104 research outputs found
The X-ray luminosity function of M37 and the evolution of coronal activity in low-mass stars
We use a 440.5 ks Chandra observation of the 500-Myr-old open
cluster M37 to derive the X-ray luminosity functions of its
stars. Combining detections of 162 M37 members with upper limits
for 160 non-detections, we find that its G, K, and M stars have a similar
median (0.57 keV) X-ray luminosity L erg/s, whereas the
L-to-bolometric-luminosity ratio (L/L) indicates that M stars
are more active than G and K stars by 1 order of magnitude at 500 Myr.
To characterize the evolution of magnetic activity in low-mass stars over their
first 600 Myr, we consolidate X-ray and optical data from the
literature for stars in six other open clusters: from youngest to oldest, the
Orion Nebula Cluster (ONC), NGC 2547, NGC 2516, the Pleiades, NGC 6475, and the
Hyades. For these, we homogenize the conversion of instrumental count rates to
L by applying the same one-temperature emission model as for M37, and
obtain masses using the same empirical mass-absolute magnitude relation (except
for the ONC). We find that for G and K stars X-ray activity decreases
2 orders of magnitude over their first 600 Myr, and for M stars,
1.5. The decay rate of the median L follows the relation L, where for G, for K, and
for M stars. In L/L space, the slopes are
, , and , respectively. These results
suggest that for low-mass stars the age-activity relation steepens after
625 Myr, consistent with the faster decay in activity observed in
solar analogs at Gyr.Comment: 2 machine readable table
Validating TGAS wide binaries with Gaia DR2 Radial Velocities and Parallaxes
Featuring greatly improved astrometry and precise radial velocities (RVs),
the second data release (DR2) from Gaia affords us a unique opportunity to test
the validity of samples of stellar wide binaries. In a previous work, we
presented a set of wide binaries identified in the joint Tycho-Gaia Astrometric
Solution catalog. Here, we use DR2 to confirm the low contamination rate of
this sample, thereby verifying the effectiveness of our algorithm for
application to DR2 data.Comment: Accepted for publication in Research Notes of the AA
Wide Binaries in Tycho-{\it Gaia}: Search Method and the Distribution of Orbital Separations
We mine the Tycho-{\it Gaia} astrometric solution (TGAS) catalog for wide
stellar binaries by matching positions, proper motions, and astrometric
parallaxes. We separate genuine binaries from unassociated stellar pairs
through a Bayesian formulation that includes correlated uncertainties in the
proper motions and parallaxes. Rather than relying on assumptions about the
structure of the Galaxy, we calculate Bayesian priors and likelihoods based on
the nature of Keplerian orbits and the TGAS catalog itself. We calibrate our
method using radial velocity measurements and obtain 6196 high-confidence
candidate wide binaries with projected separations pc. The
normalization of this distribution suggests that at least 0.6\% of TGAS stars
have an associated, distant TGAS companion in a wide binary. We demonstrate
that {\it Gaia}'s astrometry is precise enough that it can detect projected
orbital velocities in wide binaries with orbital periods as large as 10 yr.
For pairs with ~AU, characterization of random
alignments indicate our contamination to be 5\%. For ~AU, our distribution is consistent with \"{O}pik's Law. At larger
separations, the distribution is steeper and consistent with a power-law
; there is no evidence in our data of any bimodality in
this distribution for 1 pc. Using radial velocities, we
demonstrate that at large separations, i.e., of order 1 pc and beyond,
any potential sample of genuine wide binaries in TGAS cannot be easily
distinguished from ionized former wide binaries, moving groups, or
contamination from randomly aligned stars.Comment: 27 pages, 17 figures, 2 tables, submitted to MNRA
A Temporary Epoch of Stalled Spin-Down for Low-Mass Stars: Insights from NGC 6811 with Gaia and Kepler
Stellar rotation was proposed as a potential age diagnostic that is precise,
simple, and applicable to a broad range of low-mass stars (1 ).
Unfortunately, rotation period measurements of low-mass members
of open clusters have undermined the idea that stars spin down with a common
age dependence (i.e., ): K dwarfs appear to
spin down more slowly than F and G dwarfs. Ag\"ueros et al. (2018) interpreted
data for the 1.4-Gyr-old cluster NGC 752 differently, proposing that
after having converged onto a slow-rotating sequence in their first 600-700 Myr
(by the age of Praesepe), K dwarf stall on that sequence for an
extended period of time. We use data from Gaia DR2 to identify likely
single-star members of the 1-Gyr-old cluster NGC 6811 with Kepler
light curves. We measure for 171 members, more than doubling the
sample relative to the existing catalog and extending the mass limit from
0.8 to 0.6 . We then apply a gyrochronology formula
calibrated with Praesepe and the Sun to 27 single G dwarfs in NGC 6811 to
derive a precise gyrochronological age for the cluster of 1.040.07 Gyr.
However, when our new low-mass rotators are included, NGC 6811's color- sequence deviates away from the naive 1 Gyr projection down to K (K5V, 0.7 ), where it clearly overlaps with
Praesepe's. Combining these data with for other clusters, we
conclude that the assumption that mass and age are separable dependencies is
invalid. Furthermore, the cluster data show definitively that stars experience
a temporary epoch of reduced braking efficiency where stall, and
that the duration of this epoch lasts longer for lower-mass stars.Comment: 18 pages, 8 figures, 1 tabl
Poking the Beehive From Space: K2 Rotation Periods For Praesepe
We analyze {\it K2} light curves for 794 low-mass (
) members of the 650-Myr-old open cluster Praesepe, and
measure rotation periods () for 677 of these stars. We find that half
of the rapidly rotating 0.3 stars are confirmed or candidate
binary systems. The remaining fast rotators have not been
searched for companions, and are therefore not confirmed single stars. We found
previously that nearly all rapidly rotating 0.3 stars in the
Hyades are binaries, but we require deeper binary searches in Praesepe to
confirm whether binaries in these two co-eval clusters have different
distributions. We also compare the observed distribution in Praesepe
to that predicted by models of angular-momentum evolution. We do not observe
the clear bimodal distribution predicted by Brown (2014) for 0.5
stars at the age of Praesepe, but 0.250.5 stars do
show stronger bimodality. In addition, we find that 60\% of early M dwarfs
in Praesepe rotate more slowly than predicted at 650 Myr by Matt et al. (2015),
which suggests an increase in braking efficiency for these stars relative to
solar-type stars and fully convective stars. The incompleteness of surveys for
binaries in open clusters likely impacts our comparison with these models,
since the models only attempt to describe the evolution of isolated single
stars.Comment: ApJ Accepted. 17 pages, 16 figures, for PDF versions of Figures 12-15
with transparency please see the source file
Pushing Automated Abundance Derivations Into the Cool Star Regime: A Test Using Three G and Two K Stars in Praesepe
We present the results of an abundance analysis of three G and two K dwarfs
in the Praesepe open cluster based on high-resolution, moderate
signal-to-noise-ratio spectra obtained with the ARC 3.5-m Telescope at Apache
Point Observatory. Using a Principle Component Analysis and the BACCHUS
automated spectral analysis code, we determined stellar parameters and
abundances of up to 24 elements for each of our targets, which range in
temperature from 6000 to 4600 K. The average derived iron abundance for the
three G stars is 0.17+/-0.07 dex, consistent with the 0.12+/-0.04 dex derived
by Boesgaard et al. (2013) for their sample of 11 solar-type Praesepe members,
which included these G stars. To investigate the efficacy of using automated
routines to derive the abundances of cooler main-sequence stars, we compared
the abundances of the K dwarfs to those of the G dwarfs. Our abundances are
consistent to <=0.1~dex for 13 of the 18 elements we report for all five of the
stars, providing more evidence that G and K stars in a given open cluster are
chemically homogeneous. The median difference between the mean G and K stars
abundances is 0.08+/-0.05 dex, despite serious challenges with the noisier data
for the fainter K dwarfs. Our results are encouraging for chemical tagging, as
they indicate that it may be possible to use automated abundance determination
techniques to identify chemically related main-sequence stars across larger
temperature ranges than are usually considered in these experiments.Comment: 11 pages (plus 29 page long table), 3 figures; accepted by Ap
Constraining the Initial-Final Mass Relation with Wide Double White Dwarfs
In wide double white dwarf (DWD) binaries, in which the co-eval WDs evolve
independently, the more massive, faster-evolving WD can be used to obtain a
main-sequence lifetime for the less-massive WD. By converting this lifetime
into an initial mass for the less-massive WD, and combining it with the
spectroscopically derived mass for this WD, one can constrain the initial-final
mass relation (IFMR). However, the number of known and well-characterized DWDs
is small, severely limiting their usefulness for this analysis. To obtain new
constraints on the IFMR, we search for wide DWDs in the Sloan Digital Sky
Survey (SDSS) Data Release 9. We find 65 new candidate systems, thereby raising
the number of known wide DWDs to 142. We then engage in a spectroscopic
campaign to characterize these pairs, identifying 32 DA/DA pairs, two DA/DB
pairs, four DA/DAH candidate pairs, a previously unidentified candidate triple
WD system, and five DA/DC WDs. We present a reanalysis of the constraint on the
IFMR placed by Finley & Koester (1997) using the DWD PG 0922+162, and finish by
discussing how it could be expanded to a generic set of wide DWDs.Comment: 5 pages, 3 figures, to appear in the "19th European Workshop On White
Dwarfs"-ASP Conference Serie
TESS reveals that the nearby Pisces-Eridanus stellar stream is only 120 Myr old
Pisces-Eridanus (Psc-Eri), a nearby ( 80-226 pc) stellar stream
stretching across 120 degrees of the sky, was recently discovered with
Gaia data. The stream was claimed to be 1 Gyr old, which would make it
an exceptional discovery for stellar astrophysics, as star clusters of that age
are rare and tend to be distant, limiting their utility as benchmark samples.
We test this old age for Psc-Eri in two ways. First, we compare the rotation
periods for 101 low-mass members (measured using time series photometry from
the Transiting Exoplanet Survey Satellite, TESS) to those of well-studied open
clusters. Second, we identify 34 new high-mass candidate members, including the
notable stars Tauri (an Algol-type eclipsing binary) and HD 1160
(host to a directly imaged object near the hydrogen-burning limit). We conduct
an isochronal analysis of the color--magnitude data for these highest-mass
members, again comparing our results to those for open clusters. Both analyses
show that the stream has an age consistent with that of the Pleiades, i.e.,
120 Myr. This makes the Psc-Eri stream an exciting source of young
benchmarkable stars and, potentially, exoplanets located in a more diffuse
environment that is distinct from that of the Pleiades and of other dense star
clusters.Comment: 15 pages, 5 figures, 3 tables. The figure set (101 images) for Figure
2 will be available on AJ upon publicatio
A Serendipitous Pulsar Discovery in a Search for a Companion to a Low-Mass White Dwarf
We report the discovery of a previously unidentified pulsar as part of a
radio campaign to identify neutron star companions to low-mass white dwarfs
(LMWDs) using the Robert C.\ Byrd Green Bank Telescope (GBT). PSR J0802-0955,
which is coincident with the position of a WD with a mass of 0.2 solar masses,
has a pulse period of 571 ms. Because of its relatively long pulse period, the
lack of radial velocity (RV) variations in the radio data, and GBT's large beam
size at the observing frequency of 340 MHz, we conclude that PSR J0802-0955 is
unassociated with the LMWD at roughly the same position and distance.Comment: Accepted for publication in Research Notes of the AA
Today a Duo, But Once a Trio? The Double White Dwarf HS 22202146 May Be A Post-Blue Straggler Binary
For sufficiently wide orbital separations {\it a}, the two members of a
stellar binary evolve independently. This implies that in a wide double white
dwarf (DWD), the more massive WD should always be produced first, when its more
massive progenitor ends its main-sequence life, and should therefore be older
and cooler than its companion. The bound, wide DWD HS 22202146
( AU) does not conform to this picture: the more massive WD is the
younger, hotter of the pair. We show that this discrepancy is unlikely to be
due to past mass-transfer phases or to the presence of an unresolved companion.
Instead, we propose that HS 22202146 formed through a new wide DWD
evolutionary channel involving the merger of the inner binary in a hierarchical
triple system. The resulting blue straggler and its wide companion then evolved
independently, forming the WD pair seen today. Although we cannot rule out
other scenarios, the most likely formation channel has the inner binary merging
while both stars are still on the main sequence. This provides us with the
tantalizing possibility that Kozai-Lidov oscillations may have played a role in
the inner binary's merger. {\it Gaia} may uncover hundreds more wide DWDs,
leading to the identification of other systems like HS 22202146. There are
already indications that other WD systems may have been formed through
different, but related, hierarchical triple evolutionary scenarios.
Characterizing these populations may allow for thorough testing of the
efficiency with which KL oscillations induce stellar mergers.Comment: Accepted for publication in ApJ, 12 pages, 6 figures, 2 table
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