R. Lallement

Synopsis of the interstellar He parameters from combined neutral gas, pickup ion and UV scattering observations and related consequences

A coordinated effort to combine all three methods that are used to determine the physical parameters of interstellar gas in the heliosphere has been undertaken. In order to arrive at a consistent parameter set that agrees with the observations of neutral gas, pickup ions and UV backscattering we have combined data sets from coordinated observation campaigns over three years from 1998 through 2000. The key observations include pickup ions with ACE and Ulysses SWICS, neutral atoms with Ulysses GAS, as well as UV backscattering at the He focusing cone close to the Sun with SOHO UVCS and at I AU with EUVE. For the first time also the solar EUV irradiance that is responsible for photo ionization was monitored with SOHO CELIAS SEM, and the He I 58.4 nm line that illuminates He was observed simultaneously with SOHO SUMER. The solar wind conditions were monitored with SOHO, ACE, and WIND. Based on these data the modeling of the interstellar gas and its secondary products in the heliosphere has resulted in a consistent set of interstellar He parameters with much reduced uncertainties, which satisfy all observations, even extended to earlier data sets. It was also established that a substantial ionization in addition to photo ionization, most likely electron impact, is required, with increasing relative importance closer to the Sun. Furthermore, the total combined ionization rate varies significantly with solar latitude, requiring a fully three dimensional and time dependent treatment of the problem.

On the contribution of charge-exchange induced X-ray emission in the ISM and ICM

X-ray emission can be generated by charge-exchange (CXE) between highly charged ions of a hot plasma and neutral species of an interacting cool/warm gas, a phenomenon recently observed in the case of the solar wind interaction with cometary, interstellar, and geocoronal neutrals (Lisse et al. 1996; Cox 1998; Cravens 1997, 2002). Charge-exchange processes are included in most theoretical models of hot interstellar plasmas interacting with partially neutral gas, resulting in a modification of the physical states of the gases in the interaction region. However, the contribution of the charge-exchange induced X-ray emission produced at these interfaces has hitherto not been considered to be significant. The detailed calculations performed by Wise & Sarazin (1989), motivated by the observations of X-ray emission following charge- exchange in laboratory fusion devices, have shown that the emission is negligible in the case of an SNR fast shock. Our goal here is to investigate its relative importance in different astrophysical cases. We simplistically consider interfaces between partially neutral and hot gas in the following cases: (a) a supernova blast wave propagating in a neutral (or partially ionized) ISM (b) a galactic wind engulfing a halo dense cloud; (c) a high velocity cloud (HVC) moving through the halo, and (c) a dense cloud moving in intra-cluster gas. Although the phenomenon is restricted to the very narrow envelopes defined by the mean free path of the neutrals through the hot plasma, it is easy to show that its efficiency is such that the volume emissivity from these interfaces can be orders of magnitude above the emissivity of the hot gas itself, and, more important, that its relative contribution increases with decreasing hot gas density. As a consequence it should be at maximum in mixing layers in very low density hot gas. Our preliminary results suggest that the charge-exchange X-ray emission from the interface does not contribute significantly in case (a), in agreement with Wise & Sarazin, except marginally for lines of sight tangent to the interfaces, but that it can be a non-negligible fraction of the hot gas emission in cases (b-d). Detailed self-consistent models of plasma and neutral atom distributions in conduction fronts or contact discontinuities are needed for better estimates. In the easiest test case of HVCs, there is a good agreement between our predicted equivalent emission measure EM ≈

Heliospheric conditions that affect the interstellar gas inside the heliosphere

The interstellar gas that flows through the heliosphere is strongly affected by ionization close to the Sun, in particular solar photoionization, electron impact, and charge exchange. Therefore, the interpretation of any observation of interstellar gas in the inner heliosphere hinges upon the accurate knowledge of these effects and their variations. In addition, the irradiance and line profile of the relevant solar spectral line are needed to properly interpret resonant backscattering observations of the interstellar neutral gas. With instrumentation on ACE, SOHO and Wind, continuous monitoring of these important environmental conditions simultaneously with a multitude of interstellar gas observations has become possible for the first time. In this paper we present a compilation of the processes and parameters that affect the distribution of interstellar helium inside the heliosphere and their observation, including the irradiance and line profile of the He 58.4 nm line. We also make the connection to proxies for these parameters and evaluate their accuracy in order to expand the time period of coverage wherever possible.

High ions towards white dwarfs: circumstellar line shifts and stellar temperature

Aims. Our aim is to gain new insights into highly ionized gas towards nearby hot white dwarfs (WDs). The detection of absorption lines of highly ionized interstellar (IS) species in their spectra is the main diagnostic tool of the hot IS gas phase. This requires disentangling IS ions from photospheric or circumstellar (CS) ions, and thus their simultaneous study. This is particularly timely due to the recent discovery of infra-red emitting disks around WDs and the recognition that accretion from those disks can explain the presence of photospheric heavy ions, thus opening the exciting prospect of measuring the composition of extra-solar disks and planetary systems. Methods. We present far UV spectra of three nearby white dwarfs recorded with the Cosmic Object Spectrograph on board the HST. Absorption lines of several ions at various ionization stages are detected, and we investigate their origin by means of their kinematical properties, in combination with previous measurements with the FUSE satellite. We supplement with these new results a large set of archival white dwarfs UV data and we analyze the line shifts. Results. Interstellar CIV, SiIV, NV and OVI ions are likely detected at distance from the Plane towards WD1040+492 (230 pc), but within 100 parsecs there are no detections of IS ions. Small absorptions generated at cloud-hot gas interfaces may be present but are within our detection limit. The main result is the detection of the CIV, SiIV and NV counterparts to the redshifted OVI lines measured towards WD2257-073 and WD1040+492. Based on their velocities, they very likely trace absorbing circumstellar material, extending earlier results. More surprisingly, CS absorption is also detected at small redshift towards WD1942+499, and potentially towards WD2257-073. Comparisons of the COS CIII and CIV data with atmospheric models preclude a stellar origin for these features. We thus present first evidence for a temporal variability of the CS lines and for a case of double absorption. Overall these results demonstrate the ubiquity of CS absorptions and favor a large interval of CS line shifts. Based on the compilation of OVI, CIV, SiIV and NV data from IUE, FUSE, GHRS, STIS, and COS, we derive an anti-correlation between the stellar temperature and the high ion velocity shift w.r.t. to the photosphere, with positive (resp. negative) velocity shifts for the cooler (resp. hotter) white dwarfs. This trend probably reflects more than a single process, however such a dependence on the WD’s temperature again favors a CS origin for a very large fraction of those ion absorptions, and a form of balance between radiation pressure and gravitation. This is consistent with ubiquitous evaporation of orbiting dusty material. In view of these results, ion measurements close to the photospheric or the IS velocity should be interpreted with caution, especially for stars at intermediate temperatures. While tracing CS gas, they may be erroneously attributed to photospheric material or to the ISM, explaining the difficulty of finding a coherent pattern of the high ions in the local IS 3D distribution.

Far Ultraviolet Spectroscopic Explorer observations of high-velocity gas associated with the Monoceros Loop SNR

We present Far Ultraviolet Spectroscopic Explorer (

Interstellar NaI and CaII absorption observed towards the Cygnus Loop SNR

\it FUSE

Probing the inner halo and IVC gas through the Local Interstellar Chimney

) observations of high-velocity gas ( V LSR = +65 km s -1 ) seen towards the star HD 47240 which lies just behind the Monoceros Loop Supernova Remnant at a distance of ~1800 pc. This high-velocity absorption feature is detected in the far ultraviolet lines of O I, Ar I, N I, C I, Fe II and P II, in addition to being detected at visible wavelengths in Na I and Ca II and at near ultraviolet wavelengths in Mg II, Mg I, S II, O I, Si II, C II*, Al II and Fe II. High-velocity interstellar gas has not been detected in the high-ionization (high-temperature) species of O VI, C IV and Si IV. Gas phase abundances relative to that of sulphur for this high velocity feature have been derived. The refractory elements of Fe, Si and Al are all less depleted than that normally found for cold disk gas in the interstellar medium, with a pattern of relative abundance more similar to that of warm interstellar disk gas. However, the elements of N, O, and Ar show an opposite pattern of relative depletion in which their apparent elemental deficiency may be attributed to ionization effects, as also found for high-velocity gas associated with the Vela SNR by Jenkins et al. ([CITE]). The lack of detection of high-ionization gas at high velocity suggests that the Monoceros Loop remnant is more evolved than other remnants such as the Vela SNR or Cygnus Loop, and that an age of 30 000-150 000 years seems appropriate.

Exploring interstellar titanium and deuterium abundances and other correlations

We present high resolution spectra (R 5k m s 1 ) of the interstellar NaI and CaII absorption lines observed to- wards 9 early-type stars with distances ranging from 250 to 2300 pc in the line-of-sight towards the Cygnus Loop Supernova Remnant (SNR). All but one of these absorption profiles can be fit using a combination of one or more of three absorption components with average best-fit (lsr) velocities of V1=+0: 8k m s 1 , V2=+9: 0k m s 1 and V3=+19: 7k m s 1 . An additional velocity component at V4=+29: 7k m s 1 is required in order to fit the NaI profile recorded towards the star HD 198946, whose distance of 794 pc places it well in excess of the nominal 440 pc distance to the SNR. The NaI/CaII column density ratios for the three higher velocity components are typically<1.0, which are similar to values found for high-velocity gas components detected towards other evolved SNRs. Even though we have detected the three higher velocity components solely along the sight-lines towards stars with distance estimates greater than that of the Cygnus Loop, we are unable to definitely associate these components with an interaction between the expansion of the SN shock wave and the ambient interstellar medium. We suggest a more likely origin for these absorption components is that of an old pre-cursor SN neutral gas shell, within whose interstellar cavity the Cygnus Loop supernova explosion occured some 20 000 years ago.

Dense Gas Contours Surrounding the Local Cavity: Comparison with the Soft X-Ray Background Emission Map

We present an absorption study of the interstellar gas at high positive galactic latitudes in the direction close to the axis of the Local Chimney (LC), which is an extension of the rarefied local cavity that reaches out from the galactic disk to a z-distance of at least 250 pc into the lower halo region. Our study includes high-resolution (R ∼ 1. 7k m s −1 ) spectral observations of the interstellar NaI and CaII absorption lines seen towards 6 early-type stars with distances ranging from 225 to 500 pc contained within a radius of ∼6 ◦ along the sight-line (l = 160 ◦ , b =+ 55 ◦ ). These visible data are supplemented with far-ultraviolet absorption measurements of the interstellar sight-lines towards two hot white dwarf stars, RE J1043+490 (d = 230 pc) and RE J1059+512 (d = 315 pc), taken with the NASA Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. Our observations reveal interstellar gas clouds with velocities in the −20 to −60 km s −1 range that appear to be falling towards the galactic disk. In particular, we have detected absorption with a velocity of Vhelio ∼− 55 km s −1 towards two sight-lines (HD 89501 and HD 88545) that can be associated with an intermediate velocity (IV) cloud called the IV Arch. Our observations place a probable z-distance to this IVC of 275-320 pc, this being much nearer than previously thought. The far UV spectra of the two hot white dwarfs show only a few (∼10) interstellar absorption lines, which is indicative of the very low density phase of the interstellar gas contained within the LC region. We have derived relative elemental abundance ratios for C, N, O, Si, Ar and Fe for these two sight-lines and find that the abundance patterns are very similar to those found for other sight-lines in the local cavity. The high ionization line of OVIλ1032 A has been detected towards RE J1043+490 with a column density of 7 × 10 12 cm −2 , which is consistent with the average space density of this ion recently found in the Local Bubble region. Finally, we (tentatively) propose a possible connection between the formation of the Gould Belt, the Local Bubble cavity, the Local Chimney and the overlying IV Arch clouds.

ON THE DETECTION OF HELIOSPHERIC INTERFACE PROPERTIES FROM INTERSTELLAR/INTERPLANETARY LYMAN α PROFILES: EXAMPLE OF BARANOV'S TWO-SHOCK MODEL

Aims. The origin of the observed variability of the gas-phase D/H ratio in the local interstellar medium is still debated, and in particular the role of deuterium depletion onto dust grains. Here we extend the study of the relationship between deuterium and titanium, a refractory species and tracer of elemental depletion, and explore other relationships. Methods. We have acquired high resolution spectra for nine early-type stars using the VLT/UVES spectrograph, and detected the absorption lines of interstellar TiII. Using a weighted orthogonal distance regression (ODR) code and a special method to treat non symmetric errors, we compare the TiII columns with the corresponding HI, DI and also OI columns. In parallel we perform the same comparisons for available FeII data. Results. We find a significant correlation between TiII/HI and D/H in our data set, and, when combined with published results, we confirm and better constrain the previously established trends and extend the trends to low HI columns. We exclude uncertainties in HI and OI columns as the main contributor to the derived metals-deuterium correlations by showing that the TiII/HI ratio is positively correlated with DI/OI. We find a similar correlation between FeII/HI and DI/OI. The TiII gradients are similar or slightly smaller than for FeII, while one would expect larger variations on the basis of the higher condensation temperature of titanium. However we argue that ionisation effects introduce biases that affect iron and not titanium and may explain the gradient similarity. We find a less significant negative correlation between the TiII/DI ratio and the hydrogen column, possibly a sign of different evaporation of D and metals according to the cloud properties. More TiII absorption data along very low H column lines-of-sight would be useful to improve the correlation statistics.