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

The Galactic Distribution of Phosphorus: A Survey of 163 Disk and Halo Stars

Phosphorus (P) is a critical element for life on Earth yet the cosmic production sites of P are relatively uncertain. To understand how P has evolved in the solar neighborhood, we measured abundances for 163 FGK stars over a range of -1.09 $<$ [Fe/H] $<$ 0.47 using observations from the Habitable-zone Planet Finder (HPF) instrument on the Hobby-Eberly Telescope (HET). Atmospheric parameters were calculated by fitting a combination of astrometry, photometry, and Fe I line equivalent widths. Phosphorus abundances were measured by matching synthetic spectra to a P I feature at 10529.52 angstroms. Our [P/Fe] ratios show that chemical evolution models generally under-predict P over the observed metallicity range. Additionally, we find that the [P/Fe] differs by $\sim$ 0.1 dex between thin disk and thick disk stars that were identified with kinematics. The P abundances were compared with $\alpha$-elements, iron-peak, odd-Z, and s-process elements and we found that P in the disk most strongly resembles the evolution of the $\alpha$-elements. We also find molar P/C and N/C ratios for our sample match the scatter seen from other abundance studies. Finally, we measure a [P/Fe] = 0.09 $\pm$ 0.1 ratio in one low-$\alpha$ halo star and probable Gaia-Sausage-Enceladus (GSE) member, an abundance ratio $\sim$ 0.3 - 0.5 dex lower than the other Milky Way disk and halo stars at similar metallicities. Overall, we find that P is likely most significantly produced by massive stars in core collapse supernovae (CCSNe) based on the largest P abundance survey to-date.Comment: 19 pages, 10 figures, 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

Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey

The Galactic stellar halo is predicted to have formed at least partially from the tidal disruption of accreted dwarf galaxies. This assembly history should be detectable in the orbital and chemical properties of stars. The H3 Survey is obtaining spectra for 200,000 stars and, when combined with Gaia data, is providing detailed orbital and chemical properties of Galactic halo stars. Unlike previous surveys of the halo, the H3 target selection is based solely on magnitude and Gaia parallax; the survey therefore provides a nearly unbiased view of the entire stellar halo at high latitudes. In this paper we present the distribution of stellar metallicities as a function of Galactocentric distance and orbital properties for a sample of 4232 kinematically selected halo giants to 100 kpc. The stellar halo is relatively metal-rich, = -1.2, and there is no discernible metallicity gradient over the range 6 30 kpc, respectively. The Sagittarius stream dominates the metallicity distribution at 20-40 kpc for stars on prograde orbits. The Gaia-Enceladus merger remnant dominates the metallicity distribution for radial orbits to approximate to 30 kpc. Metal-poor stars with [Fe/H] < -2 are a small population of the halo at all distances and orbital categories. We associate the "in situ" stellar halo with stars displaying thick disk chemistry on halo-like orbits; such stars are confined to vertical bar z vertical bar < 10 kpc. The majority of the stellar halo is resolved into discrete features in chemical-orbital space, suggesting that the bulk of the stellar halo formed from the accretion and tidal disruption of dwarf galaxies. The relatively high metallicity of the halo derived in this work is a consequence of the unbiased selection function of halo stars and, in combination with the recent upward revision of the total stellar halo mass, implies a Galactic halo metallicity that is typical for its mass.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Debris Disks of Members of the Blanco 1 Open Cluster

We have used the Spitzer Space Telescope to obtain Multiband Imaging Photometer for Spitzer (MIPS) 24 um photometry for 37 members of the ~100 Myr old open cluster Blanco 1. For the brightest 25 of these stars (where we have 3sigma uncertainties less than 15%), we find significant mid-IR excesses for eight stars, corresponding to a debris disk detection frequency of about 32%. The stars with excesses include two A stars, four F dwarfs and two G dwarfs. The most significant linkage between 24 um excess and any other stellar property for our Blanco 1 sample of stars is with binarity. Blanco 1 members that are photometric binaries show few or no detected 24 um excesses whereas a quarter of the apparently single Blanco 1 members do have excesses. We have examined the MIPS data for two other clusters of similar age to Blanco 1 -- NGC 2547 and the Pleiades. The AFGK photometric binary star members of both of these clusters also show a much lower frequency of 24 um excesses compared to stars that lie near the single-star main sequence. We provide a new determination of the relation between V-Ks color and Ks-[24] color for main sequence photospheres based on Hyades members observed with MIPS. As a result of our analysis of the Hyades data, we identify three low mass Hyades members as candidates for having debris disks near the MIPS detection limit.Comment: Accepted to Ap

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 $\approx$600 Myr. We have compiled a sample of 720 spectra --- more than half of which are new observations --- for 516 high-confidence members of Praesepe; we have also obtained 139 new spectra for 130 high-confidence Hyads. We have collected rotation periods ($P_{rot}$) for 135 Praesepe members and 87 Hyads. To compare $H\alpha$ emission, an indicator of chromospheric activity, as a function of color, mass, and Rossby number $R_o$, we first calculate an expanded set of $\chi$ values, with which we can obtain the $H\alpha$ to bolometric luminosity ratio, $L_{H\alpha}/L_{bol}$, even when spectra are not flux-calibrated and/or stars lack reliable distances. Our $\chi$ 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 $H\alpha$ equivalent width or $L_{H\alpha}/L_{bol}$ distributions, and therefore take the merged $H\alpha$ and $P_{rot}$ data to be representative of 600-Myr-old stars. Our analysis shows that $H\alpha$ activity in these stars is saturated for $R_o\leq0.11^{+0.02}_{-0.03}$. Above that value activity declines as a power-law with slope $\beta=-0.73^{+0.16}_{-0.12}$, before dropping off rapidly at $R_o\approx0.4$...Comment: 17 pages, 15 figures, Accepted by Ap