6 research outputs found
Radius Dependent Angular Momentum Evolution in Low-Mass Stars. I
Angular momentum evolution in low-mass stars is determined by initial
conditions during star formation, stellar structure evolution, and the
behaviour of stellar magnetic fields. Here we show that the empirical picture
of angular momentum evolution arises naturally if rotation is related to
magnetic field strength instead of to magnetic flux, and formulate a corrected
braking law based on this. Angular momentum evolution then becomes a strong
function of stellar radius, explaining the main trends observed in open
clusters and field stars at a few Gyr: the steep transition in rotation at the
boundary to full convection arises primarily from the large change in radius
across this boundary, and does not require changes in dynamo mode or field
topology. Additionally, the data suggest transient core-envelope decoupling
among solar-type stars, and field saturation at longer periods in very low-mass
stars. For solar-type stars, our model is also in good agreement with the
empirical Skumanich law. Finally, in further support of the theory, we show
that the predicted age at which low-mass stars spin down from the saturated to
unsaturated field regimes in our model corresponds remarkably well to the
observed lifetime of magnetic activity in these stars.Comment: accepted by Ap
The Palomar Transient Factory Orion Project: Eclipsing Binaries and Young Stellar Objects
The Palomar Transient Factory (PTF) Orion project is an experiment within the
broader PTF survey, a systematic automated exploration of the sky for optical
transients. Taking advantage of the wide field of view available using the PTF
camera at the Palomar 48" telescope, 40 nights were dedicated in December
2009-January 2010 to perform continuous high-cadence differential photometry on
a single field containing the young (7-10Myr) 25 Ori association. The primary
motivation for the project is to search for planets around young stars in this
region. The unique data set also provides for much ancillary science. In this
first paper we describe the survey and data reduction pipeline, and present
initial results from an inspection of the most clearly varying stars relating
to two of the ancillary science objectives: detection of eclipsing binaries and
young stellar objects. We find 82 new eclipsing binary systems, 9 of which we
are candidate 25 Ori- or Orion OB1a-association members. Of these, 2 are
potential young W UMa type systems. We report on the possible low-mass (M-dwarf
primary) eclipsing systems in the sample, which include 6 of the candidate
young systems. 45 of the binary systems are close (mainly contact) systems; one
shows an orbital period among the shortest known for W UMa binaries, at
0.2156509 \pm 0.0000071d, with flat-bottomed primary eclipses, and a derived
distance consistent with membership in the general Orion association. One of
the candidate young systems presents an unusual light curve, perhaps
representing a semi-detached binary system with an inflated low-mass primary or
a star with a warped disk, and may represent an additional young Orion member.
Finally, we identify 14 probable new classical T-Tauri stars in our data, along
with one previously known (CVSO 35) and one previously reported as a candidate
weak-line T-Tauri star (SDSS J052700.12+010136.8).Comment: 66 pages, 27 figures, accepted to Astronomical Journal. Minor
typographical corrections and update to author affiliation
DRAFTS: A Deep, Rapid Archival Flare Transient Search in the Galactic Bulge
We utilize the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS)
HST/ACS dataset for a Deep Rapid Archival Flare Transient Search (DRAFTS) to
constrain the flare rate toward the older stellar population in the Galactic
bulge. During 7 days of monitoring 229,293 stars brighter than V=29.5, we find
evidence for flaring activity in 105 stars between V=20 and V=28. We divided
the sample into non-variable stars and variable stars whose light curves
contain large-scale variability. The flare rate on variable stars is \sim 700
times that of non-variable stars, with a significant correlation between the
amount of underlying stellar variability and peak flare amplitude. The flare
energy loss rates are generally higher than those of nearby well-studied single
dMe flare stars. The distribution of proper motions is consistent with the
flaring stars being at the distance and age of the Galactic bulge. If they are
single dwarfs, they span a range of \approx 1.0 - 0.25M\odot. A majority of the
flaring stars exhibit periodic photometric modulations with P <3d. If these are
tidally locked magnetically active binary systems, their fraction in the bulge
is enhanced by a factor of \sim20 compared to the local value. These stars may
be useful for placing constraints on the angular momentum evolution of cool
close binary stars. Our results expand the type of stars studied for flares in
the optical band, and suggest that future sensitive optical time-domain studies
will have to contend with a larger sample of flaring stars than the M dwarf
flare stars usually considered.Comment: accepted for publication in the Astrophysical Journa