<i>ORFEUS II</i>Echelle Observations of Molecular Hydrogen in the Galactic Halo

Ryu, Kyoung‐Seok; Dixon, W. V.; Hurwitz, M.; Seon, Kwang‐Il; Min, Kyoung Wook; Edelstein, Jerry

doi:10.1086/308256

Cited by 8 publications

(3 citation statements)

References 31 publications

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“…Several of these stars are far enough away (i.e., more than 255 pc since SN 1998A has a Galactic latitude of 34 • ) to fully probe the material in the Galactic plane. All 8 of the MW stars have a similar polarization angle (49 • < θ < 89 • ), which suggests fairly ordered but patchy dust in this direction (see Ryu et al 2000 for a detailed study of the patchy extinction in this region).…”

Section: Sn 1998amentioning

confidence: 92%

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Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi

Leonard¹,

Filippenko²

2001

PUBL ASTRON SOC PAC

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We present single-epoch spectropolarimetry of the Type II supernovae (SNe II) 1997ds, 1998A, and 1999gi. SN 1997ds and SN 1998A were both observed during the early photospheric phase, while spectropolarimetry of SN 1999gi was obtained near the start of the transition to the nebular phase. Uncorrected for interstellar polarization (ISP), SN 1997ds is characterized by p_V = 0.85 +/- 0.02%, SN 1998A has p_V = 0.24 +/- 0.05%, and SN 1999gi is polarized at p_V = 5.72 +/- 0.01%. The low continuum polarization inferred for SN 1997ds and SN 1998A and the amplitude of polarization modulations across strong line features are consistent with those measured at similar epochs for SN 1987A and the Type II-plateau SN 1999em, and supports the growing consensus that core-collapse events with hydrogen envelopes substantially intact at the time of explosion are not significantly aspherical during the early photospheric phase. The spectral shape of the high continuum polarization of SN 1999gi closely resembles a ``Serkowski'' ISP curve, and is inconsistent with the wavelength-independent nature of electron scattering expected for an aspherical SN atmosphere. Since Galactic reddening is minimal along this line-of-sight, the majority of the observed polarization in SN 1999gi is believed to be due to ISP of the host galaxy, although significant (up to p = 2%) intrinsic polarization cannot be ruled out. The potential power of SN spectropolarimetry to study the properties of interstellar dust in external galaxies is described and applied to the SN 1999gi data, where it is shown that if the polarization is indeed predominantly interstellar in origin, then R_V = 3.0 +/- 0.2 for the dust along this line-of-sight in NGC 3184.Comment: 34 pages (13 Figures, 2 Tables). Accepted by PAS

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Section: Sn 1998amentioning

confidence: 92%

“…v ! 89Њ patchy dust in this direction (see Ryu et al 2000 for a detailed study of the patchy extinction in this region).…”

Section: Havingmentioning

confidence: 99%

Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi

Leonard¹,

Filippenko²

2001

PUBL ASTRON SOC PAC

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“…The basic volume limitation on studies of H 2 persisted until the advent of new instruments. The more sensitive but short-lived instruments, HUT (Gunderson, Clayton, & Green 1998) and ORFEUS (Dixon, Hurwitz, & Bowyer 1998;Ryu et al 2000;Richter 2000), have extended the range of H 2 studies to selected distant sources in the Galaxy and the Magellanic Clouds, and the IMAPS spectrograph (Jenkins & Peimbert 1997;Jenkins et al 2000) obtained high-resolution H 2 spectra of bright targets in diverse environments.…”

Section: Introductionmentioning

confidence: 99%

AFar Ultraviolet Spectroscopic ExplorerSurvey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

Tumlinson

Shull

Rachford

et al. 2002

ApJ

212

320

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We describe a moderate-resolution FUSE survey of H 2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H 2 Lyman and Werner bands between 912 and 1120 Å. Our survey is sensitive to N(H 2 ) ≥ 10 14 cm −2 ; the highest column densities are log N(H 2 ) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H 2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions f H2 = 0.010 +0.005 −0.002 for the SMC and f H2 = 0.012 +0.006 −0.003 for the LMC, compared with f H2 = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lyα in pencil-beam sight lines as measures of N(H I). These results imply that the diffuse H 2 masses of the LMC and SMC are 8 × 10 6 M ⊙ and 2 × 10 6 M ⊙ , respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H 2 formation rate coefficient, R ∼ 3 × 10 −18 cm 3 s −1 , and incident radiation fields ranging from 10 -100 times the Galactic mean value. We find that these high-radiation, low-formation-rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H 2 column densities in low rotational states (J = 0 and 1) to derive kinetic and/or rotational temperatures of diffuse interstellar gas, and find that the distribution of rotational temperatures is similar to Galactic gas, with T 01 = 82 ± 21 K for clouds with N(H 2 ) ≥ 10 16.5 cm −2 . There is only a weak correlation between detected H 2 and far-infrared fluxes as determined by IRAS, perhaps due to differences in the survey techniques. We find that the surface density of H 2 probed by our pencil-beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps. We discuss the implications of this work for theories of star formation in low-metallicity environments.

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3D chemical structure of diffuse turbulent ISM

Bellomi

Godard²,

Hennebelle³

et al. 2020

A&A

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Context. The amount of data collected by spectrometers from radio to ultraviolet (UV) wavelengths opens a new era where the statistical and chemical information contained in the observations can be used concomitantly to investigate the thermodynamical state and the evolution of the interstellar medium (ISM). Aims. In this paper, we study the statistical properties of the HI-to-H2 transition observed in absorption in the local diffuse and multiphase ISM. Our goal is to identify the physical processes that control the probability of occurrence of any line of sight and the origins of the variations of the integrated molecular fraction from one line of sight to another. Methods. The turbulent diffuse ISM is modeled using the RAMSES code, which includes detailed treatments of the magnetohydrodynamics, the thermal evolution of the gas, and the chemistry of H2. The impacts of the UV radiation field, the mean density, the turbulent forcing, the integral scale, the magnetic field, and the gravity on the molecular content of the gas are explored through a parametric study that covers a wide range of physical conditions. The statistics of the HI-to-H2 transition are interpreted through analytical prescriptions and compared with the observations using a modified and robust version of the Kolmogorov-Smirnov test. Results. The analysis of the observed background sources shows that the lengths of the lines of sight follow a flat distribution in logarithmic scale from ~100 pc to ~3 kpc. Without taking into account any variation of the parameters along a line of sight or from one line of sight to another, the results of one simulation, convolved with the distribution of distances of the observational sample, are able to simultaneously explain the position, the width, the dispersion, and most of the statistical properties of the HI-to-H2 transition observed in the local ISM. The tightest agreement is obtained for a neutral diffuse gas modeled over ~200 pc, with a mean density n̅H̅ = 1−2 cm−3, illuminated by the standard interstellar UV radiation field, and stirred up by a large-scale compressive turbulent forcing. Within this configuration, the 2D probability histogram of the column densities of H and H2, poetically called the kingfisher diagram, is remarkably stable and is almost unaltered by gravity, the strength of the turbulent forcing, the resolution of the simulation, or the strength of the magnetic field Bx, as long as Bx < 4 μG. The weak effect of the resolution and our analytical prescription suggest that the column densities of HI are likely built up in large-scale warm neutral medium and cold neutral medium (CNM) structures correlated in density over ~20 pc and ~10 pc, respectively, while those of H2 are built up in CNM structures between ~3 and ~10 pc. Conclusions. Combining the chemical and statistical information contained in the observations of HI and H2 sheds new light on the study of the diffuse matter. Applying this new tool to several atomic and molecular species is a promising perspective to understanding the effects of turbulence, magnetic field, thermal instability, and gravity on the formation and evolution of molecular clouds.

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ORFEUS IIEchelle Observations of Molecular Hydrogen in the Galactic Halo

Cited by 8 publications

References 31 publications

Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi

Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi

AFar Ultraviolet Spectroscopic ExplorerSurvey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

3D chemical structure of diffuse turbulent ISM

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