Abstract
High-resolution UV spectra of stellar H
i
Ly
α
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 than or comparable in strength to that of the Sun, i.e.,
M
̇
≤
1
M
̇
⊙
. However, two of the M dwarfs have much stronger winds: YZ CMi (M4 Ve;
M
̇
=
30
M
̇
⊙
) and GJ 15AB (M2 V+M3.5 V;
M
̇
=
10
M
̇
⊙
). 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
M
̇
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.
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 and the speed by no more than ∼0.3 km s{sup –1}. A global fit to all acceptable He beam maps from 1994-2007 yields the following He flow parameters: V {sub ISM} = 26.08 ± 0.21 km s{sup –1}, λ = 75.54 ± 0.°19, β = –5.44 ± 0.°24, and T = 7260 ± 270 K; where λ and β 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 {sub He} = 0.0196 ± 0.0033 cm{sup –3} in the interstellar medium.
Abstract Radiation pressure exerted by solar photon output is salient to the motion of primary neutral hydrogen atoms streaming into the inner heliosphere directly from the local interstellar medium. ...The action of a time-dependent radiation pressure force, when coupled with the usual gravitational force, changes the characteristic velocities, and therefore energies, of the atoms when they reach regions in which explorer probes are present. A study is presented that uses a 2D code to backtrace neutral hydrogen trajectories from representative target points located 1 au from the Sun. It makes use of both a radiation pressure function and a function for the photoionization rate at 1 au that both oscillate with time based on measurements over a typical solar cycle, as well as a time-independent charge exchange ionization rate at 1 au. Assuming a Maxwellian distribution in the distant upwind direction, phase space data is calculated at the target points, at different moments in time. The dependence of the force on the radial particle velocity has been omitted in the analysis, such that the emphasis is on the effects of the global solar UV intensity variations through the solar cycle. This process allows for the analysis of direct and indirect Maxwellian components through time and space in the time-dependent force environment. Additionally, pseudo-bound orbits caused by energy losses associated with this force environment are observed, and their properties are evaluated with the aim of determining their effects on potential measurements by explorer probes.
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 that ...likely originates 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 ratio, 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: km s−1, , , and K; where λ and β 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.
EVIDENCE FOR A WEAK WIND FROM THE YOUNG SUN Wood, Brian E; Muller, Hans-Reinhard; Redfield, Seth ...
Astrophysical journal. Letters,
02/2014, Letnik:
781, Številka:
2
Journal Article
Recenzirano
The early history of the solar wind has remained largely a mystery due to the difficulty of detecting winds around young stars that can serve as analogs for the young Sun. Here we report on the ...detection of a wind from the 500 Myr old solar analog pi super(1) UMa (G1.5 V), using spectroscopic observations from the Hubble Space Telescope. We detect H I Ly alpha absorption from the interaction region between the stellar wind and interstellar medium, i.e., the stellar astrosphere. With the assistance of hydrodynamic models of the pi super(1) UMa astrosphere, we infer a wind only half as strong as the solar wind for this star. This suggests that the Sun and solar-like stars do not have particularly strong coronal winds in their youth.
Abstract 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 has provided ...spatially resolved observations of the warm chromospheric winds that predominate for early K to mid-M giants. The H i Ly α 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 M ̇ = ( 14 – 86 ) × 10 − 11 M ⊙ yr −1 , while the K giants with detected wind absorption have weaker winds with M ̇ = ( 1.5 – 2.8 ) × 10 − 11 M ⊙ yr −1 . We use long-slit spectra of H i Ly α for two particular red giants, α Tau (K5 III) and γ Cru (M3.5 III), to study the spatial extent of the Ly α emission. From these data we estimate limits for the extent of detectable emission, which are r = 193 R * for γ Cru and r = 44 R * for α Tau. The 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 Ly α 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 Ly α analysis.
ABSTRACT
We report on an ultraviolet spectroscopic survey of red giants observed by the
Hubble Space Telescope
, focusing on spectra of the Mg
ii
h and k lines near 2800 Å in order to study stellar ...chromospheric emission, winds, and astrospheric absorption. We focus on spectral types between K2 III and M5 III, a spectral type range with stars that are noncoronal, but possessing strong, chromospheric winds. We find a very tight relation between Mg
ii
surface flux and photospheric temperature, supporting the notion that all K2-M5 III stars are emitting at a basal flux level. Wind velocities (
V
w
) are generally found to decrease with spectral type, with
V
w
decreasing from ∼40 km s
−1
at K2 III to ∼20 km s
−1
at M5 III. We find two new detections of astrospheric absorption, for
σ
Pup (K5 III) and
γ
Eri (M1 III). This absorption signature had previously only been detected for
α
Tau (K5 III). For the three astrospheric detections, the temperature of the wind after the termination shock (TS) correlates with
V
w
, but is lower than predicted by the Rankine–Hugoniot shock jump conditions, consistent with the idea that red giant TSs are radiative shocks rather than simple hydrodynamic shocks. A full hydrodynamic simulation of the
γ
Eri astrosphere is provided to explore this further.
Motivated by recent Voyager 2 (V2) measurements of surprisingly high plasma temperatures outside the heliopause, we study the heliospheric hydrogen Lyman-a absorption observed by the Hubble Space ...Telescope for the closest available line of sight to the V2 direction, 5 Pav, which is 9° from V2. The absorption is well reproduced by a typical global heliospheric model that predicts outer heliosheath temperatures of T«20, 000 K. Arbitrarily increasing these temperatures by a factor of two to be more consistent with the initial T=30, 000-50, 000 K values measured by V2 leads to significant overprediction of absorption. Thus, the high plasma temperatures first encountered by V2 near the heliopause cannot be present throughout the outer heliosheath.
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 three-dimensional 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 sub(gr) lap 0.01 mu m are completely excluded from the inner heliosphere. Large grains, a sub(gr) gap 1.0 mu m, 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.
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