The Chemical Evolution of Phosphorus

Phosphorus is one of the few remaining light elements for which little is known about its nucleosynthetic origin and chemical evolution, given the lack of optical absorption lines in the spectra of long-lived FGK-type stars. We have identified a P I doublet in the near-ultraviolet (2135/2136 A) that is measurable in stars of low metallicity. Using archival Hubble Space Telescope-STIS spectra, we have measured P abundances in 13 stars spanning -3.3 <= [Fe/H] <= -0.2, and obtained an upper limit for a star with [Fe/H] ~ -3.8. Combined with the only other sample of P abundances in solar-type stars in the literature, which spans a range of -1 <= [Fe/H] <= +0.2, we compare the stellar data to chemical evolution models. Our results support previous indications that massive-star P yields may need to be increased by a factor of a few to match stellar data at all metallicities. Our results also show that hypernovae were important contributors to the P production in the early universe. As P is one of the key building blocks of life, we also discuss the chemical evolution of the important elements to life, C-N-O-P-S, together.Comment: Accepted for publication in Astrophysical Journal Letters. 6 pages, 4 figures; reference added to earlier versio

The Evolution of Protoplanetary Disks: Probing the Inner Disk of Very Low Accretors

We report FUV, optical, and NIR observations of three T Tauri stars in the Orion OB1b subassociation with H$\alpha$ equivalent widths consistent with low or absent accretion and various degrees of excess flux in the mid-infrared. We aim to search for evidence of gas in the inner disk in HST ACS/SBC spectra, and to probe the accretion flows onto the star using H$\alpha$ and He I $\lambda$10830 in spectra obtained at the Magellan and SOAR telescopes. At the critical age of 5 Myr, the targets are at different stages of disk evolution. One of our targets is clearly accreting, as shown by redshifted absorption at free-fall velocities in the He I line and wide wings in H$\alpha$; however, a marginal detection of FUV H$_2$ suggests that little gas is present in the inner disk, although the spectral energy distribution indicates that small dust still remains close to the star. Another target is surrounded by a transitional disk, with an inner cavity in which little sub-micron dust remains. Still, the inner disk shows substantial amounts of gas, accreting onto the star at a probably low, but uncertain rate. The third target lacks both a He I line or FUV emission, consistent with no accretion or inner gas disk; its very weak IR excess is consistent with a debris disk. Different processes occurring in targets with ages close to the disk dispersal time suggest that the end of accretion phase is reached in diverse ways.Comment: 13 pages, 3 figures. Accepted for publication in The Astrophysical Journa

Linking Signatures of Accretion with Magnetic Field Measurements - Line Profiles are not Significantly Different in Magnetic and Non-Magnetic Herbig Ae/Be Stars

Herbig Ae/Be stars are young, pre-main-sequence stars that sample the transition in structure and evolution between low- and high-mass stars, providing a key test of accretion processes in higher-mass stars. Few Herbig Ae/Be stars have detected magnetic fields, calling into question whether the magnetospheric accretion paradigm developed for low-mass stars can be scaled to higher masses. We present He I 10830 \AA\ line profiles for 64 Herbig Ae/Be stars with a magnetic field measurement in order to test magnetospheric accretion in the physical regime where its efficacy remains uncertain. Of the 5 stars with a magnetic field detection, 1 shows redshifted absorption, indicative of infall, and 2 show blueshifted absorption, tracing mass outflow. The fraction of redshifted and blueshifted absorption profiles in the non-magnetic Herbig Ae/Be stars is remarkably similar, suggesting that the stellar magnetic field does not affect gas kinematics traced by He I 10830 \AA. Line profile morphology does not correlate with the luminosity, rotation rate, mass accretion rate, or disk inclination. Only the detection of a magnetic field and a nearly face-on disk inclination show a correlation (albeit for few sources). This provides further evidence for weaker dipoles and more complex field topologies as stars develop a radiative envelope. The small number of magnetic Herbig Ae/Be stars has already called into question whether magnetospheric accretion can be scaled to higher masses; accretion signatures are not substantially different in magnetic Herbig Ae/Be stars, casting further doubt that they accrete in the same manner as classical T Tauri stars.Comment: accepted to ApJ; 17 pages, 4 figures, 3 table

Towards a Comprehensive View of Accretion, Inner Disks, and Extinction in Classical T Tauri Stars: An ODYSSEUS Study of the Orion OB1b Association

The coevolution of T Tauri stars and their surrounding protoplanetary disks dictates the timescales of planet formation. In this paper, we present magnetospheric accretion and inner disk wall model fits to near-UV (NUV) to near-IR (NIR) spectra of nine classical T Tauri stars in Orion OB1b as part of the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra (ODYSSEUS) survey. Using NUV-optical spectra from the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Director's Discretionary Program and optical-NIR spectra from the PENELLOPE VLT Large Programme, we find that the accretion rates of these targets are relatively high for the region's intermediate age of 5.0 Myr; rates are in the range of (0.5-17.2) x 10(-8) M-circle dot yr(-1), with a median value of 1.2 x 10(-8) M-circle dot yr(-1). The NIR excesses can be fit with 1200-1800 K inner disk walls located at 0.05-0.10 au from the host stars. We discuss the significance of the choice in extinction law, as the measured accretion rate depends strongly on the adopted extinction value. This analysis will be extended to the complete sample of T Tauri stars being observed through ULLYSES to characterize accretion and inner disks in star-forming regions of different ages and stellar populations

Towards a Comprehensive View of Accretion, Inner Disks, and Extinction in Classical T Tauri Stars: An ODYSSEUS Study of the Orion OB1b Association

The coevolution of T Tauri stars and their surrounding protoplanetary disks dictates the timescales of planet formation. In this paper, we present magnetospheric accretion and inner disk wall model fits to near-UV (NUV) to near-IR (NIR) spectra of nine classical T Tauri stars in Orion OB1b as part of the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra (ODYSSEUS) survey. Using NUV&ndash;optical spectra from the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Director's Discretionary Program and optical&ndash;NIR spectra from the PENELLOPE VLT Large Programme, we find that the accretion rates of these targets are relatively high for the region's intermediate age of 5.0 Myr; rates are in the range of (0.5&ndash;17.2) &times; 10&minus;8 M☉ yr&minus;1, with a median value of 1.2 &times; 10&minus;8 M☉ yr&minus;1. The NIR excesses can be fit with 1200&ndash;1800 K inner disk walls located at 0.05&ndash;0.10 au from the host stars. We discuss the significance of the choice in extinction law, as the measured accretion rate depends strongly on the adopted extinction value. This analysis will be extended to the complete sample of T Tauri stars being observed through ULLYSES to characterize accretion and inner disks in star-forming regions of different ages and stellar populations. &nbsp;</p

Haze in Pluto's atmosphere: Results from SOFIA and ground-based observations of the 2015 June 29 Pluto occultation

On UT 29 June 2015, the occultation by Pluto of a bright star (r′ = 11.9) was observed from the Stratospheric Observatory for Infrared Astronomy (SOFIA) and several ground-based stations in New Zealand and Australia. Pre-event astrometry allowed for an in-flight update to the SOFIA team with the result that SOFIA was deep within the central flash zone (~22 km from center). Analysis of the combined data leads to the result that Pluto's middle atmosphere is essentially unchanged from 2011 and 2013 (Person et al. 2013; Bosh et al. 2015); there has been no significant expansion or contraction of the atmosphere. Additionally, our multi-wavelength observations allow us to conclude that a haze component in the atmosphere is required to reproduce the light curves obtained. This haze scenario has implications for understanding the photochemistry of Pluto's atmosphere

The Last Stages of Accretion in Young Objects

Accretion from protoplanetary disks onto low mass, young stars (T Tauri stars) has been extensively studied for several decades. Theoretical, observational, and modeling efforts show that accretion from the disk onto the star follows the magnetospheric accretion paradigm, in which magnetic field lines truncate the disk and mass flows along the field lines onto the stellar surface. Studies of large samples of stars in many star-forming regions have shown that the fraction of accretors decreases as the population's age increases. Nevertheless, it is still unclear how accretion stops. Understanding the processes shutting off accretion will provide crucial information for studying the properties of the disks in which planets are forming and of the stars that host them. Here, we present a comprehensive study of stars at the last stages of their accretion phase, aiming to shed some light on the processes driving accretion to stop. By studying the gas in the inner disk using FUV observations and the dust from near-infrared excess, we found that the disks of low accretors are diverse in dust emission, suggesting that the end of accretion is reached in different ways. We also confirmed previous results that stars stop accreting as soon as the inner disk has no gas left. We showed that the He I line at 10830 Angstrom is more sensitive at detecting accretion than diagnostics using the Balmer alpha line of hydrogen. Using this accretion diagnostics, we re-classify 51 stars previously thought to be non-accretors as accretors. We studied a subset of these stars and found that, at low accretion rates, magnetospheric flows accrete mass in the unstable regime and that the geometry of the accretion region is complex. Based on the relationship between the inferred disk truncation radius, the corotation radius, and the mass accretion rate, we proposed that the dipolar fields in low accretors are weak and that the efficiency for the magnetic fields to truncate the disks is low. We found no low accretors in the propeller regime, suggesting that the propeller is not the primary process inhibiting accretion at the last stages of disk evolution. We found evidence of a physical lower limit of how much mass can be supplied to the star from its disk, at 10^-10 solar mass per year. This rate is consistent with the EUV-driven photoevaporative mass loss rate, which suggests that the properties of the outer disk control accretion from the inner disk onto the star. However, a possibility of an inner disk mass reservoir cannot be ruled out, as shown by the high mass accretion rate of PDS 70 for low mass transport efficiency. Lastly, we showed that the magnetospheric accretion model could be applied to the case of accreting giant planets, assuming that they have magnetic field strength on the order of hundreds Gauss. For the planets around PDS 70, we found that mass accreting into planets is much less than that onto the star, suggesting that giant planet formation, although important in creating gaps in the disk, does not directly starve the star of accreted mass. Our results provided essential constraints in studying protoplanetary disk evolution, star-disk interactions, and the process of planet formation. Including these constraints in simulations will provide a more complete picture of how young stars, protoplanetary disks, and planets form and evolve.PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/170073/1/thanathi_1.pd