arXiv subjects

A. Vidal-Madjar

Publications and source records attributed to A. Vidal-Madjar.

At least 109 records · Page 6Linked to original sources

arXiv2001

FUSE and HST STIS Observations of Hot and Cold Gas in the AB Aurigae System

We present the first observations of a Herbig Ae star with a circumstellar disk by the Far Ultraviolet Spectroscopic Explorer (FUSE), as well as a simultaneous observation of the star obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS). The spectra of AB Aurigae show emission and absorption features arising from gasses that have a wide range in temperature, from hot OVI emission to cold molecular hydrogen and CO absorption. Emissions from the highly ionized species OVI and CIII present in the FUSE spectrum are redshifted, while absorption features arising from low-ionization species like OI, NI, and SiII are blueshifted and show characteristic stellar wind line-profiles. We find the total column density of molecular hydrogen toward AB Aur from the FUSE apectrum, N(H_2) = (6.8 +/- 0.5) x 10^19 cm^-2. The gas kinetic temperature of the molecular hydrogen derived from the ratio N(J=1)/N(J=0) is 65 +/- 4 K. The column density of the CO observed in the STIS spectrum is N(CO) = (7.1 +/- 0.5) x 10^13 cm^-2, giving a CO/H_2 ratio of (1.04 +/- 0.11) x 10^-6. We also use the STIS spectrum to find the column density of HI, permitting us to calculate the total column density of hydrogen atoms, the fractional abundance of H_2, and the gas-to-dust ratio.

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arXiv2001

FUSE Observations of Interstellar Gas toward the SMC Star Sk 108

We discuss the interstellar absorption lines found in FUSE spectra of the Wolf-Rayet binary Sk 108, located in the northeastern part of the main ``bar'' of the Small Magellanic Cloud. The spectra cover the wavelength range 988-1187 Angstroms, at a resolution of about 12,000 and S/N of 20--40. We use detailed component information from higher resolution near-UV and optical spectra to model the far-UV lines of similarly distributed species. Both the Galactic and SMC gas toward Sk 108 seem to be predominantly neutral, though a significant fraction of the SMC gas is ionized. The column densities of P II, S II, and Ar I are consistent with essentially solar ratios, relative to N(Zn II), in both the Galactic and SMC gas; the column density of N I remains somewhat uncertain. Molecular hydrogen is present in the Galactic gas, with properties similar to those found in low mean density Galactic lines of sight and in the Galactic gas toward several other LMC and SMC stars. We report a tentative detection of H_2 in the SMC gas for J = 1 and 3, with rotational level populations consistent with an excitation temperature of order 1000 K -- similar to the H_2 found in diffuse Galactic gas toward zeta Pup. Strong absorption from N III, S III, and Fe III has revealed a significant ionized component, particularly in the SMC; O VI is present, but relatively weak, especially in the Galactic gas. The N(C IV)/N(O VI) ratio varies somewhat within the SMC --- suggesting that several processes may contribute to the observed high ion abundances.

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arXiv2000

Revealing deuterium Balmer lines in HII regions with VLT-UVES

The search for deuterium Balmer lines with VLT-UVES is reported in HII regions of the Galaxy and the Magellanic Clouds. The DI lines appear as faint, narrow emission features in the blue wings of the HI Balmer lines and can be distinguished from high-velocity HI emission. The previous identification to deuterium is re-inforced beyond doubt. The detection of D-alpha and D-beta in Orion (Hebrard et al. 2000) is confirmed and deuterium lines are now detected up to at least D-eta. The UVES observations provide the first detection of Balmer DI lines in four new HII regions (M 8, M 16, M 20, and DEM S 103 in SMC), demonstrating that these lines are of common occurence.

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arXiv2000

Type Ia supernova rate at $z \sim 0.1$

We present the EROS nearby supernova ($z \sim 0.02 - 0.2$) search and the analysis of the first year of data (1997). A total of 80 square degrees were surveyed. Eight supernovæ were detected, four of which were spectroscopically identified as type Ia supernovæ. The search efficiency was determined with a Monte-Carlo simulation taking into account the efficiencies for both supernova detection and host galaxy identification. Assuming that for a given galaxy the supernova rate is proportional to the galactic luminosity, we compute a type Ia supernova explosion rate of: ${\cal R} = 0.44 {}_{-0.21}^{+0.35} {}_{-0.07}^{+0.13} h^2: / 10^{10} \lbsun / 100 {\rm yrs}$ at an average redshift of $\sim 0.1$ where the errors are respectively statistical and systematic (type misidentification included).

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arXiv2000

No diffuse H2 in the metal deficient galaxy I Zw 18

The metal deficient starburst galaxy IZw18 has been observed with FUSE in a search for H2 molecules. The spectrum obtained with an aperture covering the full galaxy shows no absorption lines of diffuse H2 at the radial velocity of the galaxy. The upper limit for the diffuse H2 column density is found to be very low: N(H2) <~ 10^15 cm-2 (10 sigma), unlike our Galaxy where H2 is generally present for even low HI column densities. Although the HI column density is here as high as N(HI) ~ 2.10^21 cm-2, we observe 2N(H2)/N(HI)<< 10^-6. We cannot exclude the possibility that some H2 could be in very dense, small and discrete clumps which cannot be detected with the present observation. However, the remarkable absence of diffuse H2 in this metal-poor galaxy can be explained by the low abundance of dust grains (needed to form this molecule from H-atoms), the high ultraviolet flux and the low density of the HI cloud surrounding the star-forming regions. Thus having eliminated diffuse H2 as a significant contributor to the total mass, it appears that the gas of the galaxy is dominated by HI, and that the high dynamical mass is not composed of cold and diffuse baryonic dark matter.

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arXiv2000

FUSE Observations of the HD Molecule toward HD 73882

The Lyman and Werner band systems of deuterated molecular hydrogen (HD) occur in the far UV range below 1200 A. The high sensitivity of the FUSE mission can give access, at moderate resolution, to hot stars shining through translucent clouds, in the hope of observing molecular cores in which deuterium is essentially in the form of HD. Thus, the measurement of the HD/H2 ratio may become a new powerful tool to evaluate the deuterium abundance, D/H, in the interstellar medium. We report here on the detection of HD toward the high extinction star HD 73882 [E(B-V)=0.72]. A preliminary analysis is presented.

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arXiv2000

FUSE Observations of Interstellar Gas Towards the LMC Star Sk -67 05

We report on measurements of interstellar O VI, H2, P II, Si II, Ar I, and Fe II absorption along the line of sight to Sk -67 05, a B0 Ia star in a diffuse H II region in the western edge of the Large Magellanic Cloud (LMC). We find log N(O VI) = 14.40 +/- 0.04 in the Milky Way (MW) component and, using the C IV column density from previous IUE observations, N(C IV) / N(O VI) = 1.00 +/- 0.16, a value similar to other halo measurements made with FUSE. In the LMC component, log N(O VI) = 13.89 +/- 0.05, and N(C IV) / N(O VI) < 0.4 (3 sigma), since only an upper limit on N(C IV) is available. Along this sightline the LMC is rich in molecular hydrogen, log N(H2) = 19.50 +/- 0.08; in the MW log N(H2) = 14.95 +/- 0.08. A two-component fit for the excitation temperature of the molecular gas in the LMC gives T_01 = 59 +/- 5 K for J=0,1 and T_ex = 800 +/- 330 K for J=3,4,5. For the MW, T_01 = 99 (+30/-20) K; no excitation temperature could be determined for the higher rotational states. The MW and LMC gas-phase [Fe/P] abundances are ~0.6 and ~0.7 dex lower, respectively, than solar system abundances. These values are similar to [Fe/Zn] measurements for the MW and LMC towards SN 1987A.

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arXiv2000

FUSE Observations of Molecular Hydrogen in Translucent Interstellar Clouds: The Line of Sight Toward HD 73882

We report the results of initial FUSE observations of molecular hydrogen (H2) in translucent clouds. These clouds have greater optical depth than any of the diffuse clouds previously observed for far-UV H2 absorption, and provide new insights into the physics and chemistry of such regions. Our initial results involve observations of HD 73882, a well-studied southern hemisphere star lying behind substantial interstellar material (E(B-V) = 0.72; A_V = 2.44). We find a total H2 column density, N(H2) = 1.2 x 10^{21} cm^{-2}, about three times larger than the values for diffuse clouds previously measured in the far-UV. The gas kinetic temperature indicated by the ratio N(J=1)/N(J=0) is 58 +/- 10 K. With the aid of ground-based data to calculate an appropriate multi-component curve of growth, we have determined column densities for all rotational levels up to J = 7. The J >= 2 states can be reasonably fitted with a rotational excitation temperature of 307 +/- 23 K. Both the kinetic and rotational temperatures are similar to those found in previous investigations of diffuse clouds. The ratios of carbonaceous molecules to hydrogen molecules are also similar to ratios in diffuse clouds, suggesting a similar chemistry for this line of sight.

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arXiv2000

The Ionization of the Local Interstellar Medium, as Revealed by FUSE Observations of N, O and Ar toward White Dwarf Stars

FUSE spectra of the white dwarf stars G191-B2B, GD 394, WD 2211-495 and WD 2331-475 cover the absorption features out of the ground electronic states of N I, N II, N III, O I and Ar I in the far ultraviolet, providing new insights on the origin of the partial ionization of the Local Interstellar Medium (LISM), and for the case of G191-B2B, the interstellar cloud that immediately surrounds the solar system. Toward these targets the interstellar abundances of Ar I, and sometimes N I, are significantly below their cosmic abundances relative to H I. In the diffuse interstellar medium, these elements are not likely to be depleted onto dust grains. Generally, we expect that Ar should be more strongly ionized than H (and also O and N whose ionizations are coupled to that of H via charge exchange reactions) because the cross section for the photoionization of Ar I is very high. Our finding that Ar I/H I is low may help to explain the surprisingly high ionization of He in the LISM found by other investigators. Our result favors the interpretation that the ionization of the local medium is maintained by a strong EUV flux from nearby stars and hot gases, rather than an incomplete recovery from a past, more highly ionized condition.

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arXiv2000

High Resolution HST-STIS Spectra of CI and CO in the Beta Pictoris Circumstellar Disk

High resolution FUV echelle spectra showing absorption features arising from CI and CO gas in the Beta Pictoris circumstellar (CS) disk were obtained on 1997 December 6 and 19 using the Space Telescope Imaging Spectrograph (STIS). An unsaturated spin-forbidden line of CI at 1613.376 A not previously seen in spectra of Beta Pictoris was detected, allowing for an improved determination of the column density of CI at zero velocity relative to the star (the stable component), N = (2-4) x 10^{16} cm^{-2}. Variable components with multiple velocities, which are the signatures of infalling bodies in the Beta Pictoris CS disk, are observed in the CI 1561 A and 1657 A multiplets. Also seen for the first time were two lines arising from the metastable singlet D level of carbon, at 1931 A and 1463 A The results of analysis of the CO A-X (0-0), (1-0), and (2-0) bands are presented, including the bands arising from {13}^CO, with much better precision than has previously been possible, due to the very high resolution provided by the STIS echelle gratings. Only stable CO gas is observed, with a column density N(CO) = (6.3 +/- 0.3) x 10^{14} cm{-2}. An unusual ratio of the column densities of {12}^CO to {13}^CO is found (R = 15 +/- 2). The large difference between the column densities of CI and CO indicates that photodissociation of CO is not the primary source of CI gas in the disk, contrary to previous suggestion.

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arXiv2000

Detection of deuterium Balmer lines in the Orion Nebula

The detection and first identification of the deuterium Balmer emission lines, D-alpha and D-beta, in the core of the Orion Nebula is reported. Observations were conducted at the 3.6m Canada-France-Hawaii Telescope, using the Echelle spectrograph Gecko. These lines are very narrow and have identical 11 km/s velocity shifts with respect to H-alpha and H-beta. They are probably excited by UV continuum fluorescence from the Lyman (DI) lines and arise from the interface between the HII region and the molecular cloud.

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arXiv1999

Ultraviolet observations of Sirius A and Sirius B with HST-GHRS; An interstellar cloud with a possible low deuterium abundance

We present new observations of the binary Sirius A / Sirius B performed with HST-GHRS. Two interstellar clouds are detected on this sightline, one of them being identified as the Local Interstellar Cloud (LIC). The interstellar structure of this sightline is constrained by high spectral resolution data of the species OI, NI, SiII, CII, FeII and MgII. Lyman alpha interstellar lines are also observed toward the two stars. But whereas the deuterium Lyman alpha line is well detected in the LIC with an abundance in agreement with that obtained by Linsky et al. (1993 & 1995), no significant DI line is detected in the other cloud. However, the Lyman alpha lines toward Sirius A and Sirius B are not trivial. An excess of absorption is seen in the blue wing of the Sirius A Lyman alpha line and interpreted as the wind from Sirius A. In its white dwarf companion, an excess in absorption is seen in the red wing and interpreted as the core of the Sirius B photospheric Lyman alpha line. A composite Lyman alpha profile can nonetheless be constructed, and allows one to measure the deuterium abundance in the second cloud 0 < D/H_ISM < 1.6 10^-5, which is marginally in agreement with the Linsky et al. (1993 & 1995) value. This sightline appears consequently as a good candidate for a low D/H_ISM.

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arXiv1999

Possible spectral lines from the gaseous Beta Pictoris disk

We present the results of exploratory HST observations to detect the emission lines of ions in the Beta Pic disk with the spectrograph slit placed off the star. Possible emission lines from FeII have been detected at $\sim 10^{-14}$erg cm-2 s-1 at 0.5 arcsec from Beta Pic, which would suggest a total FeII density of > 2.$10^{-2}$ cm-3 at 10 AU. This detection needs confirmation. If real, it requires a large production rate of gas and dust equivalent to the total disruption of ten bodies 30 kilometers in radius per year.

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arXiv1999

Deuterium abundances

We discuss the measurements of deuterium abundances in high redshift quasar absorbers, in the solar system and in the interstellar medium. We present new results that indicate spatial variations of the deuterium abundance in the interstellar medium at the level of 50% over scales possibly as small as 10 pc, and discuss plausible causes for the origin of these variations.

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arXiv1998

Photometric stellar variation due to extra-solar comets

We performed numerical simulations of stellar occultations by extra-solar cometary tails. We find that extra-solar comets can be detected by the apparent photometric variations of the central stars. In most cases, the light curve shows a very peculiar ``rounded triangular'' shape. However, in some other cases, the curve can mimic a planetary occultation. Photometric variations due to comet occultations are mainly achromatic. Nevertheless, if comets with small periastrons have smaller particles, these occultations could be chromatic with a larger extinction in the blue by few percents. We also estimate the number of detections expected in a large photometric survey at high accuracy. By the observation of several tens of thousand of stars, it should be possible to detect several hundreds of occultation per year. We thus conclude that a spatial photometric survey would detect a large number of extra-solar comets. This would allow to explore the time evolution of cometary activity, and consequently would probe structure and evolution of extra-solar planetary systems.

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arXiv1998

Dust distribution in radiation pressure outflow. Application to the BD+31d643 disk

The radial distribution of dust has been calculated in outflows driven by radiation pressure in which the gas drag is taken into account. It is shown that the dust distribution in the newly discovered candidate disk around BD+31d643 can be explained by this simple model, provided that the ambient gas of the surrounding cluster IC 348 has a density $ρ\sim 10^4~$cm$^{-3}$. The dust is probably produced within 2300 AU from the central binary star.

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arXiv1998

Detection of spatial variations in the (D/H) ratio in the local interstellar medium

We present high spectral resolution HST-GHRS observations of the white dwarf G191-B2B, and derive the interstellar D/H ratio on the line of sight. We have observed and analysed simultaneously the interstellar lines of HI, DI, NI, OI, SiII, and SiIII. We detect three absorbing clouds, and we measure an average (D/H) ratio: N(DI)_Total / N(HI)_Total = 1.12 +/- 0.08 x 10**-5, in disagreement with the value of the D/H ratio as measured toward Capella by Linsky et al. (1993, 1995). We re-analyze the GHRS data of the Capella line of sight, and confirm their estimate, as we find: (D/H)_Capella = 1.56 +/- 0.1 x 10**-5, in the Local Interstellar Cloud in which the Sun is embedded. This shows that the D/H ratio varies by at least 30% within the local interstellar medium (not to be confused with the Local Interstellar Cloud). Furthermore, the Local Interstellar Cloud is also detected toward our target G191-B2B, and we show that the D/H ratio in this component, toward G191-B2B, can be made compatible with that derived toward Capella. However, this comes at the expense of a much smaller value for the D/H ratio as averaged over the other two components, of order 0.9 x 10**-5, and in such a way that the D/H ratio, as averaged over all three components, remains at the above value, i.e. 1.12 x 10**-5. We thus conclude that, either the D/H ratio varies from cloud to cloud, and/or the D/H ratio varies within the Local Interstellar Cloud in which the Sun is embedded, although we note that our observations neither prove nor disprove this latter possibility.

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arXiv1998

Fe II emission lines in the UV spectrum of Sirius-A and Vega

We present high-quality HST/GHRS spectra in the Hydrogen L alpha spectral region of Vega and Sirius-A. Thanks to the signal-to-noise ratio achieved in these observations and to the similarity of the two spectra, we found clear evidence of emission features in the low flux region, 1190-1222 A. These emission lines can be attributed unambiguously to Fe II and Cr II transitions. In this spectral range, silicon lines are observed in absorption. We built a series of non-LTE model atmospheres with different, prescribed temperature stratification in the upper atmosphere and treating Fe II with various degrees of sophistication in non-LTE. Emission lines are produced by the combined effect of the Schuster mechanism and radiative interlocking, and can be explained without the presence of a chromosphere. Silicon absorption lines and the L alpha profile set constraints on the presence of a chromosphere, excluding a strong temperature rise in layers deeper than tau_R ~ 10^{-4}.

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