Stellar Emission Lines Research Articles

Measuring stellar populations, dust attenuation and ionized gas at kpc scales in 10010 nearby galaxies using the integral field spectroscopy from MaNGA

As one of the three major experiments of the fourth-generation Sloan Digital Sky Survey (SDSS-IV), the Mapping Nearby Galaxies at Apatch Point Observatory (MaNGA) survey has obtained high-quality integral field spectroscopy (IFS) with a resolution of 1–2 kpc for ∼ 104 galaxies in the local universe during its six-year operation from July 2014 through August 2020. It is crucial to reliably measure the physical properties of the different components in each spectrum before one can use the data for any scientific study. In the past years we have made lots of efforts to develop a novel technique of full spectral fitting, which estimates a model-independent dust attenuation curve from each spectrum, thus allowing us to break the degeneracy between dust attenuation and stellar population properties when fitting the spectrum with stellar population synthesis models. We have applied our technique to the final data release of MaNGA, and obtained measurements of stellar population properties and emission line parameters, as well as the kinematics and dust attenuation of both stellar and ionized gas components. In this paper we describe our technique and the content and format of our data products. The whole dataset is publicly available in Science Data Bank with the link https://doi.org/10.57760/sciencedb.j00113.00088.

Field and intensity correlations: the Siegert relation from stars to quantum emitters

The Siegert relation relates field and intensity temporal correlations. After a historical review of the Siegert relation and the Hanbury Brown and Twiss effect, we discuss the validity of this relation in two different domains. We first show that this relation can be used in astrophysics to determine the fundamental parameters of stars, and that it is especially important for the observation with stellar emission lines. Second, we check the validity of this relation for moving quantum scatterers illuminated by a strong driving field.

Effects of UV Stellar Spectral Uncertainty on the Chemistry of Terrestrial Atmospheres

Abstract The upcoming deployment of the James Webb Space Telescope will dramatically advance our ability to characterize exoplanet atmospheres, both in terms of precision and sensitivity to smaller and cooler planets. Disequilibrium chemical processes dominate these cooler atmospheres, requiring accurate photochemical modeling of such environments. The host star’s UV spectrum is a critical input to these models, but most exoplanet hosts lack UV observations. For cases in which the host UV spectrum is unavailable, a reconstructed or proxy spectrum will need to be used in its place. In this study, we use the MUSCLES catalog and UV line scaling relations to understand how well reconstructed host star spectra reproduce photochemically modeled atmospheres using real UV observations. We focus on two cases: a modern Earth-like atmosphere and an Archean Earth-like atmosphere that forms copious hydrocarbon hazes. We find that modern Earth-like environments are well-reproduced with UV reconstructions, whereas hazy (Archean Earth) atmospheres suffer from changes at the observable level. Specifically, both the stellar UV emission lines and the UV continuum significantly influence the chemical state and haze production in our modeled Archean atmospheres, resulting in observable differences in their transmission spectra. Our modeling results indicate that UV observations of individual exoplanet host stars are needed to accurately characterize and predict the transmission spectra of hazy terrestrial atmospheres. In the absence of UV data, reconstructed spectra that account for both UV emission lines and continuum are the next best option, albeit at the cost of modeling accuracy.

Intrinsic Lyα Profiles of High-velocity G, K, and M Dwarfs

Abstract Observations of H i Lyman α, the brightest UV emission line of late-type stars, are critical for understanding stellar chromospheres and transition regions, modeling photochemistry in exoplanet atmospheres, and measuring the abundances of neutral hydrogen and deuterium in the interstellar medium. Yet Lyα observations are notoriously challenging owing to severe attenuation from interstellar gas, hindering our understanding of this important emission line’s basic morphology. We present high-resolution far- and near-UV spectroscopy of five G, K, and M dwarfs with radial velocities large enough to Doppler-shift the stellar Lyα emission line away from much of the interstellar attenuation, allowing the line core to be directly observed. We detect self-reversal in the Lyα emission-line core for all targets, and we show that the self-reversal depth decreases with increasing surface gravity. Mg ii self-reversed emission-line profiles provide some useful information to constrain the Lyα line core, but the differences are significant enough that Mg ii cannot be used directly as an intrinsic Lyα template during reconstructions. We show that reconstructions that neglect self-reversal could overestimate intrinsic Lyα fluxes by as much as 60%–100% for G and K dwarfs and 40%–170% for M dwarfs. The five stars of our sample have low magnetic activity and subsolar metallicity; a larger sample size is needed to determine how sensitive these results are to these factors.

Eclipse mapping of EXO 0748–676: evidence for a massive neutron star

ABSTRACT Determining the maximum possible neutron star (NS) mass places limits on the equation of state (EoS) of ultra-dense matter. The mass of NSs in low-mass X-ray binaries can be determined from the binary mass function, providing independent constraints are placed on both the binary inclination and mass ratio. In eclipsing systems, they relate via the totality duration. EXO 0748–676 is an eclipsing NS low-mass X-ray binary with a binary mass function estimated using stellar emission lines from the irradiated face of the companion. The NS mass is thus known as a function of mass ratio. Here, we model the X-ray eclipses in several energy bands, utilizing archival XMM–Newton data. We find a narrow region of absorbing material surrounding the companion star is required to explain the energy-dependent eclipses. Therefore, we suggest the companion may be experiencing ablation of its outer layers and that the system could transition into a redback millisecond pulsar. Our fit returns a mass ratio of $q=0.222^{+0.07}_{-0.08}$ and an inclination $i = 76.5 \pm ^{1.4}_{1.1}$. Combining these with the previously measured radial velocity of 410 ± 5 km s−1, derived from Doppler mapping analysis of H α emission during quiescence, returns an NS mass of ∼2 M⊙ even if the line originates as far from the NS as physically possible, favouring hard EoS. The inferred mass increases for a more realistic emission point. However, a ∼1.4 M⊙ canonical NS mass is possible when considering radial velocity values derived from other emission lines observed both during outburst and quiescence.

Planetary nebulae with Wolf–Rayet-type central stars – III. A detailed view of NGC 6905 and its central star

ABSTRACT We present a multiwavelength characterization of the planetary nebula (PN) NGC 6905 and its [Wolf–Rayet]-type ([WR]) central star (CSPN) HD 193949. Our Nordic Optical Telescope Alhambra Faint Object Spectrograph and Camera (ALFOSC) spectra and images unveil in unprecedented detail the high-ionization structure of NGC 6905. The high-quality spectra of HD 193949 allowed us to detect more than 20 WR features including the characteristic O-bump, blue bump, and red bump, which suggests a spectral type no later than a [WO2]-subtype. Moreover we detect the Ne vii and Ne viii broad emission lines, rendering HD 193949 yet another CSPN with Teff ≲ 150 kK exhibiting such stellar emission lines. We studied the physical properties (Te and ne) and chemical abundances of different regions within NGC 6905 including its low-ionization clumps; abundances are found to be homogeneous. We used the PoWR stellar atmosphere code to model the spectrum of HD 193949, which is afterwards used in a photoionization model performed with cloudy that reproduces the nebular and dust properties for a total mass in the 0.31–0.47 M⊙ range and a mass of C-rich dust of ∼2 × 10−3 M⊙. Adopting a current stellar mass of 0.6 M⊙, our model suggests an initial mass ∼1 M⊙ for HD 193949, consistent with the observations.

Spectroscopic study of the HII regions in the NGC 1232 galaxy

Context. NGC 1232 is a face-on spiral galaxy that serves as an excellent laboratory for the study of star formation due to its proximity. Recent studies have revealed interesting features about this galaxy: X-ray observations suggest that it recently collided with a dwarf galaxy, however, no apparent remnant is observed. Aims. In this study, we search for evidence of this possible collision. Methods. We used long-slit optical spectra of the galaxy in two different positions obtained with the Goodman spectrograph at the SOAR telescope. Results. We detected 18 HII regions in the north-south direction and 22 HII regions in the east-west direction and a background galaxy, NGC 1232B, for which we present the first redshift measurement and spectral analysis. We used the stellar population fitting technique to study the underlying stellar population of NGC 1232 and NGC 1232B and to subtract it from the spectra to measure the emission lines. The emission lines were used to determine the extinction, electron density, chemical abundance, and the star-formation rate gradient of NGC 1232. Conclusions. As is common in spiral galaxies, we found a stellar population gradient with older populations at the central regions and younger ones towards the outskirts, along with a negative oxygen abundance gradient of −0.16 dex/re. Due to the difficulty of measuring important emission lines, the number of objects for the abundance gradient is small, but there is a hint that this galaxy has a broken gradient profile, with a drop towards the center. Some authors have explained this effect as the result of a satellite collision, but observations of a large sample of spiral galaxies shows evidence that goes against such a mechanism. If the collision caused any disturbance in the galaxy, we believe it would be small and hard to detect with a limited number of objects. From all the other measurements, we found no deviations from a typical spiral galaxy and no significant difference between different directions in the galaxy. The stellar population and emission line analysis of NGC 1232B suggest that it is a starburst galaxy.

The Breakthrough Listen Search for Intelligent Life: Searching Boyajian's Star for Laser Line Emission

Boyajian's Star (KIC 8462852) has received significant attention, due to its unusual periodic brightness fluctuations detected by the Kepler spacecraft and subsequent ground-based observations. Possible explanations for the dips in the photometric measurements include interstellar or circumstellar dust, and it has been speculated that an artificial megastructure could be responsible. We analyze 177 high-resolution spectra of Boyajian’s Star in an effort to detect potential laser signals from extraterrestrial civilizations. The spectra were obtained by the Lick Observatory’s Automated Planet Finder (APF) telescope as part of the Breakthrough Listen Project, and cover the wavelength range of visible light from 374 to 970 nm. We calculate that the APF would be capable of detecting lasers of power greater than approximately 24 MW at the distance of Boyajian’s Star, d = 1470 ly. The top candidates from the analysis can all be explained as either cosmic-ray hits, stellar emission lines or atmospheric airglow emission lines.

1D Kinematics from Stars and Ionized Gas at z ∼ 0.8 from the LEGA-C Spectroscopic Survey of Massive Galaxies

Abstract We present a comparison of the observed, spatially integrated stellar and ionized gas velocity dispersions of ∼1000 massive ( ) galaxies in the Large Early Galaxy Astrophysics Census survey at 0.6 ≲ z ≲ 1.0. The high S/N ∼ 20 Å−1 afforded by 20 hr Very Large Telescope/Visible Multi-Object Spectrograph spectra allows for joint modeling of the stellar continuum and emission lines in all galaxies, spanning the full range of galaxy colors and morphologies. These observed integrated velocity dispersions (denoted as and ) are related to the intrinsic velocity dispersions of ionized gas or stars, but also include rotational motions through beam smearing and spectral extraction. We find good average agreement between observed velocity dispersions, with . This result does not depend strongly on stellar population, structural properties, or alignment with respect to the slit. However, in all regimes we find significant scatter between and , with an overall scatter of 0.13 dex of which 0.05 dex is due to observational uncertainties. For an individual galaxy, the scatter between and translates to an additional uncertainty of ∼0.24 dex on dynamical mass derived from , on top of measurement errors and uncertainties from Virial constant or size estimates. We measure the z ∼ 0.8 stellar mass Faber–Jackson relation and demonstrate that emission line widths can be used to measure scaling relations. However, these relations will exhibit increased scatter and slopes that are artificially steepened by selecting on subsets of galaxies with progressively brighter emission lines.

Stellar and Nebular Diagnostics in the Ultraviolet for Star-forming Galaxies

Abstract There is a long history of using optical emission and absorption lines to constrain the metallicity and ionization parameters of gas in galaxies. However, comparable diagnostics are less well developed for the ultraviolet (UV). Here, we assess the diagnostic potential of both absorption and emission features in the UV and evaluate the diagnostics against observations of local and high-redshift galaxies. We use the Flexible Stellar Population Synthesis (FSPS) nebular emission model of Byler et al., extended to include emission predictions in the UV, to evaluate the metallicity sensitivity of established UV stellar absorption indices and to identify those that include a significant contribution from nebular emission. We present model UV emission-line fluxes as a function of metallicity and ionization parameter, assuming both instantaneous bursts and constant star formation rates. We identify combinations of strong emission lines that constrain metallicity and ionization parameters, including [C iii] λ1907, C iii] λ1909, O iii] λ1661,1666, Si iii] λ1883,1892, C iv λ1548,1551, N ii] λ1750,1752, and Mg ii λ2796, and we develop UV versions of the canonical “Baldwin Phillips Terlevich” diagram. We quantify the relative contribution from stellar wind emission and nebular line emission to diagnostic line ratios that include the C iv λ1548,1551 lines, and we also develop an observationally motivated relationship for N and C enrichment that improves the performance of photoionization models. We summarize the best diagnostic choices and the associated redshift range for low-, mid-, and high-resolution rest-UV spectroscopy in preparation for the launch of the James Webb Space T elescope.

Line-dependent veiling in very active classical T Tauri stars

Context. The T Tauri stars with active accretion disks show veiled photospheric spectra. This is supposedly due to non-photospheric continuum radiated by hot spots beneath the accretion shocks at stellar surface and/or chromospheric emission lines radiated by the post-shocked gas. The amount of veiling is often considered as a measure of the mass-accretion rate. Aim. We analysed high-resolution photospheric spectra of accreting T Tauri stars LkHα 321, V1331 Cyg, and AS 353A with the aim of clarifying the nature of the line-dependent veiling. Each of these objects shows a strong emission line spectrum and powerful wind features indicating high rates of accretion and mass loss. Methods. Equivalent widths of hundreds of weak photospheric lines were measured in the observed spectra of high quality and compared with those in synthetic spectra of appropriate models of stellar atmospheres. Results. The photospheric spectra of the three T Tauri stars are highly veiled. We found that the veiling is strongly line-dependent: larger in stronger photospheric lines and weak or absent in the weakest ones. No dependence of veiling on excitation potential within 0 to 5 eV was found. Different physical processes responsible for these unusual veiling effects are discussed in the framework of the magnetospheric accretion model. Conclusions. The observed veiling has two origins: (1) an abnormal structure of stellar atmosphere heated up by the accreting matter, and (2) a non-photospheric continuum radiated by a hot spot with temperature lower than 10 000 K. The true level of the veiling continuum can be derived by measuring the weakest photospheric lines with equivalent widths down to ≈10 mÅ. A limited spectral resolution and/or low signal-to-noise ratio results in overestimation of the veiling continuum. In the three very active stars, the veiling continuum is a minor contributor to the observed veiling, while the major contribution comes from the line-dependent veiling.

Dissecting galaxy triplets in the Sloan Digital Sky Survey Data Release 10 – I. Stellar populations and emission line analysis

We identify isolated galaxy triplets in a volume-limited sample from the Sloan Digital Sky Survey Data Release 10. Our final sample has 80 galaxy systems in the redshift range 0.04$\le$z$\le$0.1, brighter than $M_r = -20.5 + 5\log h_{70}$. Spectral synthesis results and WHAN and BPT diagnostic diagrams were employed to classify the galaxies in these systems as star-forming, active nuclei, or passive/retired. Our results suggest that the brightest galaxies drive the triplet evolution, as evidenced by the strong correlations between properties as mass assembly and mean stellar population age with triplet properties. Galaxies with intermediate luminosity or the faintest one within the triplet seem to play a secondary role. Moreover, the relation between age and stellar mass of galaxies is similar for these galaxies but different for the brightest galaxy in the system. Most of the triplet galaxies are passive or retired, according to the WHAN classification. Low mass triplets present different fractions of WHAN classes when compared to higher mass triplets. A census of WHAN class combinations shows the dominance of star-forming galaxies in low mass triplets while retired and passive galaxies prevail in high-mass systems. We argue that these results suggest that the local environment, through galaxy interactions driven by the brightest galaxy, is playing a major role in triplet evolution.

FIRST CONNECTION BETWEEN COLD GAS IN EMISSION AND ABSORPTION: CO EMISSION FROM A GALAXY–QUASAR PAIR

ABSTRACT We present the first detection of molecular emission from a galaxy selected to be near a projected background quasar using the Atacama Large Millimeter/submillimeter Array (ALMA). The ALMA detection of CO(1−0) emission from the z = 0.101 galaxy toward quasar PKS 0439–433 is coincident with its stellar disk and yields a molecular gas mass of M mol ≈ 4.2 × 109 M ⊙ (for a Galactic CO-to-H2 conversion factor), larger than the upper limit on its atomic gas mass. We resolve the CO velocity field, obtaining a rotational velocity of 134 ± 11 km s−1 and a resultant dynamical mass of ≥4 × 1010 M ⊙. Despite its high metallicity and large molecular mass, the z = 0.101 galaxy has a low star formation rate, implying a large gas consumption timescale, larger than that typical of late-type galaxies. Most of the molecular gas is hence likely to be in a diffuse extended phase, rather than in dense molecular clouds. By combining the results of emission and absorption studies, we find that the strongest molecular absorption component toward the quasar cannot arise from the molecular disk, but is likely to arise from diffuse gas in the galaxy’s circumgalactic medium. Our results emphasize the potential of combining molecular and stellar emission line studies with optical absorption line studies to achieve a more complete picture of the gas within and surrounding high-redshift galaxies.

THE MUSCLES TREASURY SURVEY. I. MOTIVATION AND OVERVIEW*

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 Lyα) 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 (≡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 (≈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 cm−2 s−1 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 flare(UV) ∼ 0.3 L *Δt (Δt = 1 s). Finally, we interpret enhanced L(line)/L 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 plan/a plan) with the transition regions of their host stars.

Origin of X-shaped radio-sources: further insights from the properties of their host galaxies

We analyze the properties of a sample of X-shaped radio-sources (XRSs). These objects show, in addition to the main lobes, a pair of wings producing their peculiar radio morphology. We obtain our sample by selecting from the initial list of Cheung (2007, AJ, 133, 2097) the 53 galaxies with the better defined wings and with available SDSS images. We identified the host galaxies and measured their optical position angle, obtaining a positive result in 22 cases. The orientation of the secondary radio structures shows a strong connection with the optical axis, with all (but one) wing forming a angle larger than 40 degrees with the host major axis. The probability that this is compatible with a uniform distribution is P = 0.9 10E-4. Spectra are available from the SDSS for 28 XRSs. We modeled them to extract information on their emission lines and stellar population properties. The sample is formed by approximately the same number of high and low excitation galaxies (HEG and LEG); this classification is essential for a proper comparison with non-winged radio-galaxies. XRSs follow the same relations between radio and line luminosity defined by radio-galaxies in the 3C sample. While in HEGs a young stellar population is often present, this is not detected in the 13 LEGs, again in agreement with the properties of non XRSs. The lack of young stars in LEGs support the idea that they did not experiences a recent gas rich merger. The connection between the optical axis and the wings orientation, as well as the stellar population and emission lines properties, provide further support for an hydro-dynamic origin of the radio-wings (for example associated with the expansion of the radio cocoon in an asymmetric external medium) rather than with a change of orientation of the jet axis.

Evidence for a [WR] or WEL-type binary nucleus in the bipolar planetary nebula Vy 1-2

We present high-dispersion spectroscopic data of the compact planetary nebula Vy 1-2, where high expansion velocities up to 100 km/s are found in the Ha, [N II] and [O III] emission lines. HST images reveal a bipolar structure. Vy 1-2 displays a bright ring-like structure with a size of 2.4"x3.2" and two faint bipolar lobes in the west-east direction. A faint pair of knots is also found, located almost symmetrically on opposite sides of the nebula at PA=305 degrees. Furthermore, deep low-dispersion spectra are also presented and several emission lines are detected for the first time in this nebula, such as the doublet [Cl III] 5517, 5537 A, [K IV] 6101 A, C II 6461 A, the doublet C IV 5801, 5812 A. By comparison with the solar abundances, we find enhanced N, depleted C and solar O. The central star must have experienced the hot bottom burning (CN-cycle) during the 2nd dredge-up phase, implying a progenitor star of higher than 3 solar masses. The very low C/O and N/O abundance ratios suggest a likely post-common envelope close binary system. A simple spherically symmetric geometry with either a blackbody or a H-deficient stellar atmosphere model is not able to reproduce the ionisation structure of Vy 1-2. The effective temperature and luminosity of its central star indicate a young nebula located at a distance of ~9.7 kpc with an age of ~3500 years. The detection of stellar emission lines, C II 6461 A, the doublet C IV {\lambda}{\lambda} 5801, 5812 A and O III 5592 A, emitted from a H-deficient star, indicates the presence of a late-type Wolf-Rayet or a WEL type central star.

Detection of radial velocity shifts due to black hole binaries near merger

Abstract The barycenter of a massive black hole binary will lie outside the event horizon of the primary black hole for modest values of mass ratio and binary separation. Analogous to radial velocity shifts in stellar emission lines caused by the tug of planets, the radial velocity of the primary black hole around the barycenter can leave a tell-tale oscillation in the broad component of FeKα emission from accreting gas. Near-future X-ray telescopes such as Astro-H and Athena will have the energy resolution (δE/E ≲ 10−3) to search nearby active galactic nuclei (AGN) for the presence of binaries with mass ratios q ≳ 0.01, separated by several hundred gravitational radii. The general-relativistic and Lense–Thirring precession of the periapse of the secondary orbit imprints a detectable modulation on the oscillations. The lowest mass binaries in AGN will oscillate many times within typical X-ray exposures, leading to a broadening of the line wings and an overestimate of black hole spin in these sources. Detection of periodic oscillations in the AGN line centroid energy will reveal a massive black hole binary close to merger and will provide an early warning of gravitational radiation emission.

HUNTING FOR SUPERMASSIVE BLACK HOLES IN NEARBY GALAXIES WITH THE HOBBY–EBERLY TELESCOPE

We have conducted an optical long-slit spectroscopic survey of 1022 galaxies using the 10m Hobby-Eberly Telescope (HET) at McDonald Observatory. The main goal of the HET Massive Galaxy Survey (HETMGS) is to find nearby galaxies that are suitable for black hole mass measurements. In order to measure accurately the black hole mass, one should kinematically resolve the region where the black hole dominates the gravitational potential. For most galaxies, this region is much less than an arcsecond. Thus, black hole masses are best measured in nearby galaxies with telescopes that obtain high-spatial resolution. The HETMGS focuses on those galaxies predicted to have the largest sphere-of-influence, based on published stellar velocity dispersions or the galaxy fundamental plane. To ensure coverage over galaxy types, the survey targets those galaxies across a face-on projection of the fundamental plane. We present the sample selection and resulting data products from the long-slit observations, including central stellar kinematics and emission line ratios. The full dataset, including spectra and resolved kinematics, is available online. Additionally, we show that the current crop of black hole masses are highly biased towards dense galaxies and that especially large disks and low dispersion galaxies are under-represented. This survey provides the necessary groundwork for future systematic black hole mass measurement campaigns.

Lyman-α observations of astrospheres

Abstract. Charge-exchange reactions between outflowing stellar wind protons and interstellar neutral hydrogen atoms entering a stellar astrosphere produce a region of piled-up-decelerated neutral hydrogen called the hydrogen wall. Absorption by this gas, which is observed in stellar Lyman-α emission lines, provides the only viable technique at this time for measuring the mass-loss rates of F–M dwarf stars. We describe this technique, present an alternative way for understanding the relation of mass-loss rate with X-ray emission, and identify several critical issues.

The nature of LINER galaxies:

Galaxies, which often contain ionised gas, sometimes also exhibit a so-called low-ionisation nuclear emission line region (LINER). For 30 years this was attributed to a central mass-accreting supermassive black hole (AGN) of low luminosity, making LINER galaxies the largest AGN-sub-population, dominating in numbers over higher luminosity Seyfert galaxies and quasars. This, however, poses a serious problem. While the inferred energy balance is plausible, many LINERs clearly do not contain any other independent signatures of an AGN. Using integral field spectroscopic data from the CALIFA survey, we aim at comparing the observed radial surface brightness profiles with what is expected from illumination by an AGN. Essential for this analysis is a proper extraction of emission-lines, especially weak lines such as the Balmer Hb line which is superposed on an absorption trough. To accomplish this, we use the GANDALF code which simultaneously fits the underlying stellar continuum and emission lines. We show for 48 galaxies with LINER-like emission, that the radial emission-line surface brightness profiles are inconsistent with ionisation by a central point-source and hence cannot be due to an AGN alone. The most probable explanation for the excess LINER-like emission is ionisation by evolved stars during the short but very hot and energetic phase known as post-AGB. This leads us to an entirely new interpretation. Post-AGB stars are ubiquitous and their ionising effect should be potentially observable in every galaxy with gas present and stars older than ~1 Gyr, unless a stronger radiation field from young hot stars or an AGN outshines them. This means that galaxies with LINER-like emission are in fact not a class defined by a property, but rather by the absence of a property. It also explains why LINER emission is observed mostly in massive galaxies with old stars and little star formation.