1,643 research outputs found
The Stellar Populations of Praesepe and Coma Berenices
We present the results of a stellar membership survey of the nearby open
clusters Praesepe and Coma Berenices. We have combined archival survey data
from the SDSS, 2MASS, USNOB1.0, and UCAC-2.0 surveys to compile proper motions
and photometry for ~5 million sources over 300 deg^2. Of these sources, 1010
stars in Praesepe and 98 stars in Coma Ber are identified as candidate members
with probability >80%; 442 and 61 are identified as high-probability candidates
for the first time. We estimate that this survey is >90% complete across a wide
range of spectral types (F0 to M5 in Praesepe, F5 to M6 in Coma Ber). We have
also investigated the stellar mass dependence of each cluster's mass and radius
in order to quantify the role of mass segregation and tidal stripping in
shaping the present-day mass function and spatial distribution of stars.
Praesepe shows clear evidence of mass segregation across the full stellar mass
range; Coma Ber does not show any clear trend, but low number statistics would
mask a trend of the same magnitude as in Praesepe. The mass function for
Praesepe (t~600 Myr; M~500 Msun) follows a power law consistent with that of
the field present-day mass function, suggesting that any mass-dependent tidal
stripping could have removed only the lowest-mass members (<0.15 Msun). Coma
Ber, which is younger but much less massive (t~400 Myr; M~100 Msun), follows a
significantly shallower power law. This suggests that some tidal stripping has
occurred, but the low-mass stellar population has not been strongly depleted
down to the survey completeness limit (~0.12 Msun).Comment: Accepted to AJ; 14 pages, 10 figures, 5 tables + 2 online-only table
A Stellar Census of the Tucana-Horologium Moving Group
We report the selection and spectroscopic confirmation of 129 new late-type
(K3-M6) members of the Tuc-Hor moving group, a nearby (~40 pc), young (~40 Myr)
population of comoving stars. We also report observations for 13/17 known
Tuc-Hor members in this spectral type range, and that 62 additional candidates
are likely to be unassociated field stars; the confirmation frequency for new
candidates is therefore 129/191 = 67%. We have used RVs, Halpha emission, and
Li6708 absorption to distinguish contaminants and bona fide members. Our
expanded census of Tuc-Hor increases the known population by a factor of ~3 in
total and by a factor of ~8 for members with SpT>K3, but even so, the K-M dwarf
population of Tuc-Hor is still markedly incomplete. The spatial distribution of
members appears to trace a 2D sheet, with a broad distribution in X and Y, but
a very narrow distribution (+/-5 pc) in Z. The corresponding velocity
distribution is very small, with a scatter of +/-1.1 km/s about the mean UVW
velocity. We also show that the isochronal age (20--30 Myr) and the lithium
depletion age (40 Myr) disagree, following a trend seen in other PMS
populations. The Halpha emission follows a trend of increasing EW with later
SpT, as seen for young clusters. We find that members have been depleted of
lithium for spectral types of K7.0-M4.5. Finally, our purely kinematic and
color-magnitude selection procedure allows us to test the efficiency and
completeness for activity-based selection of young stars. We find that 60% of
K-M dwarfs in Tuc-Hor do not have ROSAT counterparts and would be omitted in
Xray selected samples. GALEX UV-selected samples using a previously suggested
criterion for youth achieve completeness of 77% and purity of 78%. We suggest
new selection criteria that yield >95% completeness for ~40 Myr
populations.(Abridged)Comment: Accepted to AJ; 28 pages, 12 figures, 5 tables in emulateapj forma
Mapping the Shores of the Brown Dwarf Desert. I. Upper Scorpius
We present the results of a survey for stellar and substellar companions to 82 young stars in the nearby OB association Upper Scorpius. This survey used nonredundant aperture mask interferometry to achieve typical contrast
limits of ΔK ~5-6 at the diffraction limit, revealing 12 new binary companions that lay below the detection limits
of traditional high-resolution imaging; we also summarize a complementary snapshot imaging survey that discovered
seven directly resolved companions. The overall frequency of binary companions (~35 +5 -4% at separations of
6-435 AU) appears to be equivalent to field stars of similar mass, but companions could be more common among
lower mass stars than for the field. The companion mass function has statistically significant differences compared to several suggested mass functions for the field, and we suggest an alternate lognormal parameterization of the mass function. Our survey limits encompass the entire brown dwarf mass range, but we only detected a single companion that might be a brown dwarf; this deficit resembles the so-called brown dwarf desert that has been observed by radial velocity planet searches. Finally, our survey’s deep detection limits extend into the top of the planetary mass function, reaching 8-12 MJup for half of our sample. We have not identified any planetary companions at high confidence (≳99.5%), but we have identified four candidate companions at lower confidence (≳97.5%) that merit additional follow-up to confirm or disprove their existence
Revising Properties of Planet-host Binary Systems II: Apparent Near-Earth Analog Planets in Binaries Are Often Sub-Neptunes
Identifying rocky planets in or near the habitable zones of their stars
(near-Earth analogs) is one of the key motivations of many past and present
planet-search missions. The census of near-Earth analogs is important because
it informs calculations of the occurrence rate of Earth-like planets, which in
turn feed into calculations of the yield of future missions to directly image
other Earths. Only a small number of potential near-Earth analogs have been
identified, meaning that each planet should be vetted carefully and then
incorporated into the occurrence rate calculation. A number of putative
near-Earth analogs have been identified within binary star systems. However,
stellar multiplicity can bias measured planetary properties, meaning that
apparent near-Earth analogs in close binaries may have different radii or
instellations than initially measured. We simultaneously fit unresolved optical
spectroscopy, optical speckle and near-infrared AO contrasts, and unresolved
photometry, and retrieved revised stellar temperatures and radii for a sample
of 11 binary Kepler targets that host at least one near-Earth analog planet,
for a total of 17 planet candidates. We found that 10 of the 17 planets in our
sample had radii that fell in or above the radius gap, suggesting that they are
not rocky planets. Only 2 planets retained super-Earth radii and stayed in the
habitable zone, making them good candidates for inclusion in rocky planet
occurrence rate calculations.Comment: Accepted to AJ. 11 pages + 1 appendix. The most up-to-date version of
the analysis code is available at
https://github.com/kendallsullivan/mcmc_spe
The Role of Multiplicity in Disk Evolution and Planet Formation
The past decade has seen a revolution in our understanding of protoplanetary
disk evolution and planet formation in single star systems. However, the
majority of solar-type stars form in binary systems, so the impact of binary
companions on protoplanetary disks is an important element in our understanding
of planet formation. We have compiled a combined multiplicity/disk census of
Taurus-Auriga, plus a restricted sample of close binaries in other regions, in
order to explore the role of multiplicity in disk evolution. Our results imply
that the tidal influence of a close (<40 AU) binary companion significantly
hastens the process of protoplanetary disk dispersal, as ~2/3 of all close
binaries promptly disperse their disks within <1 Myr after formation. However,
prompt disk dispersal only occurs for a small fraction of wide binaries and
single stars, with ~80%-90% retaining their disks for at least ~2--3 Myr (but
rarely for more than ~5 Myr). Our new constraints on the disk clearing
timescale have significant implications for giant planet formation; most single
stars have 3--5 Myr within which to form giant planets, whereas most close
binary systems would have to form giant planets within <1 Myr. If core
accretion is the primary mode for giant planet formation, then gas giants in
close binaries should be rare. Conversely, since almost all single stars have a
similar period of time within which to form gas giants, their relative rarity
in RV surveys indicates either that the giant planet formation timescale is
very well-matched to the disk dispersal timescale or that features beyond the
disk lifetime set the likelihood of giant planet formation.Comment: Accepted to ApJ; 15 pages, 3 figures, 3 tables in emulateapj forma
USco J1606-1935: An Unusually Wide Low-Mass Triple System?
We present photometric, astrometric, and spectroscopic observations of USco J160611.9-193532 AB, a candidate ultrawide (~1600 AU), low-mass (M_(tot) ~ 0.4 M_⊙) multiple system in the nearby OB association Upper Scorpius. We conclude that both components are young, comoving members of the association; we also present high-resolution observations that show that the primary is itself a close binary system. If the Aab and B components are gravitationally bound, the system would fall into the small class of young multiple systems that have unusually wide separations as compared to field systems of similar mass. However, we demonstrate that physical association cannot be assumed purely on probabilistic grounds for any individual candidate system in this separation range. Analysis of the association's two-point correlation function shows that there is a significant probability (25%) that at least one pair of low-mass association members will be separated in projection by ≾ 15", so analysis of the wide binary population in Upper Sco will require a systematic search for all wide systems; the detection of another such pair would represent an excess at the 98% confidence level
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