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

Herschel/PACS View Of Disks Around Low-Mass Stars And Brown Dwarfs In The TW Hydrae Association

We conducted Herschel/PACS observations of five very low-mass stars or brown dwarfs located in the TW Hya association with the goal of characterizing the properties of disks in the low stellar mass regime. We detected all five targets at 70 mu m and 100 mu m and three targets at 160 mu m. Our observations, combined with previous photometry from 2MASS, WISE, and SCUBA-2, enabled us to construct spectral energy distributions (SEDs) with extended wavelength coverage. Using sophisticated radiative transfer models, we analyzed the observed SEDs of the five detected objects with a hybrid fitting strategy that combines the model grids and the simulated annealing algorithm and evaluated the constraints on the disk properties via the Bayesian inference method. The modeling suggests that disks around low-mass stars and brown dwarfs are generally flatter than their higher mass counterparts, but the range of disk mass extends to well below the value found in T Tauri stars, and the disk scale heights are comparable in both groups. The inferred disk properties (i.e., disk mass, flaring, and scale height) in the low stellar mass regime are consistent with previous findings from large samples of brown dwarfs and very low-mass stars. We discuss the dependence of disk properties on their host stellar parameters and find a significant correlation between the Herschel far-IR fluxes and the stellar effective temperatures, probably indicating that the scaling between the stellar and disk masses (i.e., M-disk proportional to M-star) observed mainly in low-mass stars may extend down to the brown dwarf regime.Natural Science Foundation of Jiangsu Province of China BK20141046Youth Qianren Program of the National Science Foundation of ChinaNational Aeronautics and Space AdministrationStrategic Priority Research Program >The Emergence of Cosmological Structures> of the Chinese Academy of Sciences XDB09000000Astronom

Pyroelectric and photovoltaic properties of Nb doped PZT thin films

Nb-doped lead zirconate titanate (PZT) films with up to 12 at. % of Nb were co-sputtered from oxide PZT and metallic Nb targets at a substrate temperature of 600 °C. Up to 4 at. % of Nb was doped into the perovskite structure with the formation of B-site cation vacancies for charge compensation. The preferential (111) PZT orientation decreased with Nb-doping within the solid solution region. The ferroelectric response of the films was affected by the large values of the internal field present in the samples (e.g., −84.3 kV cm−1 in 12 at. % Nd doped films). As-deposited unpoled films showed large values of the pyroelectric coefficient due to self-poling. The pyroelectric coefficient increased with Nb-doping and showed a complex dependence on the applied bias. The photovoltaic effect was observed in the films. The value of the photocurrent increased with the A/B ratio. The combined photovoltaic–pyroelectric effect increased the values of the measured current by up to 47% upon light illumination

Characterizing Young Brown Dwarfs using Low Resolution Near-IR Spectra

We present near-infrared (1.0-2.4 micron) spectra confirming the youth and cool effective temperatures of 6 brown dwarfs and low mass stars with circumstellar disks toward the Chamaeleon II and Ophiuchus star forming regions. The spectrum of one of our objects indicates that it has a spectral type of ~L1, making it one of the latest spectral type young brown dwarfs identified to date. Comparing spectra of young brown dwarfs, field dwarfs, and giant stars, we define a 1.49-1.56 micron H2O index capable of determining spectral type to within 1 sub-type, independent of gravity. We have also defined an index based on the 1.14 micron sodium feature that is sensitive to gravity, but only weakly dependent on spectral type for field dwarfs. Our 1.14 micron Na index can be used to distinguish young cluster members (t <~ 5 Myr) from young field dwarfs, both of which may have the triangular H-band continuum shape which persists for at least tens of Myr. Using effective temperatures determined from the spectral types of our objects along with luminosities derived from near and mid-infrared photometry, we place our objects on the H-R diagram and overlay evolutionary models to estimate the masses and ages of our young sources. Three of our sources have inferred ages (t ~= 10-30 Myr) significantly older than the median stellar age of their parent clouds (1-3 Myr). For these three objects, we derive masses ~3 times greater than expected for 1-3 Myr old brown dwarfs with the bolometric luminosities of our sources. The large discrepancies in the inferred masses and ages determined using two separate, yet reasonable methods, emphasize the need for caution when deriving or exploiting brown dwarf mass and age estimates.Comment: 11 pages, Accepted to Ap

Physical Properties of Young Brown Dwarfs and Very Low-Mass Stars Inferred from High-Resolution Model Spectra

By comparing near-infrared spectra with atmosphere models, we infer the effective temperature, surface gravity, projected rotational velocity, and radial velocity for 21 very-low-mass stars and brown dwarfs. The unique sample consists of two sequences in spectral type from M6-M9, one of 5-10 Myr objects and one of >1 Gyr field objects. A third sequence is comprised of only ~M6 objects with ages ranging from 1 Gyr. Spectra were obtained in the J band at medium (R~2,000) and high (R~20,000) resolutions with NIRSPEC on the Keck II telescope. Synthetic spectra were generated from atmospheric structures calculated with the PHOENIX model atmosphere code. Using multi-dimensional least-squares fitting and Monte Carlo routines we determine the best-fit model parameters for each observed spectrum and note which spectral regions provide consistent results. We identify successes in the reproduction of observed features by atmospheric models, including pressure-broadened KI lines, and investigate deficiencies in the models, particularly missing FeH opacity, that will need to be addressed in order to extend our analysis to cooler objects. The precision that can be obtained for each parameter using medium- and high- resolution near-infrared spectra is estimated and the implications for future studies of very low mass stars and brown dwarfs are discussed.Comment: Accepted to the Astrophysical Journal Supplement Serie

Oph 1622-2405: Not a Planetary-Mass Binary

We present an analysis of the mass and age of the young low-mass binary Oph 1622-2405. Using resolved optical spectroscopy of the binary, we measure spectral types of M7.25+/-0.25 and M8.75+/-0.25 for the A and B components, respectively. We show that our spectra are inconsistent with the spectral types of M9 and M9.5-L0 from Jayawardhana & Ivanov and M9+/-0.5 and M9.5+/-0.5 from Close and coworkers. Based on our spectral types and the theoretical evolutionary models of Chabrier and Baraffe, we estimate masses of 0.055 and 0.019 Msun for Oph 1622-2405A and B, which are significantly higher than the values of 0.013 and 0.007 Msun derived by Jayawardhana & Ivanov and above the range of masses observed for extrasolar planets (M<=0.015 Msun). Planet-like mass estimates are further contradicted by our demonstration that Oph 1622-2405A is only slightly later (by 0.5 subclass) than the composite of the young eclipsing binary brown dwarf 2M 0535-0546, whose components have dynamical masses of 0.034 and 0.054 Msun. To constrain the age of Oph 1622-2405, we compare the strengths of gravity-sensitive absorption lines in optical and near-infrared spectra of the primary to lines in field dwarfs (>1 Gyr) and members of Taurus (~1 Myr) and Upper Scorpius (~5 Myr). The line strengths for Oph 1622-2405A are inconsistent with membership in Ophiuchus (<1 Myr) and instead indicate an age similar to that of Upper Sco, which is agreement with a similar analysis performed by Close and coworkers. We conclude that Oph 1622-2405 is part of an older population in Sco-Cen, perhaps Upper Sco itself.Comment: The Astrophysical Journal, in pres

Ion Mobility Shift of Isotopologues in a High Kinetic Energy Ion Mobility Spectrometer (HiKE-IMS) at Elevated Effective Temperatures

Ion mobility spectrometers (IMS) separate ions mainly by ion–neutral collision cross section and to a lesser extent by ion mass and effective temperature. When investigating isotopologues, the difference in collision cross section can be assumed negligible. Since the mobility shift of isotopologues is thus mainly caused by their difference in mass and effective temperature, the investigation of isotopologues can provide important insights into the theory of ion mobility. However, in classical IMS operated at ambient pressure, cluster formation with neutral molecules occurs, which significantly influences the mobility shift of isotopologues and thus makes a sound investigation of the effect of ion mass and effective temperature on the ion mobility difficult. In this work, the relative ion mobility of several organic compounds and their 13C-labeled isotopologues is studied in a High Kinetic Energy Ion Mobility Spectrometer (HiKE-IMS) at high reduced electric fields up to 120 Td, which allows the investigation of nonclustered ion species and thus enables a sound investigation of the mobility shift of isotopologues. The results show that the measured relative ion mobilities of isotopologues having the same effective temperature and, thus, their ion mass dominating the relative ion mobility agree well with theoretical relative ion mobilities predicted by the theory of ion mobility

Observation and modelling of dusty, low gravity L, and M dwarfs

Observational facilities allow now the detection of optical and IR spectra of young M- and L-dwarfs. This enables empirical comparisons with old M- and L- dwarfs, and detailed studies in comparison with synthetic spectra. While classical stellar atmosphere physics seems perfectly appropriate for old M-dwarfs, more physical and chemical processes, cloud formation in particular, needs to be modelled in the substellar regime to allow a detailed spectral interpretation. Not much is known so far about the details of the inset of cloud formation at the spectral transition region between M and L dwarfs. Furthermore there is observational evidence for diversity in the dust properties of objects having the same spectral type. Do we understand these differences? The question is also how young M- and L-dwarfs need to be classified, which stellar parameter do they have and whether degenerations in the stellar parameter space due to the changing atmosphere physics are present, like in the L-T transition region. The Splinter was driven by these questions which we will use to encourage interactions between observation and theory. Given the recent advances, both in observations and spectral modelling, an intensive discussion between observers and theoreticians will create new synergies in our field.Comment: Cool Stars XV splinter session summary, to appear in proceedings of Cool Stars XV Conference, 8 page