254 research outputs found
Discovery of Par 1802 as a Low-Mass, Pre-Main-Sequence Eclipsing Binary in the Orion Star-Forming Region
We report the discovery of a pre-main-sequence, low-mass, double-lined,
spectroscopic, eclipsing binary in the Orion star-forming region. We present
our observations including radial velocities derived from optical
high-resolution spectroscopy, and present an orbit solution that permits the
determination of precise empirical masses for both components of the system. We
measure that Par 1802 is composed of two equal mass (0.39+-0.03, 0.40+-0.03
Msun) stars in a circular, 4.7 day orbit. There is strong evidence, such as the
system exhibiting strong Li lines and a center-of-mass velocity consistent with
cluster membership, that this system is a member of the Orion star-forming
region and quite possibly the Orion Nebula Cluster, and therefore has an age of
only a few million years. As there are currently only a few empirical mass and
radius measurements for low-mass, PMS stars, this system presents an
interesting test for the predictions of current theoretical models of pre-main
sequence stellar evolution.Comment: 21 pages, 6 figures, 2 tables; Fig 2 caption edite
Employing a New BVIc Photometric Survey of IC 4665 to Investigate the Age of this Young Open Cluster
We present a new, BVIc photometric survey of the young open cluster IC4665,
which improves on previous studies of this young cluster by incorporating a
rigorous standardization procedure, thus providing high-fidelity colors and
magnitudes for cluster members. We use this new photometric dataset to
reevaluate the properties (age and distance) of IC4665. Namely, using a
statistical approach incorporating Tau^2 CMD modeling, we measure a
pre-main-sequence isochrone age and distance of 36+-9 Myr and 360+-12 pc, as
well as a upper-main-sequence turn-off age and distance of 42+-12 Myr and
357+-12 pc. These ages and distances are highly dependent on the isochrone
model and color used for the fitting procedure, with a possible range of ~10-20
Myr in age and ~20 pc in distance. This spread in calculated ages and distances
seen between colors and models is likely due to limitations in the individual
membership catalogs and/or systematic differences in the predicted stellar
parameters from the different sets of models. Interestingly, when we compare
the isochrone ages for IC4665 to the published lithium depletion boundary age,
28+-5 Myr, we observe that this cluster does not appear to follow the trend of
isochrone ages being 1.5 times smaller than lithium depletion boundary ages. In
addition, comparing the overall magnetic activity (X-ray and H-alpha emission)
in IC4665 with other well studied open clusters, we find the observed activity
distributions for this young cluster are best characterized by assuming an age
of 30-40 Myr, thus in agreement with our pre-main-sequence and turn-off
isochrone ages for IC4665. Overall, although some age discrepancies do exist,
particularly in the ages measured from pre-main-sequence isochrones, the range
of possible IC4665 ages derived from the various dating techniques employed
here is relatively small compared to that found for other well studied open
clusters.Comment: 17 pages, 13 figures, 6 tables; Accepted for publication in the
Astronomical Journa
Exploiting kinematics and UBVIc photometry to establish high fidelity membership of the open cluster Blanco 1
We present the results of a wide-field, high-precision UBVIc CCD photometric survey of the Galactic open cluster Blanco 1. Standardized photometry was acquired using the Y4Kcam on the SMARTS 1m telescope at CTIO. We have also determined new high-precision proper motions (σμ = 0.3 mas yr−1) over an eight square degree area down to V = 16.5. Combined with 1D kinematic data, our survey yields a complete list of cluster members down to ~ 0.5 M and new high-fidelity color-magnitude diagrams are presented for Blanco 1. Having established a bona fide membership catalog, astrophysical characteristics of solar-type cluster members such as X-ray activity and lithium abundance have been studied to gain more insights in the process of internal mixing and convection. Our new results should also help to better understand its peculiar location in the Milky Way and to unravel its dynamical histor
Constraining Stellar Rotation at the Zero-Age Main Sequence with TESS
The zero-age main sequence (ZAMS) is a critical phase for stellar angular
momentum evolution, as stars transition from contraction-dominated spin-up to
magnetic wind-dominated spin-down. We present the first robust observational
constraints on rotation for FGK stars at Myr. We have analyzed TESS
light curves for 1410 members of five young open clusters with ages between
25-55 Myr: IC 2391, IC 2602, NGC 2451A, NGC 2547, and Collinder 135. In total,
we measure 868 rotation periods, including 96 new, high-quality periods for
stars around 1 . This is an increase of ten times the existing
literature sample at the ZAMS. We then use the method to compare our
data to models for stellar angular momentum evolution. Although the ages
derived from these rotation models do not match isochronal ages, we show these
observations can clearly discriminate between different models for stellar wind
torques. Finally, fits indicate that magnetic braking and/or internal
angular momentum transport significantly impact rotational evolution even on
the pre-main sequence.Comment: 20 pages, 11 figures, 4 tables; ApJ Accepte
The Factory and The Beehive II. Activity and Rotation in Praesepe and the Hyades
Open clusters are collections of stars with a single, well-determined age,
and can be used to investigate the connections between angular-momentum
evolution and magnetic activity over a star's lifetime. We present the results
of a comparative study of the relationship between stellar rotation and
activity in two benchmark open clusters: Praesepe and the Hyades. As they have
the same age and roughly solar metallicity, these clusters serve as an ideal
laboratory for testing the agreement between theoretical and empirical
rotation-activity relations at 600 Myr. We have compiled a sample of
720 spectra --- more than half of which are new observations --- for 516
high-confidence members of Praesepe; we have also obtained 139 new spectra for
130 high-confidence Hyads. We have collected rotation periods () for
135 Praesepe members and 87 Hyads. To compare emission, an indicator
of chromospheric activity, as a function of color, mass, and Rossby number
, we first calculate an expanded set of values, with which we can
obtain the to bolometric luminosity ratio, ,
even when spectra are not flux-calibrated and/or stars lack reliable distances.
Our values cover a broader range of stellar masses and colors (roughly
equivalent to spectral types from K0 to M9), and exhibit better agreement
between independent calculations, than existing values. We find no difference
between the two clusters in their equivalent width or
distributions, and therefore take the merged
and data to be representative of 600-Myr-old stars. Our analysis
shows that activity in these stars is saturated for
. Above that value activity declines as a
power-law with slope , before dropping off rapidly
at ...Comment: 17 pages, 15 figures, Accepted by Ap
A Lower Limit on the Mass of Our Galaxy from the H3 Survey
The timing argument provides a lower limit on the mass of the Milky Way. We
find, using a sample of 32 stars at kpc drawn from the H3
Spectroscopic Survey and mock catalogs created from published numerical
simulations, that M M with 90% confidence.
We recommend using this limit to refine the allowed prior mass range in more
complex and sophisticated statistical treatments of Milky Way dynamics. The use
of such a prior would have significantly reduced many previously published
uncertainty ranges. Our analysis suggests that the most likely value of
M is M, but establishing this as the
Milky Way mass requires a larger sample of outer halo stars and a more complete
analysis of the inner halo stars in H3. The imminent growth in the sample of
outer halo stars due to ongoing and planned surveys will make this possible.Comment: 8 pages, submitted for publicatio
Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
In the CDM paradigm the Galactic stellar halo is predicted to harbor
the accreted debris of smaller systems. To identify these systems, the H3
Spectroscopic Survey, combined with , is gathering 6D phase-space and
chemical information in the distant Galaxy. Here we present a comprehensive
inventory of structure within 50 kpc from the Galactic center using a sample of
5684 giants at and kpc. We identify known structures
including the high- disk, the in-situ halo (disk stars heated to
eccentric orbits), Sagittarius (Sgr), -Sausage-Enceladus (GSE), the Helmi
Streams, Sequoia, and Thamnos. Additionally, we identify the following new
structures: (i) Aleph ([Fe/H]), a low eccentricity structure that rises
a surprising 10 kpc off the plane, (ii, iii) Arjuna ([Fe/H]) and I'itoi
([Fe/H]), which comprise the high-energy retrograde halo along with
Sequoia, and (iv) Wukong ([Fe/H]), a prograde phase-space overdensity
chemically distinct from GSE. For each structure we provide [Fe/H],
[/Fe], and orbital parameters. Stars born within the Galaxy are a major
component at 2 kpc (60), but their relative fraction
declines sharply to 5 past 15 kpc. Beyond 15 kpc, 80 of
the halo is built by two massive ()
accreted dwarfs: GSE ([Fe/H]) within 25 kpc, and Sgr ([Fe/H])
beyond 25 kpc. This explains the relatively high overall metallicity of the
halo ([Fe/H]). We attribute 95 of the sample to one
of the listed structures, pointing to a halo built entirely from accreted
dwarfs and heating of the disk.Comment: Submitted to ApJ. Key results in Figures 18-21. Summary of individual
structures in Sec. 3.3 and Table 1. Comments very welcome
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