arXiv subjects

Seth Redfield

Publications and source records attributed to Seth Redfield.

At least 145 records · Page 8Linked to original sources

arXiv2011

Properties of Turbulence in the Very Local Interstellar Clouds

We have investigated the degree to which turbulence in the Very Local Interstellar Clouds resembles the highly-studied turbulence in the solar corona and the solar wind. The turbulence diagnostics for the Local Clouds are the absorption line widths measured along 32 lines of sight to nearby stars, yielding measurements for 53 absorption components (Redfield and Linsky 2004). We have tested whether the Local Cloud turbulence has the following properties of turbulence in the solar corona or the solar wind: (a) velocity fluctuations mainly perpendicular to the average magnetic field, (b) a temperature anisotropy in the sense that the perpendicular temperature is larger than the parallel temperature (or at least enhanced relative to expectation), and (c) an ion temperature which is dependent on the ion Larmor radius, in the sense that more massive ions have higher temperatures. Our analysis of the data does not show compelling evidence for any of these properties in Local Cloud turbulence, indicating possible differences with heliospheric plasmas. In the case of anisotropy of velocity fluctuations, although the expected observational signature is not seen, we cannot exclude the possibility of relatively high degrees of anisotropy (anisotropy parameter $ε\sim 0.50 - 0.70$), if some other process in the the Local Clouds is causing variations in the turbulent line width from one line of sight to another. We briefly consider possible reasons for differences between coronal and solar wind turbulence and that in the Local Clouds.

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arXiv2010

Ion-Neutral Collisions in the Interstellar Medium: Wave Damping and Elimination of Collisionless Processes

Most phases of the interstellar medium contain neutral atoms in addition to ions and electrons. This introduces differences in plasma physics processes in those media relative to the solar corona and the solar wind at a heliocentric distance of 1 astronomical unit. In this paper, we consider two well-diagnosed, partially-ionized interstellar plasmas. The first is the Diffuse Ionized Gas (DIG) which is probably the extensive phase in terms of volume. The second is the gas that makes up the Local Clouds of the Very Local Interstellar Medium (VLISM). Ion-neutral interactions seem to be important in both media. In the DIG, ion-neutral collisions are relatively rare, but sufficiently frequent to damp magnetohydrodynamic (MHD) waves (as well as propagating MHD eddies) within less than a parsec of the site of generation. This result raises interesting questions about the sources of turbulence in the DIG. In the case of the VLISM, the ion-neutral collision frequency is higher than that in the DIG, because the hydrogen is partially neutral rather than fully ionized. We present results showing that prominent features of coronal and solar wind turbulence seem to be absent in VLISM turbulence. For example, ion temperature does not depend on ion mass. This difference may be attributable to ion-neutral collisions, which distribute power from more effectively heated massive ions such as iron to other ion species and neutral atoms.

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arXiv2010

Observational Tests of the Properties of Turbulence in the Very Local Interstellar Medium

The Very Local Interstellar Medium (VLISM) contains clouds which consist of partially-ionized plasma. These clouds can be effectively diagnosed via high resolution optical and ultraviolet spectroscopy of the absorption lines they form in the spectra of nearby stars. Among the information provided by these spectroscopic measurements are the root-mean-square velocity fluctuation due to turbulence in these clouds and the ion temperature, which may be partially determined by dissipation of turbulence. We consider whether this turbulence resembles the extensively studied and well-diagnosed turbulence in the solar wind and solar corona. Published observations are used to determine if the velocity fluctuations are primarily transverse to a large-scale magnetic field, whether the temperature perpendicular to the large scale field is larger than that parallel to the field, and whether ions with larger Larmor radii have higher temperatures than smaller gyroradius ions. Although a thorough investigation of the data is underway, a preliminary examination of the published data shows neither evidence for anisotropy of the velocity fluctuations or temperature, nor Larmor radius-dependent heating. These results indicate differences between solar wind and Local Cloud turbulence.

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arXiv2009

Plasma Physics Processes of the Interstellar Medium

We discuss the outstanding issues of the interstellar medium which will depend on the application of knowledge from plasma physics. We particularly advocate attention to recent developments in experimental plasma physics, and urge that the astronomical community consider support of these experiments in the next decade.

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arXiv2008

The Spin-Orbit Alignment of the HD17156 Transiting Eccentric Planetary System

We present high precision radial velocity observations of HD17156 during a transit of its eccentric Jovian planet. In these data, we detect the Rossiter-McLaughlin effect, which is an apparent perturbation in the velocity of the star due to the progressive occultation of part of the rotating stellar photosphere by the transiting planet. This system had previously been reported by Narita et al. (2008) to exhibit a lambda = 62 +/- 25 degree misalignment of the projected planetary orbital axis and the stellar rotation axis. We model our data, along with the Narita et al. data, and obtain lambda = 9.4 +/- 9.3 degrees for the combined data set. We thus conclude that the planetary orbital axis is actually very well aligned with the stellar rotation axis.

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arXiv2008

The Structure of the Local Interstellar Medium V: Electron Densities

We present a comprehensive survey of CII* 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 GHRS and STIS onboard the Hubble Space Telescope, we identify 13 sight lines with 23 individual CII* absorption components, which provide electron density measurements, the vast majority of which are new. We employ several strategies to determine more accurate CII column densities from the saturated CII resonance line, including, constraints of the line width from the optically thin CII* line, constraints from independent temperature measurements of the LISM gas based on line widths of other ions, and third, using measured SII column densities as a proxy for CII column densities. The sample of electron densities appears consistent with a log-normal distribution and an unweighted mean value of n_e(CII_SII) = 0.11^+0.10_-0.05 cm^-3. 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 n_e(LIC) = 0.12 +/- 0.04 cm^-3. 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. Given some simple assumptions, the range of observed electron densities translates into a range of thermal pressures, P/k = 3300^+5500_-1900 K cm^-3. This work greatly expands the number of electron density measurements and provides important constraints on the ionization, abundance, and evolutionary models of the local interstellar medium. (abridged)

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arXiv2007

Sodium Absorption From the Exoplanetary Atmosphere of HD189733b Detected in the Optical Transmission Spectrum

We present the first ground-based detection of sodium absorption in the transmission spectrum of an extrasolar planet. Absorption due to the atmosphere of the extrasolar planet HD189733b is detected in both lines of the NaI doublet. High spectral resolution observations were taken of eleven transits with the High Resolution Spectrograph (HRS) on the 9.2 meter Hobby-Eberly Telescope (HET). The NaI absorption in the transmission spectrum due to HD189733b is (-67.2 +/- 20.7) x 10^-5 deeper in the ``narrow'' spectral band that encompasses both lines relative to adjacent bands. The 1-sigma error includes both random and systematic errors, and the detection is >3-sigma. This amount of relative absorption in NaI for HD189733b is ~3x larger than detected for HD209458b by Charbonneau et al. (2002), and indicates these two hot-Jupiters may have significantly different atmospheric properties.

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arXiv2007

The Origin of Radio Scintillation In the Local Interstellar Medium

We study three quasar radio sources (B1257-326, B1519-273, and J1819+385) that show large amplitude intraday and annual scintillation variability produced by the Earth's motion relative to turbulent-scattering screens located within a few parsecs of the Sun. We find that the lines of sight to these sources pass through the edges of partially ionized warm interstellar clouds where two or more clouds may interact. From the gas flow vectors of these clouds, we find that the relative radial and transverse velocities of these clouds are large and could generate the turbulence that is responsible for the observed scintillation. For all three sight lines the flow velocities of nearby warm local interstellar clouds are consistent with the fits to the transverse flows of the radio scintillation signals.

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arXiv2007

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 local interstellar medium based on 270 radial-velocity measurements for 157 sight lines toward nearby stars. Physical-parameter measurements (i.e., temperature, turbulent velocity, depletions) are available for 90 components, or one-third of the sample, enabling initial characterizations of the physical properties of 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 in the LISM to be between ~5.5% and 19%. Relative velocities of potentially interacting clouds are often supersonic, consistent with heating, turbulent, and metal-depletion properties. Cloud-cloud collisions may be responsible for the filamentary morphologies found in ~1/3 of LISM clouds, the distribution of clouds along the boundaries of the two nearest clouds (LIC and G), the detailed shape and heating of the Mic Cloud, the location of nearby radio scintillation screens, and the location of a LISM cold cloud. Contrary to previous claims, the Sun appears to be located in the transition zone between the LIC and G Clouds.

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arXiv2007

A Low-Mass H2 Component to the AU Microscopii Circumstellar Disk

We present a determination of the molecular gas mass in the AU Microscopii circumstellar disk. Direct detection of a gas component to the AU Mic disk has proven elusive, with upper limits derived from ultraviolet absorption line and submillimeter CO emission studies. Fluorescent emission lines of H2, pumped by the OVI 1032 resonance line through the C-X (1 -- 1) Q(3) 1031.87 Å transition, are detected by the Far Ultraviolet Spectroscopic Explorer. These lines are used to derive the H2 column density associated with the AU Mic system. The derived column density is in the range N(H2) = 1.9 x 10^{17} - 2.8 x 10^{15} cm^{-2}, roughly two orders of magnitude lower than the upper limit inferred from absorption line studies. This range of column densities reflects the range of H2 excitation temperature consistent with the observations, T(H2) = 800 -- 2000 K, derived from the presence of emission lines excited by OVI in the absence of those excited by LyA. Within the observational uncertainties, the data are consistent with the H2 gas residing in the disk. The inferred N(H2) range corresponds to H2-to-dust ratios of < 1/30:1 and a total M(H2) = 4.0 x 10^{-4} - 5.8 x 10^{-6} Earth masses. We use these results to predict the intensity of the associated rovibrational emission lines of H2 at infrared wavelengths covered by ground-based instruments, HST-NICMOS, and the Spitzer-IRS.

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arXiv2007

Spitzer Limits On Dust Emission and Optical Gas Absorption Variability Around Nearby Stars with Edge-On Circumstellar Disk Signatures

We present Spitzer observations and McDonald Observatory Smith Telescope and Anglo-Australian Telescope high spectral resolution optical observations of 4 nearby stars with variable or anomalous optical absorption, likely caused by circumstellar material. The optical observations of CaII and NaI cover a 2.8 year baseline, and extend the long term monitoring of these systems by previous researchers. In addition, mini-surveys of the local interstellar medium (LISM) around our primary targets provide a reconstruction of the intervening LISM along the line of sight. We confirm that the anomalous absorption detected toward alpha Oph is not due to circumstellar material, but to a small filamentary cloud <14.3 pc from the Sun. The three other primary targets, beta Car, HD85905, and HR10 show both short and long term variability, and little of the observed absorption can be attributed to the LISM along the line of sight. The Spitzer observations did not detect infrared excesses. We are able to place upper limits on any possible fractional infrared luminosity, which range from L_IR/L_star < 2-5 10^-6, for our three disk stars. No stable gas absorption component centered at the radial velocity of the star is detected for any of our targets. Based on simple assumptions of the variable gas absorption component, we estimate limits on the circumstellar gas mass causing the variable absorption, which range from 0.4-20 10^-8 M_Earth. These multiwavelength observations place strong limits on any possible circumstellar dust, while confirming variable circumstellar gas absorption, and therefore are interesting targets to explore the origins and evolution of variable circumstellar gas. (abridged)

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arXiv2007

A Dusty Disk Around WD1150-153: Explaining the Metals in White Dwarfs by Accretion from the Interstellar Medium versus Debris Disks

We report the discovery of excess K-band radiation from a metal-rich DAV white dwarf star, WD1150-153. Our near infrared spectroscopic observations show that the excess radiation cannot be explained by a (sub)stellar companion, and is likely to be caused by a debris disk similar to the other DAZ white dwarfs with circumstellar debris disks. We find that the fraction of DAZ white dwarfs with detectable debris disks is at least 14%. We also revisit the problem 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 ISM accretion alone cannot explain the presence of metals in nearby DAZ white dwarfs. Although ISM accretion will certainly contribute, our analysis indicates that it is not the dominant source of metals for most DAZ white dwarfs. Instead, the growing number of circumstellar debris disks around DAZs suggests that circumstellar material may play a more dominant role in polluting the white dwarf atmospheres.

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arXiv2007

Gas Absorption Detected from the Edge-on Debris Disk Surrounding HD32297

Near-infrared and optical imaging of HD32297 indicate that it has an edge-on debris disk, similar to beta Pic. I present high resolution optical spectra of the NaI doublet toward HD32297 and stars in close angular proximity. A circumstellar absorption component is clearly observed toward HD32297 at the stellar radial velocity, which is not observed toward any of its neighbors, including the nearest only 0.9 arcmin away. An interstellar component is detected in all stars >90 pc, including HD32297, likely due to the interstellar material at the boundary of the Local Bubble. Radial velocity measurements of the nearest neighbors, BD+07 777s and BD+07 778, indicate that they are unlikely to be physically associated with HD32297. The measured circumstellar column density around HD32997, log N(NaI) ~ 11.4, is the strongest NaI absorption measured toward any nearby main sequence debris disk, even the prototypical edge-on debris disk, beta Pic. Assuming that the morphology and abundances of the gas component around HD32297 are similar to beta Pic, I estimate an upper limit to the gas mass in the circumstellar disk surrounding HD32297 of ~0.3 M_Earth.

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arXiv2006

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 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 I} = 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: (i) correlations of gas-phase D/H ratios with depletions of the refractory metals iron and silicon, and (ii) correlation with the molecular hydrogen 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 sigma) 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 f_d <= 1.19 +/-0.16 (1 sigma) or <= 1.12 +/- 0.14 (1 sigma) depending on the adopted light element nuclear reaction rates.

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arXiv2006

The Local Interstellar Medium

The Local Interstellar Medium (LISM) is a unique environment that presents an opportunity to study general interstellar phenomena in great detail and in three dimensions. In particular, high resolution optical and ultraviolet spectroscopy have proven to be powerful tools for addressing fundamental questions concerning the physical conditions and three-dimensional (3D) morphology of this local material. After reviewing our current understanding of the structure of gas in the solar neighborhood, I will discuss the influence that the LISM can have on stellar and planetary systems, including LISM dust deposition onto planetary atmospheres and the modulation of galactic cosmic rays through the astrosphere - the balancing interface between the outward pressure of the magnetized stellar wind and the inward pressure of the surrounding interstellar medium. On Earth, galactic cosmic rays may play a role as contributors to ozone layer chemistry, planetary electrical discharge frequency, biological mutation rates, and climate. Since the LISM shares the same volume as practically all known extrasolar planets, the prototypical debris disks systems, and nearby low-mass star-formation sites, it will be important to understand the structures of the LISM and how they may influence planetary atmospheres.

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arXiv2005

New Mass Loss Measurements from Astrospheric Lyman-alpha 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.

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arXiv2005

Rapid Dissipation of Primordial Gas from the AU Microscopii Debris Disk

The disk around AU Microscopii, an M1 star in the Beta Pictoris Moving Group, is extraordinarily well-suited for comparison to the Beta Pic debris disk (type A5V). We use far-UV absorption spectroscopy of AU Mic to probe its edge-on disk for small amounts of molecular hydrogen, the primary constituent of gas giant planets. Our conservative upper limit on the line-of-sight H_2 column density is 1.7 x 10^{19} cm^{-2}, which is 18.5 times lower than the limit obtained from non-detection of sub-mm CO emission (Liu et al. 2004). In addition, there is a hint of H_2 absorption at a column density an order of magnitude or more below our upper limit. The H_2-to-dust ratio in the AU Mic disk is < 6:1, similar to that in the Beta Pic disk. This shows that the primordial gas has largely been dissipated in less than about 12 Myr for both disks, despite their very different stellar masses and luminosities. It is extremely difficult to form a giant planet around AU Mic with current core-accretion scenarios in such a short time.

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arXiv2005

Stellar Lyman-alpha 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 Lyman-alpha emission lines, our primary purpose being to look for new detections of Lyman-alpha 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 Lyman-alpha 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 4 new detections of heliospheric absorption (70 Oph, Xi Boo, 61 Vir, and HD 165185) and 7 new detections of astrospheric absorption (EV Lac, 70 Oph, Xi Boo, 61 Vir, Delta 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 Lyman-alpha 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 Lyman-alpha fluxes from the observed stars. We use these fluxes to determine how Lyman-alpha flux correlates with coronal X-ray and chromospheric Mg II emission, and we also study how Lyman-alpha emission depends on stellar rotation.

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