CARMENES input catalogue of M dwarfs Alonso-Floriano, F J; Morales, J C; Caballero, J A ...
Astronomy and astrophysics (Berlin),
5/2015, Letnik:
577
Journal Article
Recenzirano
Odprti dostop
CARMENES is a stabilized, high-resolution, double-channel spectrograph at the 3.5 m Calar Alto telescope. It is optimally designed for radial-velocity surveys of M dwarfs with potentially habitable ...Earth-mass planets. We prepare a list of the brightest, single M dwarfs in each spectral subtype observable from the northern hemisphere, from which we will select the best planet-hunting targets for CARMENES. In this first paper on the preparation of our input catalogue, we compiled a large amount of public data and collected low-resolution optical spectroscopy with CAFOS at the 2.2 m Calar Alto telescope for 753 stars. We derived accurate spectral types using a dense grid of standard stars, a double least-squares minimization technique, and 31 spectral indices previously defined by other authors. We calculated spectral types for all 753 stars, of which 305 are new and 448 are revised. This collection of low-resolution spectroscopic data serves as a candidate target list for the CARMENES survey and can be highly valuable for other radial-velocity surveys of M dwarfs and for studies of cool dwarfs in the solar neighbourhood.
CARMENES input catalogue of M dwarfs Cortes-Contreras, M; Bejar, V J S; Caballero, J A ...
Astronomy and astrophysics (Berlin),
1/2017, Letnik:
597
Journal Article
Recenzirano
Odprti dostop
Aims. We search for low-mass companions of M dwarfs and characterize their multiplicity fraction with the purpose of helping in the selection of the most appropriate targets for the CARMENES ...exoplanet survey. Methods. We obtained high-resolution images in the I band with the lucky imaging instrument FastCam at the 1.5m Telescopio Carlos Sanchez for 490 mid- to late-M dwarfs. For all the detected binaries, we measured angular separations, position angles, and magnitude differences in the I band. We also calculated the masses of each individual component and estimated orbital periods, using the available magnitude and colour relations for M dwarfs and our own MJ-spectra l type and mass-MI relations. To avoid biases in our sample selection, we built a volume-limited sample of M0.0-M5.0 dwarfs that is complete up to 86% within 14pc. Results. From the 490 observed stars, we detected 80 companions in 76 systems, of which 30 are new discoveries. Another six companion candidates require additional astrometry to confirm physical binding. The multiplicity fraction in our observed sample is 16.7+ or -2.0%. The bias-corrected multiplicity fraction in our volume-limited sample is 19.5+ or -2.3% for angular separations of 0.2 to 5.0arcsec (1.4-65.6au), with a peak in the distribution of the projected physical separations at 2.5-7.5au. For M0.0-M3.5V primaries, our search is sensitive to mass ratios higher than 0.3 and there is a higher density of pairs with mass ratios over 0.8 compared to those at lower mass ratios. Binaries with projected physical separations shorter than 50au also tend to be of equal mass. For 26 of our systems, we estimated orbital periods shorter than 50a, 10 of which are presented here for the first time. We measured variations in angular separation and position angle that are due to orbital motions in 17 of these systems. The contribution of binaries and multiples with angular separations shorter than 0.2arcsec, longer than 5.0arcsec, and of spectroscopic binaries identified from previous searches, although not complete, may increase the multiplicity fraction of M dwarfs in our volume-limited sample to at least 36%.
Context. Stellar jets are believed to play a key role in star formation, but the question of how they originate is still being debated. Aims. We derive the physical properties at the base of the jet ...from DG Tau both along and across the flow and as a function of velocity. Methods. We analysed seven optical spectra of the DG Tau jet, taken with the Hubble Space Telescope Imaging Spectrograph. The spectra were obtained by placing a long-slit parallel to the jet axis and stepping it across the jet width. The resulting position-velocity diagrams in optical forbidden emission lines allowed access to plasma conditions via calculation of emission line ratios. In this way, we produced a 3D map (2D in space and 1D in velocity) of the jet’s physical parameters i.e. electron density ne, hydrogen ionisation fraction xe, and total hydrogen density nH. The method used is a new version of the BE-technique. Results. A fundamental improvement is that the new diagnostic method allows us to overcome the upper density limit of the standard S ii diagnostics. As a result, we find at the base of the jet high electron density, ne ~ 105, and very low ionisation, xe ~ 0.02−0.05, which combine to give a total density up to nH ~ 3 × 106. This analysis confirms previous reports of variations in plasma parameters along the jet, (i.e. decrease in density by several orders of magnitude, increase of xe from 0.05 to a plateau at 0.7 downstream at 2′′ from the star). Furthermore, a spatial coincidence is revealed between sharp gradients in the total density and supersonic velocity jumps. This strongly suggests that the emission is caused by shock excitation. No evidence was found of variations in the parameters across the jet, within a given velocity interval. The position-velocity diagrams indicate the presence of both fast accelerating gas and slower, less collimated material. We derive the mass outflow rate, Ṁj, in the blue-shifted lobe in different velocity channels, that contribute to a total of Ṁj ~ 8±4 × 10-9 M⊙ yr-1. We estimate that a symmetric bipolar jet would transport at the low and intermediate velocities probed by rotation measurements, an angular momentum flux of L̇j ~ 2.9 ± 1.5 × 10-6 M⊙ yr-1 AU km s-1. We discuss implications of these findings for jet launch theories. Conclusions. The derived properties of the DG Tau jet are demonstrated to be consistent with magneto-centrifugal theory. However, non-stationary modelling is required in order to explain all of the features revealed at high resolution.
Aims. Rotational studies at a variety of ages and masses are important for constraining the angular momentum evolution of young stellar objects (YSO). Of particular interest are the very low mass ...(VLM) stars and brown dwarfs (BDs), because of the significant lack of known rotational periods in that mass range. We aim to extend previous studies well down into the substellar regime, providing for the first time information on rotational periods for a large sample of young VLM stars and BDs. Methods. This extensive rotational period study of YSOs in the 1 Myr old Orion Nebula Cluster (ONC) is based on a deep photometric monitoring campaign using the Wide Field Imager (WFI) camera on the ESO/MPG 2.2 m telescope on La Silla, Chile. Time series data was obtained with about 95 data points spread over 19 nights. Accurate I-band photometry of 2908 stars was obtained within a magnitude range of 13 to 21 mag, i.e. extending three magnitudes deeper than previous studies in the ONC. Two different power spectral analysis techniques were used to search for periodic variability. In addition, the $\chi^{2}$ variability test was used for the detection of irregular variables. Results. We found 487 periodic variables with estimated masses between 0.5 $M_\odot$ and 0.015 $M_\odot$, 124 of which are BD candidates. This is by far the most extensive and complete rotational period data set for young VLM stars and BDs. In addition to the periodic variables, 808 objects show non-periodic brightness variations. We study the dependence of the period distribution on mass and variability level and compare this with known objects in the ONC with masses up to 1.5 $M_\odot$ and with the ~2 Myr old cluster NGC 2264. We find that substellar objects rotate on average faster than the VLM stars. In addition, our rotational data suggest a dependence of the rotational periods on position within the field, which can be explained by a possible age spread in the ONC with a somewhat younger central region. The results of a comparison between the period distributions of the ONC and NGC 2264 favours this hypothesis. In addition, periodic variables with larger peak-to-peak amplitudes rotate on average slower than those with small peak-to-peak amplitude variations, which can possibly be explained by different magnetic field topologies.
Aims. We aim at identifying the least massive population of the solar metallicity, young (120 Myr), nearby (133.5 pc) Pleiades star cluster with the ultimate goal of understanding the physical ...properties of intermediate-age, free-floating, low-mass brown dwarfs and giant planetary-mass objects, and deriving the cluster substellar mass function across the deuterium-burning mass limit at ≈0.012 M⊙. Methods. We performed a deep photometric and astrometric J- and H-band survey covering an area of ~0.8 deg2 in the Pleiades cluster. The images with completeness and limiting magnitudes of J,H ≈ 20.2 and ≈21.5 mag were acquired ~9 yr apart, allowing us to derive proper motions with a typical precision of ±6 mas yr-1. For the cluster distance and age, the survey is sensitive to Pleiades members with masses in the interval ≈0.2−0.008 M⊙. J- and H-band data were complemented with Z, K, and mid-infrared magnitudes up to 4.6 μm coming from the UKIRT Infrared Deep Sky Survey (UKIDSS), the WISE catalog, and follow-up observations of our own. Pleiades member candidates were selected to have proper motions compatible with that of the cluster, and colors following the known Pleiades sequence in the interval J = 15.5−18.8 mag, and ZUKIDSS − J ≥ 2.3 mag or Z nondetections for J> 18.8 mag. Results. We found a neat sequence of astrometric and photometric Pleiades substellar member candidates with two or more proper motion measurements and with magnitudes and masses in the intervals J = 15.5−21.2 mag and ≈0.072−0.008 M⊙. The faintest objects show very red near- and mid-infrared colors exceeding those of field high-gravity dwarfs by ≥0.5 mag. This agrees with the reported properties of field young L-type dwarfs and giant planets orbiting stars of ages of ~100 Myr. The Pleiades photometric sequence does not show any color turn-over because of the presence of photospheric methane absorption down to J = 20.3 mag, which is about 1 mag fainter than predicted by the combination of evolutionary models and colors computed from model atmospheres. The astrometric data suggest that Pleiades brown dwarfs have a proper motion dispersion of 6.4−7.5 mas yr-1, and are dynamically relaxed at the age of the cluster. The Pleiades mass function extends down to the deuterium burning-mass threshold, with a slope fairly similar to that of other young star clusters and stellar associations. The new discoveries may become benchmark objects for interpreting the observations of the emerging young ultracool population and giant planets around stars in the solar neighborhood.
A wide field imager attached to the MPG/ESO 2.2 m telescope on La Silla has been used to monitor the Orion Nebula Cluster on 45 nights between 25 Dec. 1998 and 28 Feb. 1999. Ninety-two images were ...obtained during this period through an intermediate band filter centered at 815.9 nm. More than 1500 sources with I magnitudes between 12.5 and 20 were monitored. We find that essentially every star brighter than 16th mag (where the precision is <0.01 mag) is a variable, with about half having a peak-to-peak variation of ~0.2 mag or more. A clear correlation is found between the level of variability and infrared excess emission, in the sense that stars with evidence for circumstellar disks have larger amplitudes of variation. A search for periodic variables was carried out and 369 such stars were discovered, most or all of which are rotating, spotted T Tauri stars. Periodic variables are most commonly found among the low amplitude variables. 46% of the stars with magnitudes between 12.5 and 16 and standard deviation, σ < 0.1 mag, were found to be periodic, whereas only 24% of the stars in the same magnitude range with $\sigma > 0.1$ yielded periods. Our work confirms the existence of a bimodal period distribution, with peaks near 2 and 8 days, for stars with $M > 0.25~M_\odot$ and a unimodal distribution peaked near 2 days, for lower mass stars. We show that a statistically significant correlation exists between infrared excess emission and rotation in the sense that slower rotators are more likely to show evidence of circumstellar disks. Slowly rotating stars, with angular velocities, ω < 1 radian/d, corresponding to rotation periods longer than 6.28 d, have a mean infrared excess emission, $\Delta (I-K) = 0.55 \pm 0.05$, indicative of the presence of inner disks, while rapid rotators, with $\omega > 2$ radians/d, corresponding to rotation periods shorter than 3.14 d, have a much smaller mean of $0.17 \pm 0.05$. This supports the hypothesis that disks are involved in regulating stellar rotation during the pre-main sequence phase. We explore a simple and commonly adopted model of rotational evolution in which stars conserve angular velocity while locked to a disk and conserve angular momentum once released. If these assumptions are valid, and if the locking period is 8 days, we find that more than half of the stars in the ONC are no longer locked to disks and that an exponential decay model with a disk-locking half-life of about 0.5–1 My fits the observations well. Assuming that the mean ages of the higher and lower mass stars are the same, the faster rotation of the lower mass stars can be understood as either a consequence of a shorter disk-locking time or a shorter initial disk-locking period, or both.
We present the results of an extensive search for periodic and irregular variable pre-main sequence (PMS) stars in the young (2–4 Myr) open cluster NGC 2264, based on photometric monitoring using the ...Wide Field Imager (WFI) on the 2.2 m telescope on La Silla (Chile). In total, about 10 600 stars with $I_{{\rm C}}$ magnitudes between 9.8 mag and 21 mag have been monitored in our $34\arcmin$ $\times$ $33\arcmin$ field. Time series data were obtained in the $I_{{\rm C}}$ band in 44 nights between Dec. 2000 and March 2001; altogether we obtained 88 data points per star. Using two different time series analysis techniques (Scargle periodogram and CLEAN) we found 543 periodic variable stars with periods between 0.2 days and 15 days. Also, 484 irregular variable stars were identified using a $\chi^2$-test. In addition we have carried out nearly simultaneous observations in V, $R_{{\rm C}}$ and a narrow-band Hα filter. The photometric data enable us to reject background and foreground stars from our sample of variable stars according to their location in the $I_{{\rm C}}$ vs. $(R_{{\rm C}}-I_{{\rm C}})$ colour–magnitude and $(R_{{\rm C}}-{\rm H}\alpha)$ vs. $(R_{{\rm C}}-I_{{\rm C}})$ colour–colour diagrams. We identified 405 periodic variable and 184 irregular variable PMS stars as cluster members using these two different tests. In addition 35 PMS stars for which no significant variability were detected could be identified as members using an $\rm H\alpha$ emission index criterion. This yields a total of 624 PMS stars in NGC 2264, of which only 182 were previously known. Most of the newly found PMS stars are fainter than $I_{{\rm C}} \simeq 15~{\rm mag}$ and of late spectral type ($\ga$M 2). We find that the periodic variables, as a group, have a smaller degree of variability and smaller $\rm H\alpha$ index than the irregular variables. This suggests that the sample of periodic variables is biased towards weak-line T Tauri stars (WTTSs) while most of the irregular variables are probably classical T Tauri stars (CTTSs). We have quantified this bias and estimated that the expected fraction of WTTSs among PMS stars in the cluster is $77\%$. This is relatively close to the fraction of WTTSs among the periodic variables which is $85\%$. We also estimated the total fraction of variables in the cluster using only two well selected concentrations of PMS stars called NGC 2264 N & S in which we can easily estimate the total number of PMS stars. We find that at least $74\%$ of the PMS stars in the cluster with $I_{{\rm C}}\leq 18.0~{\rm mag}$ were found to be variable (either periodic or irregular) by our study. This number shows that our search for PMS stars in NGC 2264 through extensive and accurate photometric monitoring is very efficient in detecting most PMS stars down to at least $I_{{\rm C}}=18.0~{\rm mag}$.
Context. A severe problem for research in star-formation is that the masses of young stars are almost always estimated from evolutionary tracks alone. Since the tracks published by different groups ...differ, it is often only possible to give a rough estimate of the masses of young stars. It is thus crucial to test and calibrate the tracks. Up to now, only a few tests of the tracks could be carried out. However, it is now possible with the VLTI to set constrains on the tracks by determining the masses of many young binary stars precisely. Aims. In order to use the VLTI efficiently, a first step is to find suitable targets, which is the purpose of this work. Given the distance of nearby star-forming regions, suitable VLTI targets are binaries with orbital periods between at least 50 days and a few years. Although a number of surveys for detecting spectroscopic binaries have been carried out, most of the binaries found so far have periods that are too short. Methods. We thus surveyed the Chamaeleon, Corona Australis, Lupus, Sco- Cen, and \rho Ophiuci star-forming regions in order to search for spectroscopic binaries with periods longer than 50 days, which are suitable for the VLTI observations. Results. As a result of the 8 year campaign, we discovered 8 binaries with orbital periods longer than 50 days. Amongst the newly discovered long-period binaries is CS Cha, which is one of the few classical T Tauri stars with a circumbinary disk. The survey is limited to objects with masses higher than 0.1 to 0.2 M_\odot for periods between 1 and 8 years. Conclusions. We find that the frequency of binaries with orbital periods \leq3000 days is of 20\pm5%. The frequency of long and short period pre-main sequence spectroscopic binaries is about the same as for stars in the solar neighbourhood. In total 14 young binaries are now known that are suitable for mass determination with the VLTI.
We report on the near-infrared (JHK-bands) low-resolution spectroscopy and red optical (Z-band) photometry of seven proper-motion, very low-mass substellar member candidates of the Pleiades cluster ...with magnitudes in the interval J = 17.5−20.8 and K = 16.1−18.5 mag. Spectra were acquired for six objects with the LIRIS and NIRSPEC instruments mounted on the 4.2-m William Herschel and the 10-m Keck II telescopes, respectively. Z-band images of two of the faintest candidates were collected with the ACAM instrument on the WHT. The new data confirm the low temperatures of all seven Pleiades proper motion candidates. From the imaging observations, we find extremely red Z − J and Z − K colors that suggest that the faintest target, Calar Pleiades 25, has a Galactic rather than extragalactic nature. We tentatively classify the spectroscopic targets from early-L to ~T0 and suggest that the L/T transition, which accounts for the onset of methane absorption at 2.1 μm, may take place at J ≈ 20.3 and K ≈ 17.8 mag in the Pleiades (absolute values of MJ ≈ 14.7 and MK ≈ 12.2 mag). We find evidence of likely low-gravity atmospheres based on the presence of triangular-shape H-band fluxes and the high flux ratio K/H (compatible with red H − K colors) of Calar Pleiades 20, 21, and 22, which is a feature also seen in field low-gravity dwarfs. Weak K i absorption lines at around 1.25 μm are probably seen in two targets. These observations add support to the cluster membership of all seven objects in the Pleiades. The trend delineated by the spectroscopic sequence of Pleiades late-M and L dwarfs resembles that of the field. With masses estimated at 0.012−0.015 M⊙ (solar metallicity and 120 Myr), Calar Pleiades 20 (L6±1), 21 (L7±1), and 22 (L/T) may become the coolest and least massive Pleiades members that are corroborated with photometry, astrometry, and spectroscopy. Calar Pleiades 25 (<0.012 M⊙) is a firm free-floating planetary-mass candidate in the Pleiades.
PDF
