- ID:
- ivo://CDS.VizieR/J/AJ/160/19
- Title:
- 827 ultracool dwarfs with K2
- Short Name:
- J/AJ/160/19
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The occurrence of planets orbiting ultracool dwarfs is poorly constrained. We present results from a guest observer program on NASA's K2 spacecraft to search for transiting planets orbiting a sample of 827 ultracool dwarfs. Having found no transiting planets in our sample, we determined an upper limit on the occurrence of planets. We simulated planets orbiting our sample for a range of orbital periods and sizes. For the simulated planets that transit their host, we injected the transit light curve into the real K2 light curves, then attempted to recover the injected planets. For a given occurrence rate, we calculated the probability of seeing no planets, and use the results to place an upper limit on planet occurrence as a function of planet radius and orbital period. We find that short-period, mini-Neptune and Jupiter-sized planets are rare around ultracool dwarfs, consistent with results for early and mid-type M dwarf stars. We constrain the occurrence rate {eta} for planets between 0.5 and 10 R{earth} with orbital periods between 1 and 26.3days.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/564/A55
- Title:
- 1.1-2.4um spectra of 7 young M and L dwarfs
- Short Name:
- J/A+A/564/A55
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Gravity modifies the spectral features of young brown dwarfs (BDs). A proper characterization of these objects is crucial for the identification of the least massive and latest-type objects in star-forming regions, and to explain the origin(s) of the peculiar spectrophotometric properties of young directly imaged extrasolar planets and BD companions. We obtained medium-resolution (R~1500-1700) near-infrared (1.1-2.5um) spectra of seven young M9.5-L3 dwarfs classified at optical wavelengths. We aim to empirically confirm the low surface gravity of the objects in the near-infrared. We also test whether self-consistent atmospheric models correctly represent the formation and the settling of dust clouds in the atmosphere of young late-M and L dwarfs. We used the Infrared Spectrometer And Array Camera (ISAAC) at the Very Large Telescope (VLT) to obtain the spectra of the targets. We compared the spectra to those of mature and young BDs, and to young late-type companions to nearby stars with known ages, to identify and study gravity-sensitive features. We computed spectral indices weakly sensitive to the surface gravity to derive near-infrared spectral types. Finally, we found the best fit between each spectrum and synthetic spectra from the BT-Settl 2010 and 2013 atmospheric models. Using the best fit, we derived the atmospheric parameters of the objects and identified which spectral characteristics the models do not reproduce.
- ID:
- ivo://CDS.VizieR/J/A+A/640/A66
- Title:
- Updated X-ray view of the Hyades cluster
- Short Name:
- J/A+A/640/A66
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We revisit the X-ray properties of the main sequence Hyades members and the relation between X-ray emission and stellar rotation. As an input catalog for Hyades members, we combined three recent membership lists derived from Gaia DR2 data that include the Hyades core and its tidal tails. We searched for X-ray detections of the main sequence Hyades members in the ROSAT all-sky survey (RASS), and pointings from ROSAT, the Chandra X-Ray Observatory, and XMM-Newton. Furtmermore, we adopted rotation periods derived from Kepler's K2 mission and other resources. We find an X-ray detection for 281 of 1066 bona fide main sequence Hyades members and provide statistical upper limits for the undetected sources. The majority of the X-ray detected stars are located in the Hyades core because of its generally smaller distance to the sun. F- and G-type stars have the highest detection fraction (72%), while K- and M-type dwarfs have lower detection rates (22 %). The X-ray luminosities of the detected members range from about 2x10^27^ for late M-type dwarfs to approximately 2x10^30^erg/s for active binaries. The X-ray luminosity distribution functions formally differ for the members in the core and tidal tails, which is likely caused by a larger fraction of field stars in our Hyades tails sample. Compared to previous studies, our sample is slightly fainter in X-rays due to differences in the Hyades membership list used; furthermore, we extend the X-ray luminosity distribution to fainter luminosities. The X-ray activity of F- and G-type stars is well defined at F_X_/F_bol_=10^-5^. The fractional X-ray luminosity and its spread increases to later spectral types reaching the saturation limit (F_X_/F_bol_=10^-3^) for members later than spectral type M3. Confirming previous results, the X-ray flux varies by less than a factor of three between epochs for the 104 Hyades members with multiple epoch data, significantly less than expected from solar-like actvity cycles. Rotation periods are found for 204 Hyades members, with about half of them being detected in X-rays. The activity-rotation relation derived for the coeval Hyades members has properties very similar to those obtained by other authors investigating stars of different ages.
- ID:
- ivo://CDS.VizieR/J/MNRAS/431/2063
- Title:
- UV/X-ray activity of M dwarfs within 10pc
- Short Name:
- J/MNRAS/431/2063
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- M dwarfs are the most numerous stars in the Galaxy. They are characterized by strong magnetic activity. The ensuing high-energy emission is crucial for the evolution of their planets and the eventual presence of life on them. We systematically study the X-ray and ultraviolet emission of a subsample of M dwarfs from a recent proper-motion survey, selecting all M dwarfs within 10pc to obtain a nearly volume-limited sample (~90 percent completeness). Archival ROSAT, XMM-Newton and GALEX data are combined with published spectroscopic studies of H{alpha} emission and rotation to obtain a broad picture of stellar activity on M dwarfs. We make use of synthetic model spectra to determine the relative contributions of photospheric and chromospheric emission to the ultraviolet flux. We also analyse the same diagnostics for a comparison sample of young M dwarfs in the TW Hya association (~10Myr). We find that generally the emission in the GALEX bands is dominated by the chromosphere but the photospheric component is not negligible in early-M field dwarfs. The surface fluxes for the H{alpha}, near-ultraviolet, far-ultraviolet and X-ray emission are connected via a power-law dependence. We present here for the first time such flux-flux relations involving broad-band ultraviolet emission for M dwarfs. Activity indices are defined as flux ratio between the activity diagnostic and the bolometric flux of the star in analogy to the CaII R'HK index. For given spectral type, these indices display a spread of 2-3dex which is largest for M4 stars. Strikingly, at mid-M spectral types, the spread of rotation rates is also at its highest level. The mean activity index for fast rotators, likely representing the saturation level, decreases from X-rays over the FUV to the NUV band and H{alpha}, i.e. the fractional radiation output increases with atmospheric height. The comparison to the ultraviolet and X-ray properties of TWHya members shows a drop of nearly three orders of magnitude for the luminosity in these bands between ~10Myr and few Gyr age. A few young field dwarfs (<1Gyr) in the 10-pc sample bridge the gap indicating that the drop in magnetic activity with age is a continuous process. The slope of the age decay is steeper for the X-ray than for the UV luminosity.
- ID:
- ivo://CDS.VizieR/J/A+AS/126/21
- Title:
- Velocities and duplicity of F-type dwarfs
- Short Name:
- J/A+AS/126/21
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present new radial and rotational velocities for 595 nearby early F dwarfs, based on digital spectra cross-correlated with individually optimised synthetic template spectra. The selection of optimum templates, the determination of rotational velocities, and the extraction of velocities from the blended spectra of double-lined spectroscopic binaries are discussed in some detail. We find 170 spectroscopic binaries in the sample and determine orbits for 18 double-lined and 2 single-lined binaries, including some spectroscopic triples. 73 stars are listed with too rapid rotation to yield useful radial velocities (i.e. vsini>120km/s). We discuss the binary frequency in the sample, and the influence of unrecognised binaries on the definition of clean metallicity groups of young F dwarfs and the determination of their kinematical properties.
- ID:
- ivo://CDS.VizieR/J/A+A/485/95
- Title:
- Velocities of F-G-K stars in Blanco 1
- Short Name:
- J/A+A/485/95
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The nearby open cluster Blanco 1 is of considerable astrophysical interest for formation and evolution studies of open clusters because it is the third highest Galactic latitude cluster known. It has been observed often, but so far no definitive and comprehensive membership determination is readily available. An observing programme was carried out to study the stellar population of Blanco 1, and especially the membership and binary frequency of the F5-K0 dwarfs. We obtained radial-velocities with the CORAVEL spectrograph in the field of Blanco 1 for a sample of 148 F-G-K candidate stars in the magnitude range 10<V<14. New proper motions and UBVI CCD photometric data from two extensive surveys were obtained independently and are used to establish reliable cluster membership assignments in concert with radial-velocity data. The membership of 68 stars is confirmed on the basis of proper motion, radial velocity, and photometric criteria. Fourteen spectroscopic- and suspected binaries (2 SB2s, 9 SB1s, 3SB?) have been discovered among the confirmed members. Thirteen additional stars are located above the main sequence or close to the binary ridge, with radial velocities and proper motions supporting their membership. These are probable binaries with wide separations. Nine binaries (7 SB1 and 2 SB2) were detected among the field stars. The spectroscopic binary frequency among members is 20% (14/68); however, the overall binary rate reaches 40% (27/68) if one includes the photometric binaries. The cluster mean heliocentric radial velocity is +5.53+/-0.11km/s based on the most reliable 49 members. The Vsini distribution is similar to that of the Pleiades, confirming the age similarities between the two clusters.
- ID:
- ivo://CDS.VizieR/J/A+A/498/949
- Title:
- Velocities of 1253 F-K dwarfs in open clusters
- Short Name:
- J/A+A/498/949
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A long-term monitoring of solar-type dwarfs in 13 nearby open clusters, d<500pc, was conducted during 19 years, but most individual radial velocities were not published, with the exception of a small number of spectroscopic binaries. The program was designed to study the membership, the duplicity, and to search for new members. We obtained 6070 radial velocities during 19 years at the Haute-Provence Observatory (France) and 1130 during 13 years at ESO La Silla (Chile) for 1253 stars in the field of 13 open clusters. The zero point of the CORAVEL radial velocity system has been improved and the new values supersede those published previously. The membership and binarity of solar-type stars in NGC 1976 (Orion), IC 2602, NGC 7092, and NGC 2682 are briefly discussed. For the whole sample, we confirm the membership of 894 stars and discovered 150 new spectroscopic binaries among them. Added to those already known in the Hyades and Coma Berenices, the number of member spectroscopic binaries amounts to 188. The overall binary frequency is 30% (188/618) for stars with at least 2 measurements. IC 2602 may represent an exception because no spectroscopic binaries were found among the 26 members. New orbital elements were computed with the updated values of the radial velocities for 66 systems, representing 55 members and 11 non-members. This paper is the last one from our team reporting CORAVEL radial velocities of solar-type dwarfs in nearby open clusters. All CORAVEL observations for dwarfs and red giants in open clusters are now available in electronic form.
- ID:
- ivo://CDS.VizieR/J/A+A/526/A127
- Title:
- Velocities of M67 main-sequence and giant stars
- Short Name:
- J/A+A/526/A127
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Precise analyses of stellar radial velocities is able to reveal intrinsic causes of the wavelength shifts of spectral lines (other than Doppler shifts due to radial motion), such as gravitational redshifts and convective blueshifts. Aims. Gravitational redshifts in solar-type main-sequence stars are expected to be some 500m/s greater than those in giants. We search for this difference in redshifts among groups of open-cluster stars that share the same average space motion and thus have the same average Doppler shift.
- ID:
- ivo://CDS.VizieR/J/A+A/512/A37
- Title:
- Velocities of 43 nearby L dwarfs
- Short Name:
- J/A+A/512/A37
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present radial velocity measurements of a sample of L0-L8 dwarfs observed with VLT/UVES and Keck/HIRES. We combine these measurements with distance and proper motion from the literature to determine space motions for 43 of our targets. We identify nine candidate members of young moving groups, which have ages of 50-600Myr according to their space motion.
- ID:
- ivo://CDS.VizieR/J/A+A/601/A79
- Title:
- VLTS. Properties of O dwarf in 30 Dor
- Short Name:
- J/A+A/601/A79
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The VLT-FLAMES Tarantula Survey has observed hundreds of O-type stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). We study the properties of 105 apparently single O-type dwarfs. To determine stellar and wind parameters, we used the IACOB-GBAT package, an automatic procedure based on a large grid of atmospheric models calculated with the FASTWIND code. In addition to classical techniques, we applied the Bayesian BONNSAI tool to estimate evolutionary masses. We provide a new calibration of effective temperature vs. spectral type for O-type dwarfs in the LMC, based on our homogeneous analysis of the largest sample of such objects to date and including all spectral subtypes. Good agreement with previous results is found, although the sampling at the earliest subtypes could be improved. Rotation rates and helium abundances are studied in an evolutionary context. We find that most of the rapid rotators (vsini higher than 300km/s) in our sample have masses below 25 MSun and intermediate rotation-corrected gravities (logg_c_ between 3.9 and 4.1). Such rapid rotators are scarce at higher gravities (i.e. younger ages) and absent at lower gravities (larger ages). This is not expected from theoretical evolutionary models, and does not appear to be due to a selection bias in our sample. We compare the estimated evolutionary and spectroscopic masses, finding a trend that the former is higher for masses below 20M_{sun}_. This can be explained as a consequence of limiting our sample to the O-type stars, and we see no compelling evidence for a systematic mass discrepancy. For most of the stars in the sample we were unable to estimate the wind-strength parameter (hence mass-loss rates) reliably, particularly for objects with luminosity lower than logL/L_{sun}_ about 5.1. Ultraviolet spectroscopy is needed to undertake a detailed investigation of the wind properties of these dwarfs.
