Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres*
TL;DR: In this paper, the authors search the Hubble Space Telescope (HST) archive for previously unanalyzed observations of stellar H I Lyα emission lines, their primary purpose being to look for new detections of Lyα absorption from the outer heliosphere and to also search for analogous absorption of the astrospheres surrounding the observed stars.
Abstract: 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.
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TL;DR: In this paper, mass loss generally increases with coronal activity, but winds suddenly weaken at a certain activity threshold, suggesting that the magnetic field geometry associated with these spots may be inhibiting the winds.
Abstract: 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.
461 citations
Cites background from "Stellar Lyα Emission Lines in the H..."
...We have recently analyzed all appropriate Lyα spectra in the HST archive to search for new astrospheric detections (Wood et al. 2005, hereafter Paper 2)....
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TL;DR: In this paper, the authors calculate the rate of photoevaporation of a circumstellar disk by energetic radiation (far-UV), 6 eV 0.1 keV) from its central star.
Abstract: We calculate the rate of photoevaporation of a circumstellar disk by energetic radiation (far-UV (FUV), 6 eV 0.1 keV) from its central star. We focus on the effects of FUV and X-ray photons since EUV photoevaporation has been treated previously, and consider central star masses in the range 0.3-7 M ?. Contrary to the EUV photoevaporation scenario, which creates a gap at about rg ~ 7(M */M ?) AU and then erodes the outer disk from inside out, we find that FUV photoevaporation predominantly removes less bound gas from the outer disk. Heating by FUV photons can cause significant erosion of the outer disk where most of the mass is typically located. X-rays indirectly increase the mass-loss rates (by a factor of ~2) by ionizing the gas, thereby reducing the positive charge on grains and polycyclic aromatic hydrocarbons and enhancing FUV-induced grain photoelectric heating. FUV and X-ray photons may create a gap in the disk at ~10 AU under favorable circumstances. Photoevaporation timescales for M * ~ 1 M ? stars are estimated to be ~106 years, after the onset of disk irradiation by FUV and X-rays. Disk lifetimes do not vary much for stellar masses in the range 0.3-3 M ?. More massive stars (7 M ?) lose their disks rapidly (in ~105 years) due to their high EUV and FUV fields. Disk lifetimes are shorter for shallow surface density distributions and when the dust opacity in the disk is reduced by processes such as grain growth or settling. The latter suggests that the photoevaporation process may accelerate as the dust disk evolves.
391 citations
TL;DR: In this article, the authors presented an empirical dynamical model of the LISM clouds based on 270 radial velocity measurements for 157 sight lines toward nearby stars, including 15 warm clouds located within 15 pc of the Sun, each with a different velocity vector.
Abstract: 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: stars
357 citations
Cites background from "Stellar Lyα Emission Lines in the H..."
...…Liu, & Matthews 2004) which shows no circumstellar absorption in H2 (Roberge et al. 2005; France et al. 2007) or other UV lines, including Lyman-α (Wood et al. 2005b) and the single UV observation does not allow for any constraint on the constancy of the observed absorption (Redfield & Linsky…...
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...These models include heating by ultraviolet (UV) photons on grains and polycyclic aromatic hydrocarbon (PAH) molecules and cooling by various forbidden lines and the hydrogen Lyman-α line....
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...…Linsky 2002), which represent 32% of the components in our sample, and (3) the moderate-resolution UV database of HST observations of LISM absorption toward stars within 100 pc (Wood, Alexander, & Linsky 1996; Wood et al. 2000, 2005b), representing the remaining 13% of the components in our sample....
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...Only two other stars in the LISM database have known edge-on circumstellar disks: (1) β Car (Lagrange-Henri et al. 1990) in which only the Na I absorption feature was observed to vary, whereas the Ca II absorption is relatively steady, and match the UV observations (Redfield, Kessler-Silacci, & Cieza 2007; Redfield & Linsky 2002), and (2) AU Mic (Kalas, Liu, & Matthews 2004) which shows no circumstellar absorption in H2 (Roberge et al. 2005; France et al. 2007) or other UV lines, including Lyman-α (Wood et al. 2005b) and the single UV observation does not allow for any constraint on the constancy of the observed absorption (Redfield & Linsky 2002)....
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...The solar wind 3.5 billions years ago may have been ∼35-fold stronger than it is today (Wood et al. 2005a)....
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University of Colorado Boulder1, European Space Research and Technology Centre2, University of Washington3, University of Texas at Austin4, Goddard Space Flight Center5, National Scientific and Technical Research Council6, Western Washington University7, Cornell University8, University of Maryland, College Park9, University of Nice Sophia Antipolis10, Wesleyan University11, University of St Andrews12, Tsinghua University13, Adler Planetarium14
TL;DR: In this article, a panchromatic (X-ray to mid-IR) study of the spectral energy distribution (SED) of 11 nearby planet-hosting stars is performed, based on the measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) survey.
Abstract: Ground- and space-based planet searches employing radial velocity techniques and transit photometry have detected thousands of planet-hosting stars in the Milky Way. With so many planets discovered, the next step toward identifying potentially habitable planets is atmospheric characterization. While the Sun-Earth system provides a good framework for understanding the atmospheric chemistry of Earth-like planets around solar-type stars, the observational and theoretical constraints on the atmospheres of rocky planets in the habitable zones (HZs) around low-mass stars (K and M dwarfs) are relatively few. The chemistry of these atmospheres is controlled by the shape and absolute flux of the stellar spectral energy distribution (SED), however, flux distributions of relatively inactive low-mass stars are poorly understood at present. To address this issue, we have executed a panchromatic (X-ray to mid-IR) study of the SEDs of 11 nearby planet-hosting stars, the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) Treasury Survey. The MUSCLES program consists visible observations from Hubble and ground-based observatories. Infrared and astrophysically inaccessible wavelengths (EUV and Lyalpha) are reconstructed using stellar model spectra to fill in gaps in the observational data. In this overview and the companion papers describing the MUSCLES survey, we show that energetic radiation (X-ray and ultraviolet) is present from magnetically active stellar atmospheres at all times for stars as late as M6. The emission line luminosities of C IV and Mg II are strongly correlated with band-integrated luminosities and we present empirical relations that can be used to estimate broadband FUV and XUV (is equivalent to X-ray + EUV) fluxes from individual stellar emission line measurements. We find that while the slope of the SED, FUV/NUV, increases by approximately two orders of magnitude form early K to late M dwarfs (approximately equal 0.01-1), the absolute FUV and XUV flux levels at their corresponding HZ distances are constant to within factors of a few, spanning the range 10-70 erg per (sq cm) s in the HZ. Despite the lack of strong stellar activity indicators in their optical spectra, several of the M dwarfs in our sample show spectacular UV flare emission in their light curves. We present an example with flare/quiescent ultraviolet flux ratios of the order of 100:1 where the transition region energy output during the flare is comparable to the total quiescent luminosity of the star E(sub flare)(UV) approximately 0.3 L(sub *) delta (t) (delta t = 1 s). Finally, we interpret enhanced L(line)/L(sub Bol) ratios for C IV and N V as tentative observational evidence for the interaction of planets with large planetary mass-to-orbital distance ratios (M(sub plan)/A(sub plan)) with the transition regions of their host stars.
353 citations
TL;DR: In this paper, the authors detect a transit signature in the Lyman-α light curve with a transit depth of 5.05 ± 0.75%, which exceeds the occultation depth produced by the planetary disk alone at the 3.5σ level.
Abstract: We observed three transits of the extrasolar planet HD 189733b in H i Lyman-α and in a few other lines in the ultraviolet with HST/ACS, in the search for atmospheric signatures. We detect a transit signature in the Lyman-α light curve with a transit depth of 5.05 ± 0.75%. This depth exceeds the occultation depth produced by the planetary disk alone at the 3.5σ level (statistical). Other stellar emission lines are less bright, and, taken individually, they do not show the transit signature, while the whole spectra redward of the Lyman-α line has enough photons to show a transit signature consistent with the absorption by the planetary disk alone. The transit depth's upper limits in the emission lines are 11.1% for O i λ 1305 A and 5.5% for C ii λ 1335 A lines. The presence of an extended exosphere of atomic hydrogen around HD 189733b producing 5% absorption of the full unresolved Lyman-α line flux shows that the planet is losing gas. The Lyman-α light curve is well-fitted by a numerical simulation of escaping hydrogen in which the planetary atoms are pushed by the stellar radiation pressure. We constrain the escape rate of atomic hydrogen to be between 109 and 1011 g s-1 and the ionizing extreme UV flux between 2 and 40 times the solar value (1-σ ), with higher escape rates corresponding to larger EUV flux. The best fit is obtained for dM /dt = 1010 g s-1 and an EUV flux F EUV = 20 times the solar value. HD 189733b is the second extrasolar planet for which atmospheric evaporation has been detected.
348 citations
References
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01 Jun 1969
TL;DR: In this paper, Monte Carlo techniques are used to fit dependent and independent variables least squares fit to a polynomial least-squares fit to an arbitrary function fitting composite peaks direct application of the maximum likelihood.
Abstract: Uncertainties in measurements probability distributions error analysis estimates of means and errors Monte Carlo techniques dependent and independent variables least-squares fit to a polynomial least-squares fit to an arbitrary function fitting composite peaks direct application of the maximum likelihood. Appendices: numerical methods matrices graphs and tables histograms and graphs computer routines in Pascal.
12,737 citations
TL;DR: In this paper, the solution of the non-LTE optically thick transfer equations for hydrogen, carbon, and other constituents to determine semi-empirical models for six components of the quiet solar chromosphere was investigated.
Abstract: The described investigation is concerned with the solution of the non-LTE optically thick transfer equations for hydrogen, carbon, and other constituents to determine semiempirical models for six components of the quiet solar chromosphere. For a given temperature-height distribution, the solution is obtained of the equations of statistical equilibrium, radiative transfer for lines and continua, and hydrostatic equilibrium to find the ionization and excitation conditions for each atomic constituent. The emergent spectrum is calculated, and a trial and error approach is used to adjust the temperature distribution so that the emergent spectrum is in best agreement with the observed one. The relationship between semiempirical models determined in this way and theoretical models based on radiative equilibrium is discussed by Avrett (1977). Harvard Skylab EUV observations are used to determine models for a number of quiet-sun regions.
2,205 citations
TL;DR: In this paper, the mean level of Ca n H and K emission (averaged over 15 years) is correlated with rotation period, as expected, but there is a further dependence of the emission on spectral type.
Abstract: Rotation periods are reported for 14 main-sequence stars, bringing the total number of such stars with well-determined rotation periods to 41. It is found that the mean level of their Ca n H and K emission (averaged over 15 years) is correlated with rotation period, as expected. However, there is a further dependence of the emission on spectral type. When expressed as the ratio of chromospheric flux to total bolometric flux, the emission is well correlated with the parameter Pohs/Tc, where Pohs is the observed rotation period and tc(B—V) is a theoretically-derived convective overturn time, calculated assuming a mixing length to scale height ratio a ~ 2. This finding is consonant with general predictions of dynamo theory, if the relation between chromospheric emission and dynamo-generated magnetic fields is essentially independent of rotation rate and spectral type for the stars considered. The dependence of mean chromospheric emission on rotation and spectral type is essentially the same for stars above and below the Vaughan-Preston “gap,” thus casting doubt on explanations of the gap in terms of a discontinuity in dynamo characteristics. Subject headings: Ca n emission — convection — stai
1,650 citations
TL;DR: High-precision, high-cadence photometric measurements of the star HD 209458 are reported, which is known from radial velocity measurements to have a planetary-mass companion in a close orbit and the detailed shape of the transit curve due to both the limb darkening of thestar and the finite size of the planet is clearly evident.
Abstract: We report high-precision, high-cadence photometric measurements of the star HD 209458, which is known from radial velocity measurements to have a planetary-mass companion in a close orbit. We detect two separate transit events at times that are consistent with the radial velocity measurements. In both cases, the detailed shape of the transit curve due to both the limb darkening of the star and the finite size of the planet is clearly evident. Assuming stellar parameters of 1.1 R⊙ and 1.1 M⊙, we find that the data are best interpreted as a gas giant with a radius of 1.27 ± 0.02 RJup in an orbit with an inclination of 871 ± 02. We present values for the planetary surface gravity, escape velocity, and average density and discuss the numerous observations that are warranted now that a planet is known to transit the disk of its parent star.
1,494 citations
"Stellar Lyα Emission Lines in the H..." refers background in this paper
...HD 209458 (#59): This star has a planetary companion that transits in front of the star once every 3.52 days (Charbonneau et al. 2000)....
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TL;DR: The detection of atomic hydrogen absorption in the stellar Lyman α line during three transits of HD209458b is reported, showing that this absorption should take place beyond the Roche limit and therefore can be understood in terms of escaping hydrogen atoms.
Abstract: The planet in the system HD209458 is the first one for which repeated transits across the stellar disk have been observed1,2. Together with radial velocity measurements3, this has led to a determination of the planet's radius and mass, confirming it to be a gas giant. But despite numerous searches for an atmospheric signature4,5,6, only the dense lower atmosphere of HD209458b has been observed, through the detection of neutral sodium absorption7. Here we report the detection of atomic hydrogen absorption in the stellar Lyman α line during three transits of HD209458b. An absorption of 15 ± 4% (1σ) is observed. Comparison with models shows that this absorption should take place beyond the Roche limit and therefore can be understood in terms of escaping hydrogen atoms.
1,218 citations
"Stellar Lyα Emission Lines in the H..." refers background in this paper
...By comparing STIS G140M Lyα spectra taken during and outside transit times, Vidal-Madjar et al. (2003) find that the Lyα profile decreases as much as 15% at some wavelengths during transit, suggesting absorption from an extended, evaporating planetary atmosphere....
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