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Register a new userORFEUS II echelle spectra: deuterium and molecular hydrogen in the ISM towards BD +39 3226
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Authors
H. Bluhm
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In ORFEUS II spectra of the sdO star BD +39 3226 interstellar hydrogen and deuterium is detected. From Ly alpha profile fitting and a curve of growth analysis of the Lyman series of H I and D I we derive the column densities N(H)=1.20(+0.28/-0.22)*10^20 cm^(-2) and N(D)=1.45(+0.50/-0.38)*10^(15) cm^(-2). From the analysis of metal absorption lines in ORFEUS and IUE spectra we obtain column densities for 11 elements. In addition, we examine absorption lines of H_2 for rotational excitation states up to J=7. We find an H_2 ortho-to-para ratio of 2.5, the fractional abundance of molecular hydrogen has a low value of log f=-4.08 for a total amount of N(H_2)=4.8(+2.0/-1.6)*10^15 cm^(-2). The column densities of the excitation states reveal a moderate Boltzmann excitation temperature of 130 K and an equivalent excitation temperature for the excited upper states due to UV pumping of <1800 K.
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In front of the LMC molecular hydrogen is found in absorption near 0 km/s, being local disk gas, near +60 km/s in an intermediate velocity cloud, and near +120 km/s, in a high velocity halo cloud. The nature of the gas is discussed based on four ORFEUS far UV spectra of LMC stars and including data from the ground and from the IUE satellite. The local gas is cool and, given a span of sight lines of only 2.5 deg, rather fluffy. The fractional abundance of H_2 varies from log(f)=-5.4 to -3.3. Metal depletions (up to -1.7 dex for Fe) are typical for galactic disk gas. In the IV and HV gas an apparent underabundance of neutral oxygen points to an ionization level of the gas of about 90%. H_2 is detected in IV and HV gas toward HD 269546. In the IV gas we find an H_2 column density of log(N)simeq15.6. The H_2 excitation indicates that the line of sight samples a cloud at a temperature below 150 K. Column densities are too small to detect the higher UV pumped excitation levels. The high velocity H_2 (log(N)simeq15.6) is highly excited and probably exposed to a strong radiation field. Its excitation temperature exceeds 1000 K. Due to the radial velocity difference between the halo gas and the Milky Way disk, the unattenuated disk radiation is available for H_2 excitation in the halo. We do not find evidence for an intergalactic origin of this gas; a galactic as well as a Magellanic Cloud origin is possible.
During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spectrometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane instruments of the ORFEUS telescope. We present the in-flight performance and the principles of the data reduction for this instrument. The wavelength range is 90 nm to 140 nm, the spectral resolution is significantly better than lambda/(Delta lambda) = 10000, where Delta lambda is measured as FWHM of the instrumental profile. The effective area peaks at 1.3 cm^2 near 110 nm. The background is dominated by straylight from the Echelle grating and is about 15% in an extracted spectrum for spectra with a rather flat continuum. The internal accuracy of the wavelength calibration is better than +/- 0.005 nm.
We present a measurement of the D/H abundance ratio in a metal-poor damped Lyman alpha (DLA) system along the sightline of QSO SDSS1558-0031. The DLA system is at redshift z = 2.70262, has a neutral column density of log(NHI)=20.67+/-0.05 cm^2, and a gas-phase metallicity [O/H]= -1.49 which indicates that deuterium astration is negligible. Deuterium absorption is observed in multiple Lyman series with a column density of log(NDI)=16.19+/-0.04 cm^2, best constrained by the deuterium Lyman-11 line. We measure log(D/H) = -4.48+/-0.06, which when combined with previous measurements along QSO sightlines gives a best estimate of log(D/H) = -4.55+/-0.04, where the 1-sigma error estimate comes from a jackknife analysis of the weighted means. Using the framework of standard big bang nucleosynthesis, this value of D/H translates into a baryon density of Omega_b h^2 = 0.0213 +/- 0.0013 +/- 0.0004 where the error terms represent the 1-sigma errors from D/H and the uncertainties in the nuclear reaction rates respectively. Combining our new measurement with previous measurements of D/H, we no longer find compelling evidence for a trend of D/H with NHI.
We study the effect of temperature up to 1000K on the structure of dense molecular para-hydrogen and ortho-deuterium, using the path-integral Monte Carlo method. We find a structural phase transition from orientationally disordered hexagonal close packed (hcp) to an orthorhombic structure of Cmca symmetry before melting. The transition is basically induced by thermal fluctuations, but quantum fluctuations of protons (deuterons) are important in determining the transition temperature through effectively hardening the intermolecular interaction. We estimate the phase line between hcp and Cmca phases as well as the melting line of the Cmca solid.
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