arXiv subjects

Hans-Reinhard Mueller

Publications and source records attributed to Hans-Reinhard Mueller.

18 recordsLinked to original sources

arXiv2026

HST Observations of HD 166620 and Tau Ceti: First UV Spectra of a Magnetic Grand Minimum Star and the Extent of Tau Ceti's Astrosphere

We present new Hubble Space Telescope (HST) UV spectra of the K2 V star HD 166620, the first star clearly recognized to be in a "magnetic grand minimum" state analogous to the Sun's "Maunder Minimum" in the late 1600's. The stellar H I Lyman-alpha surface fluxes are extremely low, about a factor of two below fluxes observed during solar minimum, and also significantly lower than those of Tau Ceti (G8 V) and HD 191408 (K2.5 V), two stars more similar to HD 166620 in spectral type and age (~10 Gyr) than the Sun. The Tau Ceti data that are compared with HD 166620 include both old archival data and a new HST observation as well. The Lyman alpha data are used to confirm a nondetection of astrospheric Lyman-alpha absorption for this star, suggesting a very weak wind with Mdot<0.1 Mdot_sun. The very compact astrosphere inferred for Tau Ceti indicates that the star's debris disk is at least partly exposed to the ISM, and we discuss possible consequences.

astro-ph.SR↗

arXiv2024

Resolving Red Giant Winds with the Hubble Space Telescope

We describe recent spectroscopic observations of red giant stars made by the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope, which have provided spatially resolved observations of the warm chromospheric winds that predominate for early K to mid-M giants. The H I Lyman-alpha lines of a set of 11 red giants observed with the STIS/E140M echelle grating are first analyzed to ascertain wind H I column densities and total wind mass-loss rates. The M giants have estimated mass-loss rates of Mdot=(14-86)e-11 Msun/yr, while the K giants with detected wind absorption have weaker winds with Mdot=(1.5-2.8)e-11 Msun/yr. We use long-slit spectra of H I Lyman-alpha for two particular red giants, Alpha Tau (K5 III) and Gamma Cru (M3.5 III), to study the spatial extent of the Lyman-alpha emission. From these data we estimate limits for the extent of detectable emission, which are r=193 Rstar for Gamma Cru and r=44 Rstar for Alpha Tau. Cross-dispersion emission profiles in the STIS echelle spectra of the larger sample of red giants also show evidence for spatial resolution, not only for H I Lyman-alpha but for other lines with visible wind absorption, such as Fe II, Mg II, Mg I, O I, and C II. We characterize the nature of these spatial signatures. The spatial extent is far more apparent for the M giants than for the K giants, consistent with the stronger winds found for the M giants from the Lyman-alpha analysis.

astro-ph.SR↗

arXiv2021

New Observational Constraints on the Winds of M Dwarf Stars

High resolution UV spectra of stellar H I Lyman-alpha lines from the Hubble Space Telescope (HST) provide observational constraints on the winds of coronal main sequence stars, thanks to an astrospheric absorption signature created by the interaction between the stellar winds and the interstellar medium. We report the results of a new HST survey of M dwarf stars, yielding six new detections of astrospheric absorption. We estimate mass-loss rates for these detections, and upper limits for nondetections. These new constraints allow us to characterize the nature of M dwarf winds and their dependence on coronal activity for the first time. For a clear majority of the M dwarfs, we find winds that are weaker or comparable in strength to that of the Sun, i.e. Mdot<=1 Mdot_sun. However, two of the M dwarfs have much stronger winds: YZ CMi (M4 Ve; Mdot=30 Mdot_sun) and GJ 15AB (M2 V+M3.5 V; Mdot=10 Mdot_sun). Even these winds are much weaker than expectations if the solar relation between flare energy and coronal mass ejection (CME) mass extended to M dwarfs. Thus, the solar flare/CME relation does not appear to apply to M dwarfs, with important ramifications for the habitability of exoplanets around M dwarfs. There is evidence for some increase in Mdot with coronal activity as quantified by X-ray flux, but with much scatter. One or more other factors must be involved in determining wind strength besides spectral type and coronal activity, with magnetic topology being one clear possibility.

astro-ph.SR↗

arXiv2019

Evidence for Asymmetry in the Velocity Distribution of the Interstellar Neutral Helium Flow Observed by IBEX and Ulysses

We use observations from the Interstellar Boundary Explorer (IBEX) and Ulysses to explore the possibility that the interstellar neutral helium flowing through the inner solar system possesses an intrinsic non-Maxwellian velocity distribution. In fitting the IBEX and Ulysses data, we experiment with both a kappa distribution and a bi-Maxwellian, instead of the usual Maxwellian assumption. The kappa distribution does not improve the quality of fit to either the IBEX or Ulysses data, and we find lower limits to the kappa parameter of kappa>12.1 and kappa>6.0 from the IBEX and Ulysses analyses, respectively. In contrast, we do find evidence that a bi-Maxwellian improves fit quality. For IBEX, there is a clear preferred bi-Maxwellian solution with T_perp/T_par=0.62+/-0.11 oriented about an axis direction with ecliptic coordinates (lambda_axis,b_axis)=(57.2+/-8.9 deg,-1.6+/-5.9 deg). The Ulysses data provide support for this result, albeit with lower statistical significance. The axis direction is close to the ISM flow direction, in a heliocentric rest frame, and is therefore unlikely to be indicative of velocity distribution asymmetries intrinsic to the ISM. It is far more likely that these results indicate the presence of asymmetries induced by interactions in the outer heliosphere.

astro-ph.SR↗

arXiv2017

A Ulysses Detection of Secondary Helium Neutrals

The Interstellar Boundary EXplorer (IBEX) mission has recently studied the flow of interstellar neutral He atoms through the solar system, and discovered the existence of a secondary He flow likely originating in the outer heliosheath. We find evidence for this secondary component in Ulysses data. By coadding hundreds of Ulysses He beam maps together to maximize signal-to-noise, we identify a weak signal that is credibly associated with the secondary component. Assuming a laminar flow from infinity, we infer the following He flow parameters: V=12.8+/-1.9 km/s, lambda=74.4+/-1.8 deg, beta=-10.5+/-4.1 deg, and T=3000+/-1100 K; where lambda and beta are the ecliptic longitude and latitude direction in J2000 coordinates. The secondary component has a density that is 4.9+/-0.9% that of the primary component. These measurements are reasonably consistent with measurements from IBEX, with the exception of temperature, where our temperature is much lower than IBEX's T=9500 K. Even the higher IBEX temperature is suspiciously low compared to expectactions for the outer heliosheath source region. The implausibly low temperatures are due to the incorrect assumption of a laminar flow instead of a diverging one, given that the flow in the outer heliosheath source region will be deflecting around the heliopause. As for why the IBEX and Ulysses T values are different, difficulties with background subtraction in the Ulysses data are a potential source of concern, but the discrepancy may also be another effect of the improper laminar flow assumption, which could affect the IBEX and Ulysses analyses differently.

astro-ph.SR↗

arXiv2015

Ulysses and IBEX Constraints on the Interstellar Neutral Helium Distribution

We relax the usual assumption of Maxwellian velocity distributions in the interstellar medium (ISM) in the analysis of neutral He particle data from Ulysses and the Interstellar Boundary Explorer (IBEX). For Ulysses, the possibility that a narrow component from heavy neutrals is contaminating the He signal is considered, which could potentially explain the lower ISM temperature measured by Ulysses compared to IBEX. The expected heavy element contribution is about an order of magnitude too small to resolve that discrepancy. For IBEX, we find that modest asymmetries in the ISM velocity distribution can potentially improve the quality of fit to the first two years of data, and perhaps improve agreement with the Ulysses measurements.

astro-ph.SR↗

arXiv2015

Revisiting Ulysses Observations of Interstellar Helium

We report the results of a comprehensive reanalysis of Ulysses observations of interstellar He atoms flowing through the solar system, the goal being to reassess the interstellar He flow vector and to search for evidence of variability in this vector. We find no evidence that the He beam seen by Ulysses changes at all from 1994-2007. The direction of flow changes by no more than ~0.3 deg and the speed by no more than ~0.3 km/s. A global fit to all acceptable He beam maps from 1994-2007 yields the following He flow parameters: V_ISM=26.08+/-0.21 km/s, lambda=75.54+/-0.19 deg, beta=-5.44+/-0.24 deg, and T=7260+/-270 K; where lambda and beta are the ecliptic longitude and latitude direction in J2000 coordinates. The flow vector is consistent with the original analysis of the Ulysses team, but our temperature is significantly higher. The higher temperature somewhat mitigates a discrepancy that exists in the He flow parameters measured by Ulysses and the Interstellar Boundary Explorer, but does not resolve it entirely. Using a novel technique to infer photoionization loss rates directly from Ulysses data, we estimate a density of n_He=0.0196+/-0.0033 cm^-3 in the interstellar medium.

astro-ph.SR↗

arXiv2012

Trajectories and Distribution of Interstellar Dust Grains in the Heliosphere

The solar wind carves a bubble in the surrounding interstellar medium (ISM), known as the heliosphere. Charged interstellar dust grains (ISDG) encountering the heliosphere may be diverted around the heliopause or penetrate it depending on their charge-to-mass ratio. We present new calculations of trajectories of ISDG in the heliosphere, and the dust density distributions that result. We include up-to-date grain charging calculations using a realistic UV radiation field and full 3-D magnetohydrodynamic fluid + kinetic models for the heliosphere. Models with two different (constant) polarities for the solar wind magnetic field (SWMF) are used, with the grain trajectory calculations done separately for each polarity. Small grains a_gr ~ 0.01 micron are completely excluded from the inner heliosphere. Large grains, a_gr ~ 1.0 micron pass into the inner solar system and are concentrated near the Sun by its gravity. Trajectories of intermediate size grains depend strongly on the SWMF polarity. When the field has magnetic north pointing to ecliptic north, the field de-focuses the grains resulting in low densities in the inner heliosphere, while for the opposite polarity the dust is focused near the Sun. The ISDG density outside the heliosphere inferred from applying the model results to in situ dust measurements is inconsistent with local ISM depletion data for both SWMF polarities, but is bracketed by them. This result points to the need to include the time variation in the SWMF polarity during grain propagation. Our results provide valuable insights for interpretation of the in situ dust observations from Ulysses.

astro-ph.SR↗

arXiv2012

Direct modeling of neutral helium in the heliosphere

Several years of neutral particle measurements by the NASA/IBEX mission have yielded direct observations of interstellar neutral helium and oxygen. The data indicate the presence of secondary neutral helium and oxygen, which are created within the heliosphere by charge exchange involving helium or oxygen ions. This contribution describes a detailed conserving calculation method based on Keplerian orbits that has been developed to characterize helium distribution functions throughout the heliosphere, in particular in the innermost heliosphere, while accounting for loss and production of neutral particles along their path. Coupled with global heliosphere models of plasma distributions, this code is useful for predicting the fluxes of heavy neutral atoms at spacecraft detectors, so enabling inferences on the characteristics of the interstellar medium.

astro-ph.SR↗

arXiv2012

Primary neutral helium in the heliosphere

Two years of neutral measurements by IBEX-Lo have yielded several direct observations of interstellar neutral helium and oxygen during preferred viewing seasons. Besides the interstellar signal, there are indications of the presence of secondary neutral helium and oxygen created in the heliosphere. Detailed modeling of these particle species is necessary to connect the measured fluxes to the pristine local interstellar medium while accounting for loss and production of neutral particles during their path through the heliosphere. In this contribution, global heliosphere models are coupled to analytic calculations of neutral trajectories to obtain detailed estimates of the neutral distribution function of primary interstellar helium atoms in the heliosphere, in particular in the inner heliosphere.

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arXiv2011

Time-variability in the Interstellar Boundary Conditions of the Heliosphere: Effect of the Solar Journey on the Galactic Cosmic Ray Flux at Earth

During the solar journey through galactic space, variations in the physical properties of the surrounding interstellar medium (ISM) modify the heliosphere and modulate the flux of galactic cosmic rays (GCR) at the surface of the Earth, with consequences for the terrestrial record of cosmogenic radionuclides. One phenomenon that needs studying is the effect on cosmogenic isotope production of changing anomalous cosmic ray fluxes at Earth due to variable interstellar ionizations. The possible range of interstellar ram pressures and ionization levels in the low density solar environment generate dramatically different possible heliosphere configurations, with a wide range of particle fluxes of interstellar neutrals, their secondary products, and GCRs arriving at Earth. Simple models of the distribution and densities of ISM in the downwind direction give cloud transition timescales that can be directly compared with cosmogenic radionuclide geologic records. Both the interstellar data and cosmogenic radionuclide data are consistent with cloud transitions during the Holocene, with large and assumption-dependent uncertainties. The geomagnetic timeline derived from cosmic ray fluxes at Earth may require adjustment to account for the disappearance of anomalous cosmic rays when the Sun is immersed in ionized gas.

astro-ph.GA↗

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.

astro-ph↗

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.

astro-ph↗

arXiv2002

Investigating a Possible Spectral Signature of the Wind-ISM Interaction Region of Alpha Tau

Ultraviolet spectra from the GHRS instrument on board the Hubble Space Telescope reveal the presence of a mysterious absorption feature in the Mg II h & k lines of the nearby (d=20.0 pc) K5 III star Alpha Tau. The narrow absorption looks like an interstellar absorption feature but it is in the wrong location based on our knowledge of the local ISM flow vector. Since the absorption is close to the rest frame of the star, it has been interpreted as being from the interaction region between Alpha Tau's massive, cool wind and the interstellar medium, i.e., Alpha Tau's "astrosphere". We compute hydrodynamic models of the Alpha Tau astrosphere in order to see if the models can reproduce the Mg II absorption feature. These models do predict that stellar wind material heated, decelerated, and compressed after passing through a termination shock a few thousand AU from the star should produce a Mg II absorption feature with about the right width at roughly the right velocity. However, our first models underestimate the Mg II column density by an order of magnitude. A much larger parameter search is necessary to see whether the observed Mg II absorption can be reproduced by acceptable changes to the adopted stellar wind and ISM properties.

astro-ph↗

arXiv2002

Measured Mass Loss Rates of Solar-like Stars as a Function of Age and Activity

Collisions between the winds of solar-like stars and the local ISM result in a population of hot hydrogen gas surrounding these stars. Absorption from this hot H I can be detected in high resolution Lyman-alpha spectra of these stars from the Hubble Space Telescope. The amount of absorption can be used as a diagnostic for the stellar mass loss rate. We present new mass loss rate measurements derived in this fashion for four stars (Epsilon Eri, 61 Cyg A, 36 Oph AB, and 40 Eri A). Combining these measurements with others, we study how mass loss varies with stellar activity. We find that for the solar-like GK dwarfs, the mass loss per unit surface area is correlated with X-ray surface flux. Fitting a power law to this relation yields Mdot ~ Fx^(1.15+/-0.20). The active M dwarf Proxima Cen and the very active RS CVn system Lambda And appear to be inconsistent with this relation. Since activity is known to decrease with age, the above power law relation for solar-like stars suggests that mass loss decreases with time. We infer a power law relation of Mdot ~ t^(-2.00+/-0.52). This suggests that the solar wind may have been as much as 1000 times more massive in the distant past, which may have had important ramifications for the history of planetary atmospheres in our solar system, that of Mars in particular.

astro-ph↗

arXiv2000

Observational Estimates for the Mass-Loss Rates of Alpha Centauri and Proxima Centauri Using HST Lyman-alpha Spectra

We study H I Lyman-alpha absorption observed by the Hubble Space Telescope toward the nearby binary system Alpha Cen (G2 V+K0 V) and its distant companion star Proxima Cen (M5.5 Ve). Absorption from heliospheric H I heated by the solar wind/ISM interaction is observed toward both Alpha Cen and Proxima Cen. Absorption from analogous "astrospheric" material surrounding the stars is detected toward Alpha Cen, but not Proxima Cen. The nondetection of astrospheric absorption toward Proxima Cen suggests that the stellar wind of Proxima Cen must be significantly weaker than that of the Alpha Cen system. We use hydrodynamic models of the astrospheres computed assuming different mass-loss rates to predict astrospheric Lyman-alpha absorption for comparison with the observations. The model that best matches the Alpha Cen data has a mass-loss rate of twice the solar rate, and the models suggest an upper limit of 0.2 solar for Proxima Cen. Finally, we note that the heliospheric absorption observed toward Proxima Cen in 2000 May is identical to the heliospheric absorption observed toward Alpha Cen in 1995 May, implying that the structure of the outer heliosphere does not change significantly during the solar activity cycle.

astro-ph↗

arXiv2000

Hydrogen Lyman-alpha Absorption Predictions by Boltzmann Models of the Heliosphere

We use self-consistent kinetic/hydrodynamic models of the heliosphere to predict H I Lyman-alpha absorption profiles for various lines of sight through the heliosphere. These results are compared with Lyman-alpha absorption lines of six nearby stars observed by the Hubble Space Telescope. The directions of these lines of sight range from nearly upwind (36 Oph) to nearly downwind (Epsilon Eri). Only three of the Lyman-alpha spectra (36 Oph, Alpha Cen, and Sirius) actually show evidence for the presence of heliospheric absorption, which is blended with the ubiquitous interstellar absorption, but the other three spectra still provide useful upper limits for the amount of heliospheric absorption for those lines of sight. Most of our models use a Boltzmann particle code for the neutrals, allowing us to estimate neutral velocity distributions throughout the heliosphere, from which we compute model Lyman-alpha absorption profiles. In comparing these models with the data, we find they predict too much absorption in sidewind and downwind directions, especially when higher Mach numbers are assumed for the interstellar wind. Models created assuming different values of the interstellar temperature and proton density fail to improve the agreement. Somewhat surprisingly, a model that uses a multi-fluid treatment of the neutrals rather than the Boltzmann particle code is more consistent with the data, and we speculate as to why this may be the case.

astro-ph↗

arXiv1997

Toward numerical and analytical studies of first order phase transitions

Discrete lattice simulations of an one-dimensional phi^4 theory coupled to an external heat bath are being carried out. Great care is taken to remove the effects of lattice discreteness and finite size and to establish the correct correspondence between simulations and the desired, finite-temperature continuum limit.

hep-lat↗