Sergio B. Dieterich

THE SOLAR NEIGHBORHOOD. XXXIII. PARALLAX RESULTS FROM THE CTIOPI 0.9 m PROGRAM: TRIGONOMETRIC PARALLAXES OF NEARBY LOW-MASS ACTIVE AND YOUNG SYSTEMS

We present basic observational data and association membership analysis for 45 young and active low-mass stellar systems from the ongoing RECONS photometry and astrometry program at the Cerro Tololo Inter-American Observatory. Most of these systems have saturated X-ray emission (log(Lx/Lbol) > -3.5) based on X-ray fluxes from the ROSAT All-Sky Survey, and many are significantly more luminous than main-sequence stars of comparable color. We present parallaxes and proper motions, Johnson-Kron-Cousins VRI photometry, and multiplicity observations from the CTIOPI program on the CTIO 0.9m telescope. To this we add low-resolution optical spectroscopy and line measurements from the CTIO 1.5m telescope, and interferometric binary measurements from the Hubble Space Telescope Fine Guidance Sensors. We also incorporate data from published sources: JHKs photometry from the 2MASS point source catalog; X-ray data from the ROSAT All-Sky Survey; and radial velocities from literature sources. Within the sample of 45 systems, we identify 21 candidate low-mass pre-main-sequence members of nearby associations, including members of beta Pictoris, TW Hydrae, Argus, AB Doradus, two ambiguous 30 Myr old systems, and one object that may be a member of the Ursa Major moving group. Of the 21 candidate young systems, 14 are newly identified as a result of this work, and six of those are within 25 parsecs of the Sun.

THE SOLAR NEIGHBORHOOD. XXX. FOMALHAUT C

LP 876-10 is a nearby active M4 dwarf in Aquarius at a distance of 7.6 pc. The star is a new addition to the 10 pc census, with a parallax measured via the REsearch Consortium On Nearby Stars (RECONS) astrometric survey on the Small and Moderate Aperture Research Telescope System’s 0.9 m telescope. We demonstrate that the astrometry, radial velocity, and photometric data for LP 876-10 are consistent with the star being a third bound stellar component to the Fomalhaut multiple system, despite the star lying nearly 6 ◦ away from Fomalhaut A in the sky. The three-dimensional separation of LP 876-10 from Fomalhaut is only 0.77 ± 0.01 pc, and 0.987 ± 0.006 pc from TW PsA (Fomalhaut B), well within the estimated tidal radius of the Fomalhaut system (1.9 pc). LP 87610 shares the motion of Fomalhaut within ∼ 1k m s −1 , and we estimate an interloper probability of ∼10 −5 . Neither our echelle spectroscopy nor astrometry are able to confirm the close companion to LP 876-10 reported in the Washington Double Star Catalog (WSI 138). We argue that the Castor Moving Group to which the Fomalhaut system purportedly belongs, is likely to be a dynamical stream, and hence membership to the group does not provide useful age constraints for group members. LP 876-10 (Fomalhaut C) has now risen from obscurity to become a rare example of a field M dwarf with well-constrained age (440 ± 40 Myr) and metallicity. Besides harboring a debris disk system and candidate planet, Fomalhaut now has two of the widest known stellar companions.

The Solar Neighborhood. XXXII. The Hydrogen Burning Limit

We construct a Hertzsprung-Russell diagram for the stellar/substellar boundary based on a sample of 63 objects ranging in spectral type from M6V to L4. We report newly observed VRI photometry for all 63 objects and new trigonometric parallaxes for 37 objects. The remaining 26 objects have trigonometric parallaxes from the literature. We combine our optical photometry and trigonometric parallaxes with 2MASS and WISE photometry and employ a novel spectral energy distribution fitting algorithm to determine effective temperatures, bolometric luminosities, and radii. Our uncertainties range from ~20?K to ~150?K in temperature, ~0.01 to ~0.06 in log (L/L ?) and ~3% to ~10% in radius. We check our methodology by comparing our calculated radii to radii directly measured via long baseline optical interferometry. We find evidence for the local minimum in the radius-temperature and radius-luminosity trends that signals the end of the stellar main sequence and the start of the brown dwarf sequence at T eff ~ 2075 K, log (L/L ?) ~ ?3.9, and (R/R ?) ~ 0.086. The existence of this local minimum is predicted by evolutionary models, but at temperatures ~400?K cooler. The minimum radius happens near the locus of 2MASS?J0523?1403, an L2.5 dwarf with V ? K = 9.42. We make qualitative arguments as to why the effects of the recent revision in solar abundances accounts for the discrepancy between our findings and the evolutionary models. We also report new color-absolute magnitude relations for optical and infrared colors which are useful for estimating photometric distances. We study the optical variability of all 63 targets and find an overall variability fraction of 36% at a threshold of 15?mmag in the I band, which is in agreement with previous studies.

THE SOLAR NEIGHBORHOOD. XXVIII. THE MULTIPLICITY FRACTION OF NEARBY STARS FROM 5 TO 70 AU AND THE BROWN DWARF DESERT AROUND M DWARFS

We report on our analysis of Hubble Space Telescope/NICMOS snapshot high-resolution images of 255 stars in 201 systems within ∼10 pc of the Sun. Photometry was obtained through filters F110W, F180M, F207M, and F222M using NICMOS Camera 2. These filters were selected to permit clear identification of cool brown dwarfs through methane contrast imaging. With a plate scale of 76 mas pixel −1 , NICMOS can easily resolve binaries with subarcsecond separations in the 19. 5×19. �� 5 field of view. We previously reported five companions to nearby M and L dwarfs from this search. No new companions were discovered during the second phase of data analysis presented here, confirming that stellar/substellar binaries are rare. We establish magnitude and separation limits for which companions can be ruled out for each star in the sample, and then perform a comprehensive sensitivity and completeness analysis for the subsample of 138 M dwarfs in 126 systems. We calculate a multiplicity fraction of 0.0 +3.5 −0.0 % for L companions to M dwarfs in the separation range of 5‐70 AU, and 2.3 +5.0 −0.7 % for L and T companions to M dwarfs in the separation range of 10‐70 AU. We also discuss trends in the color‐magnitude diagrams using various color combinations and present astrometry for 19 multiple systems in our sample. Considering these results and results from several other studies, we argue that the so-called brown dwarf desert extends to binary systems with low-mass primaries and is largely independent of primary mass, mass ratio, and separations. While focusing on companion properties, we discuss how the qualitative agreement between observed companion mass functions and initial mass functions suggests that the paucity of brown dwarfs in either population may be due to a common cause and not due to binary formation mechanisms.

A Spectroscopic Orbit for Regulus

We present a radial velocity study of the rapidly rotating B star Regulus that indicates the star is a single-lined spectroscopic binary. The orbital period (40.11 days) and probable semimajor axis (0.35 AU) are large enough that the system is not interacting at present. However, the mass function suggests that the secondary has a low mass (M2 > 0.30 M☉), and we argue that the companion may be a white dwarf. Such a star would be the remnant of a former mass donor that was the source of the large spin angular momentum of Regulus itself.

The solar neighborhood. XXXV. Distances to 1404 M dwarf systems within 25 PC in the southern sky

We present trigonometric, photometric, and photographic distances to 1748 southern (d  ⩽ 0 ) M dwarf systems with  ⩾ μ 01 8 · yr �1 , of which 1404 are believed to lie within 25 pc of the Sun. The stars have ⩽⩽ V 6.67 21.38 J and ⩽⩽ VK 3.50 ( ) 9.27 Js , covering the entire M dwarf spectral sequence from M0.0 V through M9.5 V. This sample therefore provides a comprehensive snapshot of our current knowledge of the southern sky for the nearest M dwarfs that dominate the stellar population of the Galaxy. Roughly one-third of the 1748 systems, each of which has an M dwarf primary, have published high quality parallaxes, including 179 from the REsearch Consortium On Nearby Stars astrometry program. For the remaining systems, we offer photometric distance estimates that have well-calibrated errors. The bulk of these (∼700) are based on new V RI J KC KC photometry acquired at the CTIO/ SMARTS 0.9 m telescope, while the remaining 500 primaries have photographic plate distance estimates calculated using SuperCOSMOS B RI JF 59 IVN photometry. Confirmed and candidate subdwarfs in the sample have been identified, and a census of companions is included.

THE SOLAR NEIGHBORHOOD. XXXVI. THE LONG-TERM PHOTOMETRIC VARIABILITY OF NEARBY RED DWARFS IN THE VRI OPTICAL BANDS

We present an analysis of long-term photometric variability for nearby red dwarfs at optical wavelengths. The sample consists of 264 M dwarfs south of DEC = +30 with V-K = 3.96-9.16 and Mv~10-20 (spectral types M2V-M8V), most of which are within 25 pc. The stars have been observed in the VRI filters for ~4-14 years at the CTIO/SMARTS 0.9m telescope. Of the 238 red dwarfs within 25 pc, we find that only ~8% are photometrically variable by at least 20 mmag (~2%) in the VRI bands. We find that high variability at optical wavelengths over the long-term can be used to identify young stars. Overall, however, the fluxes of most red dwarfs at optical wavelengths are steady to a few percent over the long term. The low overall rate of photometric variability for red dwarfs is consistent with results found in previous work on similar stars on shorter timescales, with the body of work indicating that most red dwarfs are only mildly variable. We highlight 17 stars that show long-term changes in brightness, sometimes because of flaring activity or spots, and sometimes because of stellar cycles similar to our Suns solar cycle. Remarkably, two targets show brightnesses that monotonically increase (G 169-029) or decrease (WT 460AB) by several percent over a decade. We also provide long-term variability measurements for seven M dwarfs within 25 pc that host exoplanets, none of which vary by more than 20 mmag. Both as a population, and for the specific red dwarfs with exoplanets observed here, photometric variability is therefore often not a concern for planetary environments, at least at the optical wavelengths where they emit much of their light.

The discovery and mass measurement of a new ultra-short-period Planet: K2-131b

We report the discovery of a new ultra-short-period planet and summarize the properties of all such planets for which the mass and radius have been measured. The new planet, EPIC 228732031b, was discovered in K2 Campaign 10. It has a radius of 1.81-0.12+0.16 R_Earth and orbits a G dwarf with a period of 8.9 hr. Radial velocities obtained with Magellan/PFS and TNG/HARPS-N show evidence for stellar activity along with orbital motion. We determined the planetary mass using two different methods: (1) the “floating chunk offset” method, based only on changes in velocity observed on the same night; and (2) a Gaussian process regression based on both the radial velocity and photometric time series. The results are consistent and lead to a mass measurement of 6.5+/- 1.6 M_Earth and a mean density of 6.0-2.7+3.0 g cm‑3.

The Solar Neighborhood XLIV: RECONS Discoveries within 10 parsecs

We describe the 44 systems discovered to be within 10 parsecs of the Sun by the RECONS team, primarily via the long-term astrometry program at CTIO that began in 1999. The systems --- including 41 with red dwarf primaries, 2 white dwarfs, and 1 brown dwarf --- have been found to have trigonometric parallaxes greater than 100 milliarcseconds (mas), with errors of 0.4--2.4 mas in all but one case. We provide updated astrometric, photometric (VRIJHK magnitudes), spectral type, and multiplicity information here. Among these are 14 systems that are new entries to the 10 parsec sample based on parallaxes measured at the CTIO/SMARTS 0.9m telescope. These are the first parallaxes for nine systems, while the remaining five systems had previously measured parallaxes with errors greater than 10 mas or values placing them beyond 10 parsecs. We also present parallaxes from URAT for seven of these systems, providing additional evidence that they are closer than 10 parsecs. In addition, we provide new data for 22 systems that were previously known to lie within 10 parsecs and 9 systems reported to be closer than that horizon but for which new parallaxes place them further away. In total, we provide data for 75 systems, for which 71 have new or updated parallaxes here. The 44 systems added by RECONS comprise one of every seven systems known within 10 parsecs. We illustrate the evolution of the 10 parsec sample from the 191 systems known when the final Yale Parallax Catalog (YPC) was published in 1995 to the 316 systems known today. Even so close to the Sun, additional discoveries of red and brown dwarfs (and perhaps even white dwarfs) are likely, both as primaries and secondaries, although we estimate that at least 90% of the stellar systems closer than 10 parsecs have now been identified.

Dynamical Masses of ε Indi B and C: Two Massive Brown Dwarfs at the Edge of the Stellar–substellar Boundary

We report individual dynamical masses for the brown dwarfs Epsilon Indi B and C, which have spectral types of T1.5 and T6, respectively, measured from astrometric orbit mapping. Our measurements are based on a joint analysis of astrometric data from the Carnegie Astrometric Planet Search and the Cerro Tololo Inter-American Observatory Parallax Investigation as well as archival high resolution imaging, and use a Markov Chain Monte Carlo method. We find dynamical masses of 75.0+-0.82 Mjup for the T1.5 B component and 70.1+-0.68 Mjup for the T6 C component. These masses are surprisingly high for substellar objects and challenge our understanding of substellar structure and evolution. We discuss several evolutionary scenarios proposed in the literature and find that while none of them can provide conclusive explanations for the high substellar masses, evolutionary models incorporating lower atmospheric opacities come closer to approximating our results. We discuss the details of our astrometric model, its algorithm implementation, and how we determine parameter values via Markov Chain Monte Carlo Bayesian inference.