Seth Redfield

New mass-loss measurements from astrospheric Lyα absorption

Measurements of stellar mass-loss rates are used to assess how wind strength varies with coronal activity and age for solar-like stars. Mass loss generally increases with activity, but we find evidence that winds suddenly weaken at a certain activity threshold. Very active stars are often observed to have polar starspots, and we speculate that the magnetic field geometry associated with these spots may be inhibiting the winds. Our inferred mass-loss/age relation represents an empirical estimate of the history of the solar wind. This result is important for planetary studies as well as solar/stellar astronomy, since solar wind erosion may have played an important role in the evolution of planetary atmospheres.

The Structure of the Local Interstellar Medium. IV. Dynamics, Morphology, Physical Properties, and Implications of Cloud-Cloud Interactions*

We present an empirical dynamical model of the LISM based on 270 radial velocity measurements for 157 sight lines toward nearby stars. Physical parameter measurements (i.e., temperature, turbulent velocity, depletions) are availablefor90components,orone-thirdof thesample,enablinginitialcharacterizationsof thephysicalpropertiesof LISM clouds. The model includes 15 warm clouds located within 15 pc of the Sun, each with a different velocity vector. We derive projected morphologies of all clouds and estimate the volume filling factor of warm partially ionized material intheLISMtobebetween � 5.5%and 19%.Relative velocitiesof potentially interactingclouds areoftensupersonic, consistent with heating, turbulent, and metal depletion properties. Cloud-cloud collisions may be responsible for the filamentary morphologies found in � 1 of LISM clouds, the distribution of clouds along the boundaries of the two nearestclouds(LICandG),thedetailedshapeandheatingof theMicCloud,thelocationof nearbyradioscintillation screens, and the location of an LISM cold cloud. Contrary to previous claims, the Sun appears to be located in the transition zone between the LIC and G Cloud. Subject headingg ISM: atoms — ISM: clouds — ISM: structure — line: profiles — ultraviolet: ISM — ultraviolet: starsWe present a comprehensive survey of C II absorption detections toward stars within 100 pc in order to measure the distribution of electron densities present in the local interstellar medium (LISM). Using high spectral resolution observations of nearby stars obtained by the Goddard High-Resolution Spectrograph (GHRS) and the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST), we searched for all detections of LISM C II absorption. We identify 13 sight lines with 23 individual C II absorption components, which provide electron density measurements, the vast majority of which are new. We employ several strategies to determine more accurate C II column densities from the saturated C II resonance line, including, constraints of the line width from the optically thin C II line, constraints from independent temperature measurements of the LISM gas based on line widths of other ions, and third, using measured S II column densities as a proxy for C II column densities. The distribution of electron densities based on using S II as a proxy for C II is similar to the distribution based on carbon alone, while significantly tighter, and proves to be a promising technique to avoid grossly overestimating the C II column density based on the saturated line profile. The sample of electron densities appears consistent with a log-normal distribution and an unweighted mean value of ne(C IISII) = 0.11 +0.10 −0.05 cm. Seven individual sight lines probe the Local Interstellar Cloud (LIC), and all present a similar value for the electron density, with a weighted mean of ne(LIC) = 0.12±0.04 cm . Two clouds, the NGP and Gem clouds, show similar electron density properties as the LIC. The Hyades Cloud, a decelerated cloud at the leading edge of the platoon of LISM clouds, has a significantly higher electron density than the LIC. Observed toward G191-B2B, the high electron density may be caused by the lack of shielding from such a strong radiation source.

What Is the Total Deuterium Abundance in the Local Galactic Disk

Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, together with spectra from the Copernicus and interstellar medium absorption profile spectrograph (IMAPS) instruments, reveal an unexplained, very wide range in the observed deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk beyond the Local Bubble. We argue that spatial variations in the depletion of deuterium onto dust grains can explain these local variations in the observed gas-phase D/H ratios. We present a variable deuterium depletion model that naturally explains the constant measured values of D/H inside the Local Bubble, the wide range of gas-phase D/H ratios observed in the intermediate regime [log N(H ) = 19.2-20.7], and the low gas-phase D/H ratios observed at larger hydrogen column densities. We consider empirical tests of the deuterium depletion hypothesis: (1) correlations of gas-phase D/H ratios with depletions of the refractory metals iron and silicon, and (2) correlation with the H2 rotational temperature. Both of these tests are consistent with deuterium depletion from the gas phase in cold, not recently shocked regions of the ISM, and high gas-phase D/H ratios in gas that has been shocked or otherwise heated recently. We argue that the most representative value for the total (gas plus dust) D/H ratio within 1 kpc of the Sun is ≥23.1 ± 2.4(1 σ) parts per million (ppm). This ratio constrains Galactic chemical evolution models to have a very small deuterium astration factor, the ratio of primordial to total (D/H) ratio in the local region of the Galactic disk, which we estimate to be fd ≤ 1.19(1 σ) or ≤1.12 ± 0.14(1 σ) depending on the adopted light-element nuclear reaction rates.

Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres*

We search the Hubble Space Telescope (HST) archive for previously unanalyzed observations of stellar H I Lyα emission lines, our primary purpose being to look for new detections of Lyα absorption from the outer heliosphere and to also search for analogous absorption from the astrospheres surrounding the observed stars. The astrospheric absorption is of particular interest because it can be used to study solar-like stellar winds that are otherwise undetectable. We find and analyze 33 HST Lyα spectra in the archive. All the spectra were taken with the E140M grating of the Space Telescope Imaging Spectrograph (STIS) instrument on board HST. The HST STIS spectra yield four new detections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD 165185) and seven new detections of astrospheric absorption (EV Lac, 70 Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling the previous number of heliospheric and astrospheric detections. When combined with previous results, 10 of 17 lines of sight within 10 pc yield detections of astrospheric absorption. This high detection fraction implies that most of the ISM within 10 pc must be at least partially neutral, since the presence of H I within the ISM surrounding the observed star is necessary for an astrospheric detection. In contrast, the detection percentage is only 9.7% (3 out of 31) for stars beyond 10 pc. Our Lyα analyses provide measurements of ISM H I and D I column densities for all 33 lines of sight, and we discuss some implications of these results. Finally, we measure chromospheric Lyα fluxes from the observed stars. We use these fluxes to determine how Lyα flux correlates with coronal X-ray and chromospheric Mg II emission, and we also study how Lyα emission depends on stellar rotation.

The Three-dimensional Structure of the Warm Local Interstellar Medium. II. The Colorado Model of the Local Interstellar Cloud

In this second paper in a series on the structure of the local interstellar medium (LISM), we construct a three-dimensional model of the local interstellar cloud (LIC) based on Hubble Space Telescope (HST), Extreme Ultraviolet Explorer (EUVE), and ground-based Ca II spectra. Starting with hydrogen column densities derived from deuterium column densities measured with the Goddard High Resolution Spectrograph instrument on HST for 16 lines of sight to nearby stars, we derive a model consisting of the sum of nine spherical harmonics that best fit the data. We then rederive the model by including the lines of sight to three hot white dwarfs observed by EUVE and 13 lines of sight with Ca II column densities at the projected LIC velocity. The LIC model is clearly not a long thin filamentary structure like optical images of some interstellar clouds (e.g., reflection nebulae in the Pleiades), but neither is it spherical in shape. As seen from the north Galactic pole, the LIC is egg-shaped with an axis of symmetry that points in the direction l ≈ 315°. Since the direction of the center of the Scorpius-Centaurus association is l = 320°, the shape of the LIC could be determined by the flow of hot gas from Sco-Cen. The model shows that the Sun is located just inside the LIC in the direction of the Galactic center and toward the north Galactic pole. The absence of Mg II absorption at the LIC velocity toward α Cen indicates that the distance to the edge of the LIC in this direction is ≤0.05 pc and the Sun should cross the boundary between the LIC and the Galactic (G) cloud in less than 3000 yr. We estimate that the volume of the LIC is about 93 pc3 and its mass is about 0.32 M☉. The physical parameters and hydrogen column density of the LIC are roughly consistent with theoretical models of the warm interstellar medium that assume pressure and ionization equilibrium. However, the empirical hydrogen ionization of the LIC is much higher and the gas temperature lower than the theoretical models predict. Therefore, the ionization is likely due to shock activity from a nearby supernova that has not yet reached equilibrium. The higher ionization increases the gas cooling, which can explain why the gas is 2400 K cooler than the ionization equilibrium models predict. Computed and observed temperatures are in agreement for a model with the observed LIC electron density.

Rocky planetesimals as the origin of metals in DZ stars

The calcium and hydrogen abundances, Galactic positions and kinematics of 146 DZ white dwarfs from the Sloan Digital Sky Survey are analysed to constrain the possible origin of their externally polluted atmospheres. There are no correlations found between their accreted calcium abundances and spatial–kinematical distributions relative to interstellar material. Furthermore, two thirds of the stars are currently located above the Galactic gas and dust layer, and their kinematics indicate multi-Myr residences in this region where interstellar material is virtually absent. Where detected, the hydrogen abundances for 37 DZA stars show little or no correlation with accreted calcium or spatial–kinematical distributions, though there is a general trend with cooling age. It is found that Eddington-type accretion of interstellar hydrogen can reproduce the observed hydrogen abundances, yet simultaneously fails to account for calcium. The calcium-to-hydrogen ratios for the DZA stars are dominated by supersolar values, as are the lower limits for the remaining 109 DZ stars. All together, these polluted white dwarfs currently contain 10 20±2 g of calcium in their convective envelopes, commensurate with the masses of calcium inferred for large asteroids. A census of current T eff 12 000 K, helium-rich stars from the Sloan Digital Sky Survey suggests the DZ and DC white dwarfs belong to the same stellar population, with similar basic atmospheric compositions, effective temperatures, spatial distributions and Galactic space velocities. Based on this result, pollution by the interstellar medium cannot simultaneously account for both the polluted and non-polluted subpopulations. Rather, it is probable that these white dwarfs are contaminated by circumstellar matter; the rocky remains of terrestrial planetary systems. In this picture, two predictions emerge. First, at least 3.5 per cent of all white dwarfs harbour the remnants of terrestrial planetary systems; this is a concrete lower limit and the true fraction is almost certainly, and perhaps significantly, higher. Therefore, one can infer that at least 3.5 per cent of main-sequence A- and F-type stars build terrestrial planets. Secondly, the DZA stars are externally polluted by both metals and hydrogen, and hence constrain the frequency and mass of water rich, extrasolar planetesimals.

The Structure of the Local Interstellar Medium. III. Temperature and Turbulence

We present 50 individual measurements of the gas temperature and turbulent velocity in the local interstellar medium (LISM) within 100 pc. By comparing the absorption line widths of many ions with different atomic masses, we can satisfactorily discriminate between the two dominant broadening mechanisms, thermal broadening and macroscopic nonthermal, or turbulent, broadening. We find that the successful use of this technique requires a measurement of a light ion, such as D I, and an ion at least as heavy as Mg II. However, observations of more lines provide an important consistency check and can also improve the precision and accuracy of the measurement. Temperature and turbulent velocity measurements are vital to understanding the physical properties of the gas in our local environment and can provide insight into the three-dimensional morphological structure of the LISM. The weighted mean gas temperature in the LISM warm clouds is 6680 K and the dispersion about the mean is 1490 K. The weighted mean turbulent velocity is 2.24 km s-1 and the dispersion about the mean is 1.03 km s-1. The ratio of the mean thermal pressure to the mean turbulent pressure is PT/Pξ ~ 26. Turbulent pressure in LISM clouds cannot explain the difference in the apparent pressure imbalance between warm LISM clouds and the surrounding hot gas of the Local Bubble. Pressure equilibrium among the warm clouds may be the source of a moderately negative correlation between temperature and turbulent velocity in these clouds. However, significant variations in temperature and turbulent velocity are observed. The turbulent motions in the warm partially ionized clouds of the LISM are definitely subsonic, and the weighted mean turbulent Mach number for clouds in the LISM is 0.19 with a dispersion of 0.11. These measurements provide important constraints on models of the evolution and origin of warm partially ionized clouds in our local environment.

The Structure of the Local Interstellar Medium. I. High-Resolution Observations of Fe II, Mg II, and Ca II toward Stars within 100 Parsecs

High-resolution absorption measurements (λ/Δλ 100,000) of the resonance lines of Fe II, Mg II, and Ca II are presented for all available observed targets within 100 pc. The Fe II and Mg II spectra were obtained with the Goddard High Resolution Spectrograph (GHRS) and the Space Telescope Imaging Spectrograph (STIS) instruments aboard the Hubble Space Telescope (HST). Of the 63 observations of targets within 100 pc, we present new measurements for 24 lines of sight. We also compiled all published absorption measurements based on Ca II spectra obtained by various ground-based instruments. For each observation we provide measurements of the central velocity, Doppler parameter, and column density for each absorption component. These three ions provide the best opportunity to measure the component velocity structure. Because these are the heaviest ions observed in absorption through the warm local interstellar medium (LISM), the narrow line widths minimize significant blending of components and allow for accurate measurements of the central velocity. We present a statistical analysis of the LISM absorption measurements, which provides an overview of some physical characteristics of warm clouds in the LISM, such as, temperature, turbulent velocity, ionization degree, and depletion. The complete collection and reduction of all LISM absorption measurements provides an important database for studying the structure of nearby warm clouds. Subsequent papers will present models for the morphology and physical properties of individual structures (clouds) in the LISM.

A Dusty Disk around WD 1150–153: Explaining the Metals in White Dwarfs by Accretion from the Interstellar Medium versus Debris Disks

WereportthediscoveryofexcessK-bandradiationfromametal-richDAVwhitedwarfstar,WD1150� 153.Our near-infrared spectroscopic observations show that the excess radiation cannot be explained by a (sub)stellar companion,andislikelytobecausedbyadebrisdisksimilartotheotherDAZwhitedwarfswithcircumstellardebrisdisks. Wefindthatthefractionof DAZwhitedwarfs withdetectabledebrisdisksisatleast14%.Wealsorevisittheproblem of explaining the metals in white dwarf photospheres by accretion from the interstellar medium (ISM). We use the observed interstellar column densities toward stars in close angular proximity and similar distance as DAZ white dwarfs to constrain the contribution of accretion from the ISM. We find no correlation between the accretion density required to supply metals observed in DAZs with the densities observed in their interstellar environment, indicating that ISMaccretion alonecannot explain thepresence ofmetals innearbyDAZ white dwarfs. Although ISMaccretion willcertainlycontribute,ouranalysisindicatesthatitisnotthedominantsourceofmetalsformostDAZwhitedwarfs. Instead,thegrowing numberofcircumstellardebrisdisksaroundDAZssuggeststhatcircumstellarmaterialmayplay a more dominant role in polluting the white dwarf atmospheres. Subject headingg accretion, accretion disks — circumstellar matter — stars: individual (WD 1150� 153) — white dwarfs Online material: color figures

The Three-dimensional Structure of the Warm Local Interstellar Medium. I. Methodology*

In this first in a series of papers, we develop a methodology for constructing three-dimensional models of the local interstellar cloud (LIC) and adjacent warm clouds in the local interstellar medium (LISM). Our models are based on the column density of neutral hydrogen gas (N) inferred primarily from measurements of the deuterium column density toward nearby stars obtained from the analysis of Hubble Space Telescope ultraviolet spectra. We also use values of N inferred from spectra of hot white dwarfs and B-type stars obtained by the Extreme Ultraviolet Explorer satellite. These very different methods give consistent results for the three white dwarf stars in common. We assume that along each line of sight all interstellar gas moving with a speed consistent with the LIC velocity vector has a constant density, N = 0.10 cm-3, and extends from the heliosphere to an edge determined by the value of N moving at this speed. A number of stars have velocities and/or depletions that indicate absorption by other warm clouds in their lines of sight. On this basis α Cen A and B and probably also Ind lie inside the Galactic center (G) cloud, HZ 43 and 31 Com lie inside what we call the north Galactic pole cloud, and β Cet is located inside what we call the south Galactic pole cloud. We show the locations of these clouds in Galactic coordinates. The Sun is located very close to the edge of the LIC toward the Galactic center and the north Galactic pole. The absence of Mg II absorption at the LIC velocity toward α Cen indicates that the distance to the edge of the LIC in this direction is ≤0.05 pc and the Sun should leave the LIC and perhaps enter the G cloud in less than 3000 yr. Comparison of LIC and total values of N toward pairs of stars with separations between 09 and 20° reveals a pattern of good agreement so long as both stars lie within 60 pc of the Sun. Thus the LIC and perhaps also other nearby warm clouds have shapes that are smooth on these angular scales. In our second paper we will therefore fit the shape of the LIC with a set of smooth basis functions (spherical harmonics).