Rotation modulation of Kepler light curves in mid-A to late-B stars is shown to be present. This is demonstrated by the high correlation of projected rotational velocities with photometric frequencies in 30 stars. The time-frequency diagrams show stochastic variations in all respects similar to those in spotted cool stars. This disposes of any explanation in terms of binary proximity effects. More than half of the sample of stars with effective temperatures in the range of 8300-12000K show rotational modulation, indicating that starspots are the rule rather than the exception among A stars. The periodograms of a subset of these stars show a characteristic pattern in which a broad peak is flanked by a sharp peak at a slightly higher frequency. It is demonstrated that the sharp peak has the same width as the spectral window, indicating a stable period over the duration of the 4-yr Kepler observations. It is speculated that this might be a signature of a reflection effect in a non-transiting planet. These observations suggest that the presence of localized magnetic fields in A and B stars and that current views of radiative stellar envelopes need to be revised.
Stellar X-ray emission plays an important role in the study of exoplanets as a proxy for stellar winds and as a basis for the prediction of extreme ultraviolet (EUV) flux, unavailable for direct measurements, which in their turn are important factors for the mass-loss of planetary atmospheres. Unfortunately, the detection thresholds limit the number of stars with the directly measured X-ray fluxes. At the same time, the known connection between the sunspots and X-ray sources allows using of the starspot variability as an accessible proxy for the stellar X-ray emission. To realize this approach, we analysed the light curves of 1729 main-sequence stars with rotation periods 0.5<P<30d and effective temperatures 3236<Teff<7166K observed by the Kepler mission. It was found that the squared amplitude of the first rotational harmonic of a stellar light curve may be used as a kind of activity index. This averaged index revealed practically the same relation with the Rossby number as that in the case of the X-ray to bolometric luminosity ratio R_x_. As a result, the regressions for stellar X-ray luminosity L_x_(P, T_eff_) and its related EUV analogue L_EUV_ were obtained for the main-sequence stars. It was shown that these regressions allow prediction of average (over the considered stars) values of log(L_x_) and log(L_EUV_) with typical errors of 0.26 and 0.22dex, respectively. This, however, does not include the activity variations in particular stars related to their individual magnetic activity cycles.
A list of the 2027 stars that have the largest photometric amplitudes in Hipparcos Photometry shows that the most variables stars are all Miras. The percentage of variable types change as a function of amplitude. This compilation should also be of value to photometrists looking for relatively unstudied, but large amplitude stars.
The Chinese Small Telescope Array (CSTAR) carried out high-cadence time-series observations of ~20.1 square degrees centered on the South Celestial Pole during the 2008, 2009, and 2010 winter seasons from Dome A in Antarctica. The nearly continuous six months of dark conditions during each observing season allowed for >10^6^ images to be collected through gri and clear filters, resulting in the detection of >10^4^ sources over the course of three years of operation. The nearly space-like conditions in the Antarctic plateau are an ideal testbed for the suitability of very small-aperture (<20cm) telescopes to detect transient events, variable stars, and stellar flares. We present the results of a robust search for such objects using difference image analysis of the data obtained during the 2009 and 2010 winter seasons. While no transients were found, we detected 29 flaring events and find a normalized flaring rate of 5+/-4*10^-7^flare/hr for late-K dwarfs, 1+/-1*10^-6^flare/hr for M dwarfs and 7+/-1*10^-7^flare/hr for all other stars in our sample. We suggest future small-aperture telescopes planned for deployment at Dome A would benefit from a tracking mechanism, to help alleviate effects from ghosting, and a finer pixel scale, to increase the telescope's sensitivity to faint objects. We find that the light curves of non-transient sources have excellent photometric qualities once corrected for systematics, and are limited only by photon noise and atmospheric scintillation.
Stellar population of the young star cluster NGC 6231
Short Name:
J/AJ/154/87
Date:
21 Oct 2021
Publisher:
CDS
Description:
NGC 6231 is a young cluster (age ~2-7 Myr) dominating the Sco OB1 association (distance ~1.59 kpc) with ~100 O and B stars and a large pre-main-sequence stellar population. We combine a reanalysis of archival Chandra X-ray data with multiepoch near-infrared (NIR) photometry from the VISTA Variables in the Via Lactea (VVV) survey and published optical catalogs to obtain a catalog of 2148 probable cluster members. This catalog is 70% larger than previous censuses of probable cluster members in NGC 6231. It includes many low-mass stars detected in the NIR but not in the optical and some B stars without previously noted X-ray counterparts. In addition, we identify 295 NIR variables, about half of which are expected to be pre-main-sequence stars. With the more complete sample, we estimate a total population in the Chandra field of 5700-7500 cluster members down to 0.08 M_{sun}_ (assuming a universal initial mass function) with a completeness limit at 0.5 M_{sun}_. A decrease in stellar X-ray luminosities is noted relative to other younger clusters. However, within the cluster, there is little variation in the distribution of X-ray luminosities for ages less than 5 Myr. The X-ray spectral hardness for B stars may be useful for distinguishing between early-B stars with X-rays generated in stellar winds and B-star systems with X-rays from a pre-main-sequence companion (>35% of B stars). A small fraction of catalog members have unusually high X-ray median energies or reddened NIR colors, which might be explained by absorption from thick or edge-on disks or being background field stars.
We present the results of a five month photometric time-series survey for stellar rotation over a 40'x40' field centered on the 150Myr open cluster M35 (=NGC 2168). We report rotation periods for 441 stars within this field and determine their cluster membership and binarity based on a decade-long radial velocity survey, proper-motion measurements, and multiband photometric observations. We find that 310 of the stars with measured rotation periods are late-type members of M35. The distribution of rotation periods for cluster members span more than 2 orders of magnitude from ~0.1 to 15 days, not constrained by the sampling frequency and the timespan of the survey. With an age between the zero-age main sequence and the Hyades, and with ~6 times more rotation periods than measured in the Pleiades, M35 permit detailed studies of early rotational evolution of late-type stars. Nearly 80% of the 310 rotators lie on two distinct sequences in the color-period plane, and define clear relations between stellar rotation period and color (mass). The M35 color-period diagram enables us to determine timescales for the transition between the two rotational states, of ~60Myr and ~140Myr for G and K dwarfs, respectively.
Rotation period measurements of stars observed with the Kepler mission have revealed a lack of stars at intermediate rotation periods, accompanied by a decrease of photometric variability. Whether this so-called dearth region is a peculiarity of stars in the Kepler field, or reflects a general manifestation of stellar magnetic activity, is still under debate. The K2 mission has the potential to unravel this mystery by measuring stellar rotation and photometric variability along different fields in the sky. Our goal is to measure stellar rotation periods and photometric variabilities for tens of thousands of K2 stars, located in different fields along the ecliptic plane, to shed light on the relation between stellar rotation and photometric variability. We use Lomb-Scargle periodograms, auto-correlation and wavelet functions to determine consistent rotation periods. Stellar brightness variability is assessed by computing the variability range, R_var_, from the light curve. We further apply Gaussian mixture models to search for bimodality in the rotation period distribution. Combining measurements from all K2 campaigns, we detect rotation periods in 29860 stars. The reliability of these periods was estimated from stars observed more than once. We find that 75-90% of the stars show period deviation smaller than 20% between different campaigns, depending on the peak height threshold in the periodograms. For effective temperatures below 6000K, the variability range shows a local minimum at different periods, consistent with an isochrone age of ~750Myr. Additionally, the rotation period distribution shows evidence for bimodality, although the dearth region in the K2 data is less pronounced compared to the Kepler field. The period at the dip of the bimodal distribution shows good agreement with the period at the local variability minimum. We conclude that the rotation period bimodality is present in different fields of the sky, and is hence a general manifestation of stellar magnetic activity. The reduced variability in the dearth region is interpreted as a cancelation between dark spots and bright faculae. Our results strongly advocate that the role of faculae has been underestimated so far, suggesting a more complex dependence of the brightness variability on the rotation period.
We present results from a 5 night wide-field time-series photometric survey that detects variable field stars. We find that the fraction of stars whose light curves show variations depends on color and magnitude, reaching 17% for the brightest stars in this survey (V~4) for which the photometric precision is best. The fraction of stars found to be variable is relatively high at colors bluer than the Sun and relatively low at colors similar to the Sun and increases again for stars redder than the Sun. We present light curves for a sample of the pulsating and eclipsing variables. Most of the stars identified as pulsating variables have low amplitudes ({Delta}V=0.01-0.05), relatively blue colors, and multiple periods. There are 13 stars we identify as either SX Phoenicis or {delta} Scuti stars. These classes represent a significant contribution to the total number of blue variables found in this survey. Another 17 stars are identified as eclipsing variables, which have a wide range in color, magnitude, and amplitude. Two variable giants are observed, and both show night-to-night ~1% variations. We present data for 222 variables in total, most of which are not classified. Implications of surveys for stellar variability and interferometry are briefly discussed. On 2000 March 16-20 UT we observed a time series of images in V and one or two images each in UBRI toward two 59'x59' fields using the NOAO Mosaic Camera at the Kitt Peak 0.9 m telescope.
We report the results of a diffraction-limited, photometric variability study of the central 5"x5" of the Galaxy conducted over the past 10-years using speckle imaging techniques on the W.M. Keck I 10 m telescope. Within our limiting magnitude of m_K_<16mag for images made from a single night of data, we find a minimum of 15K[2.2um]-band variable stars out of 131 monitored stars. The only periodic source in our sample is the previously identified variable IRS 16SW, for which we measure an orbital period of 19.448+/-0.002-days.
The Table "stars.dat" contains the catalogue of variable stars discovered and/or observed for the 12 low-extinction fields towards the galactic bulge, with the IAG/USP Meridian Circle. The first column of the catalog indicates the star label, formed by a name that identifies the window (see Table 1) and a sequential number that indicates the position of the object in the corresponding database. The following columns display successively the mean right ascension, the mean declination, their standard deviations (in seconds and in arcseconds, respectively) (J2000); the mean magnitude observed (m_{Val}), the difference between the maximum and the minimum magnitude value observed, the number of observations, a estimative for the period, if possible, or a indication "NF" when the stars show periodic characteristics but we aren't able to found a period, or "NC" when the star can be aperiodic or have few observations. The next column have the tentative classification and the last are the remarks about the previous known variables. Finding charts will be available upon request from the authors.
