Analyzing H I Lyman-alpha lines

4.1 Analyzing H I Lyman-alpha lines

Spectroscopic analyses of stellar H I Lyman- $α$ lines have proven to be the best way so far to clearly detect and measure weak solar-like winds, but analysis of this line is complex. The history of Ly $α$ absorption observations and analyses is summarized here, with emphasis on how these studies eventually led to the detection of solar-like stellar winds.

The Ly $α$ line at 1216 Å is the most fundamental transition of the most abundant atom in the universe. As such, the line is a valuable diagnostic for many purposes. For cool stars, the Ly $α$ line is a strong radiative coolant for stellar chromospheres, and is therefore an important chromospheric diagnostic. Cool stars produce very little continuum emission at 1216 Å, so Ly $α$ lines observed from cool stars are strong, isolated emission lines. However, these lines are always heavily contaminated by interstellar absorption.

Figure 5:

HST/GHRS spectra of the Ly $α$ lines of $α$ Cen A and B, showing broad absorption from interstellar H I and narrow absorption from D I (Linsky and Wood, 1996

Figure 5 shows the Ly $α$ lines observed from the two members of the $α$ Cen binary, which is the nearest star system to us at a distance of only 1.3 pc. The centers of the stellar emission lines are obliterated by broad, saturated H I absorption. Narrower absorption from deuterium (D I) is visible $− 0.33˚A$ from the H I absorption. The D I absorption is entirely from interstellar material in between $α$ Cen and the Sun, and interstellar absorption also accounts for much of the H I absorption. The H I and D I absorption lines are therefore valuable diagnostics for the LISM, and stellar Ly $α$ data like that in Figure 5 have proven to be useful for measuring the properties of warm, neutral ISM gas, and also for mapping out the distribution of this gas in the vicinity of the Sun (see Linsky et al., 2000 ; Redfield and Linsky, 2000 ).

The importance of these data is magnified further by the measurements the data provide of D/H ratios. The LISM D/H ratio has important applications for both cosmology and our understanding of Galactic chemical evolution (see McCullough, 1992 ; Linsky, 1998 ; Moos et al., 2002 ; Wood et al., 2004 ). The light element abundances in the universe are powerful diagnostics for Big Bang nucleosynthesis calculations, with the exact abundances depending on the cosmic baryon density, $Ωb$ . The deuterium abundance is particularly sensitive to $Ωb$ , so measuring the primordial D/H ratio and thereby constraining cosmological models has been a major goal in astronomy (see Boesgaard and Steigman, 1985 ; Burles et al., 2001 ). The most accurate meaningful D/H measurements come from analyses of LISM absorption like that in Figure 5 . Unfortunately, the LISM D/H ratio only provides a lower limit to the primordial D/H ratio. Since deuterium is destroyed in stellar interiors, the D/H ratio is expected to have decreased with time, so Galactic chemical evolution models are required to extrapolate back to a primordial value (see Prantzos, 1996 ; Tosi et al., 1998 ; Chiappini et al., 2002 ). Primordial D/H values can also be measured more directly by measuring D/H in more pristine intergalactic material (see Kirkman et al., 2003 ).

The desire to improve our understanding of the LISM and measure D/H has provided the primary impetus behind stellar Ly $α$ analyses. This work dates back to Copernicus (York and Rogerson, 1976 ; Dupree et al., 1977 ), which was the first ultraviolet astronomical satellite to provide high quality stellar Ly $α$ spectra. Copernicus was followed by the long-lived International Ultraviolet Explorer (IUE) satellite, which also provided Ly $α$ spectra that could be analyzed for LISM and D/H purposes (see Murthy et al., 1987 ; Diplas and Savage, 1994 ). However, it was the Goddard High Resolution Spectrograph (GHRS) instrument aboard HST that was the first UV spectrometer capable of fully resolving the D I and H I absorption line profiles. The GHRS spectrometer was replaced by the Space Telescope Imaging Spectrometer (STIS) in 1997.

The first Ly $α$ analyses from HST data were for the lines of sight to Capella and Procyon (Linsky et al., 1993 , 1995 ). However, the third analysis, which was of the $α$ Cen data shown in Figure 5 , presented a dilemma. The observed H I absorption is simply inconsistent with D I and other ISM absorption lines (Mg II, Fe II, etc.). The D I absorption and the other non-H I lines are centered at a heliocentric velocity of $− 1 v = − 18.0 ± 0.2 km s$ , and the widths of these lines suggest an interstellar temperature of $T = 5400 ± 500 K$ . However, the H I absorption implies $v = − 15.8 ± 0.2 km s−1$ and $T = 8350 K$ . In other words, the H I absorption is broader than it should be, and it is also redshifted by $− 1 2.2km s$ from where it should be. (Linsky and Wood, 1996 ). Interestingly enough, the H I redshift was also discerned earlier in much lower quality IUE spectra (Landsman et al., 1984 ).

Linsky and Wood (1996 ) interpreted the problem as being due to the presence of an extra H I absorption component that contributes no absorption to any of the weaker ISM lines. This was also one of the hypotheses suggested by Landsman et al. (1984 ) based on the IUE data. Two-component fits to the HST/GHRS data from Linsky and Wood (1996 ) suggest that the extra H I absorption can be explained by the existence of a redshifted H I absorption component with a temperature of $T ≈ 30000 K$ and a column density (in $cm −2$ ) of $logN (H I) ≈ 15.0$ . This temperature is much hotter than typical LISM material, and the column density is almost three orders of magnitude lower than the ISM H I column density towards $α$ Cen. The low column density would explain why the absorption component is only seen in the H I line and not in any of the other ISM lines from atomic species with much lower abundances. However, the interpretation for this absorption component was initially a mystery.

Fortuitously, the $α$ Cen Ly $α$ analysis was being performed at about the same time as the first heliospheric models including neutrals in a self-consistent manner were being developed (see Section 2.3). One session of the 1995 IUGG (International Union of Geodesy and Geophysics) General Assembly brought together interstellar and heliospheric experts, and it was realized during that meeting that the heated heliospheric hydrogen predicted by the new heliospheric models had precisely the right properties to account for the extra $α$ Cen absorption component. It was quickly realized that if hot hydrogen existed around the Sun, then it should exist around other solar-like stars as well, so the initial $α$ Cen analysis suggested that the excess H I absorption could be partly due to astrospheric as well as heliospheric absorption (Linsky and Wood, 1996 ). Thus was born a new way to detect and study stellar winds.

Figure 6:

Schematic diagram showing how a stellar Ly $α$ profile changes from its initial appearance at the star and then through various regions that absorb parts of the profile before it reaches an observer at Earth: the stellar astrosphere, the LISM, and finally the heliosphere (Wood et al., 2003b ). The lower panel shows the actual observed Ly $α$ profile of $α$ Cen B. The upper solid line is the assumed stellar emission profile and the dashed line is the ISM absorption alone. The excess absorption is due to heliospheric H I (green shading) and astrospheric H I (red shading).

More work was required to verify this interpretation of the Ly $α$ data. Gayley et al. (1997 ) made the first direct comparison between the $α$ Cen Ly $α$ data and the predictions of various heliospheric models. This work demonstrated that heliospheric Ly $α$ absorption can only account for the excess absorption seen on the red side of the Ly $α$ line, and that astrospheric absorption is required to explain the excess absorption seen on the blue side of the line. A more refined interpretation for the $α$ Cen Ly $α$ absorption was therefore developed, and is shown schematically in Figure 6 . The four middle panels in Figure 6 show what happens to the $α$ Cen B Ly $α$ line profile as it journeys from the star towards the Sun. The Ly $α$ profile first has to traverse the hot hydrogen in the star’s astrosphere, which erases the central part of the line. The Ly $α$ emission then makes the long interstellar journey, resulting in additional absorption, including some from D I. Finally, the profile has to travel through the heliosphere, resulting in additional absorption on the red side of the line. Most of the astrospheric and heliospheric absorption is from material in the “hydrogen wall” region mentioned at the end of Section 2.3 (see Figure 4 ).

Why is the heliospheric absorption redshifted relative to the interstellar absorption? For the $α$ Cen line of sight, which is roughly in the upwind direction relative to the LISM flow seen by the Sun, the heliospheric absorption is redshifted primarily because of the deceleration and deflection of interstellar material as it crosses the bow shock. Heliospheric models predict that heliospheric Ly $α$ absorption should always be redshifted relative to the ISM absorption, even in downwind directions, although the physical explanation for the redshift in the downwind direction is more complicated (Izmodenov et al., 1999b ; Wood et al., 2000b ). Conversely, astrospheric absorption will always be blueshifted relative to the ISM absorption, since we are viewing that absorption from outside the astrosphere rather than inside. It is very fortunate that heliospheric and astrospheric material produce excess absorption on opposite sides of the Ly $α$ line, as this makes it possible to identify the source of the absorption.

The bottom panel of Figure 6 shows the observed Ly $α$ profile of $α$ Cen B (Linsky and Wood, 1996 ). As mentioned above, the non-H I ISM lines observed towards $α$ Cen suggest $−1 v = − 18.0 ± 0.2km s$ and $T = 5400 ± 500 K$ for the ISM material in this direction. The dashed line in the bottom panel of Figure 6 shows what the H I absorption looks like when forced to be consistent with these results. No matter what is assumed for the stellar line profile, and no matter what is assumed for the ISM H I column density, there is always excess absorption on both sides of the H I absorption feature that cannot be explained by ISM absorption. As suggested above, the red side excess is best interpreted as heliospheric absorption and the blue side excess is best interpreted as astrospheric absorption.

This example illustrates how heliospheric and astrospheric absorption is detected. The ISM H I absorption is estimated by forcing the H I fit parameters to be consistent with D I and other ISM lines. In many cases, this still leads to excellent fits to the data, but in some cases there is evidence for excess H I absorption on one or both sides of the line, indicating the presence of heliospheric and/or astrospheric absorption. The Ly $α$ analyses can be simplified even further if one assumes that $−5 D ∕H = 1.5 × 10$ , in addition to forcing $v$ and $T$ to be consistent for D I and H I. This assumption should be valid for nearby stars, since recent work suggests that $D ∕H ≈ 1.5 × 10− 5$ throughout the Local Bubble, with no evidence for variation (Linsky, 1998 ; Moos et al., 2002 ; Wood et al., 2004 ).

The heliospheric/astrospheric interpretation of the excess Ly $α$ absorption has strong theoretical support, but additional evidence for the validity of this interpretation is still valuable. The best purely empirical demonstration that it is correct comes by comparing the Ly $α$ absorption observed towards $α$ Cen with that observed towards a distant M dwarf companion of the $α$ Cen system called Proxima Cen (Wood et al., 2001 ). This comparison is made in Figure 7 . The Ly $α$ absorption profiles agree well on the red side of the line where the heliospheric absorption is located. However, the blue-side excess absorption seen towards $α$ Cen is not seen towards Proxima Cen. This means that the blue-side excess absorption seen towards $α$ Cen has to be from circumstellar material surrounding $α$ Cen that does not extend as far as the distant companion Proxima Cen ( $∼ 12000 AU$ away), consistent with the astrospheric interpretation. Apparently, Proxima Cen must have a weaker wind than the $α$ Cen binary, which results in a much smaller astrosphere and much less astrospheric Ly $α$ absorption. (The two $α$ Cen stars are close enough that they will share the same astrosphere, and the astrospheric absorption will therefore be characteristic of the combined winds of both stars.) This example suggests how the astrospheric absorption might be used as a diagnostic for the mass loss rates of solar-like stars, which is a subject that is discussed in detail in Section 4.3.