like cooler stars, have starspots and flares.
- ID:
- ivo://CDS.VizieR/J/A+A/621/A21
- Title:
- Activity of Kepler stars
- Short Name:
- J/A+A/621/A21
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The study of stellar activity cycles is crucial to understand the underlying dynamo and how it causes magnetic activity signatures such as dark spots and bright faculae. Having knowledge about the dominant source of surface activity might allow us to draw conclusions about the stellar age and magnetic field topology, and to put the solar cycle in context. We investigate the underlying process that causes magnetic activity by studying the appearance of activity signatures in contemporaneous photometric and chromospheric time series. Lomb-Scargle periodograms are used to search for cycle periods present in the photometric and chromospheric 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 to be 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 >~2.55Gyr. The data further suggest that the Vaughan-Preston gap cannot explain the previously detected dearth of Kepler rotation periods between 15 and 25 days. Nevertheless, our results led us to propose an explanation for the lack of rotation periods to be due to the non-detection of periodicity caused by the cancelation of dark spots and bright faculae at ~800Myr.
