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Register a new userRocket and FUSE Observations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen
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We present far-ultraviolet spectroscopy of the emission/reflection nebula IC 405 obtained by a rocket-borne long-slit spectrograph and the Far Ultraviolet Spectroscopic Explorer. Both data sets show a rise in the ratio of the nebular surface brightness to stellar flux (S/F_*) of approximately two orders of magnitude towards the blue end of the far-UV bandpass. Scattering models using simple dust geometries fail to reproduce the observed S/F_* for realistic grain properties. The high spectral resolution of the FUSE data reveals a rich fluorescent molecular hydrogen spectrum ~1000 north of the star that is clearly distinguished from the steady blue continuum. The S/F_* remains roughly constant at all nebular pointings, showing that fluorescent molecular hydrogen is not the dominant cause for the blue rise. We discuss three possible mechanisms for the ``Blue Dust: differential extinction of the dominant star (HD 34078), unusual dust grain properties, and emission from nebular dust. We conclude that uncertainties in the nebular geometry and the degree of dust clumping are most likely responsible for the blue rise. As an interesting consequence of this result, we consider how IC 405 would appear in a spatially unresolved observation. If IC 405 was observed with a spatial resolution of less than 0.4 pc, for example, an observer would infer a far-UV flux that was 2.5 times the true value, giving the appearance of a stellar continuum that was less extinguished than radiation from the surrounding nebula, an effect that is reminiscent of the observed ultraviolet properties of starburst galaxies.
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We present far-ultraviolet observations of IC 63, an emission/reflection nebula illuminated by the B0.5IV star gamma Cassiopeia, located 1.3 pc from the nebula. Molecular hydrogen fluorescence was detected first in IC 63 by IUE and later at shorter wavelengths by ORFEUS. Here we present Far Ultraviolet Spectroscopic Explorer (FUSE) observations towards three locations in the nebula, complemented by Hopkins Ultraviolet Telescope (HUT) data on the central nebular position. In addition, we present a sounding rocket calibration of a FUSE spectrum of gamma Cas. Molecular hydrogen fluorescence is detected in all three FUSE pointings. The intensity of this emission as well as the contributions from other species are seen to vary with position. The absolute flux calibration of the sounding rocket data allows us to reliably predict the radiation field incident on IC 63. We use these data to test models of the fluorescent process. Our modeling resolves the perceived discrepancy between the existing ultraviolet observations and achieves a satisfactory agreement with the H_2 rotational structure observed with FUSE.
We describe a moderate-resolution FUSE mini-survey of H2 in the Milky Way and Magellanic Clouds, using four hot stars and four AGN as background sources. FUSE spectra of nearly every stellar and extragalactic source exhibit numerous absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. One extragalactic sightline, PKS 2155-304, with low N(HI) shows no detectable H2 and could be the Lockman Hole of molecular gas, of importance for QSO absorption-line studies. We measure H2 column densities in low rotational states (J = 0 and 1) to derive rotational and/or kinetic temperatures of diffuse interstellar gas. The higher-J abundances can constrain models of the UV radiation fields and gas densities. In three optically thick clouds toward extragalactic sources, we find n(H) ~ 30-50 cm(-3) and cloud thicknesses of 2-3 pc. The rotational temperatures for H2 at high Galactic latitude, <T_01> = 107 +/- 17 K (seven sightlines) and 120 +/- 13 K (three optically thick clouds), are higher than those in the Copernicus sample composed primarily of targets in the disk. We find no evidence for great differences in the abundance or state of excitation of H2 between sight lines in the Galaxy and those in the SMC and LMC. In the future, we will probe the distribution and physical parameters of diffuse molecular gas in the disk and halo and in the lower-metallicity environs of the LMC and SMC.
We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of HD141569A, a transitional object known to possess a circumstellar disk. We observe two components of gas at widely different temperatures along the line of sight. We detect cold H2, which is thermalized up to J=2 at a kinetic temperature of 51K. Such low temperatures are typical of the diffuse interstellar medium. Since the line of sight to HD141569A does not pass through its disk, it appears that we are observing the cold H2 in a low extinction envelope associated with the high Galactic latitude dark cloud complex L134N, which is in the same direction and at nearly the same distance as HD141569A. The column densities of the higher J-levels of H2 suggest the presence of warm gas along the line of sight. The excitation conditions do not seem to be consistent with what is generally observed in diffuse interstellar clouds. The observed radial velocity of the gas implies that the UV spectral lines we observe are likely interstellar in origin rather than circumstellar, although our absorption line study does not definitely rule out the possibility that the warm gas is close to the star. The discovery of such warm gas along the line of sight may provide evidence for turbulent phenomena in the dark cloud L134N.
We report results from a FUSE survey of interstellar molecular hydrogen (H2) along 45 sight lines to AGN at high Galactic latitudes (|b| > 20 degrees). Most (39 of 45) of the sight lines show detectable Galactic H2 absorption from Lyman and Werner bands between 1000 and 1126 A, with column densities ranging from N(H2) = 10^(14.17-19.82) cm^-2. In the northern Galactic hemisphere, we identify many regions of low column, N(H2) < 10^15 cm^-2, between longitude l = 60-180 degrees and at b > 54 degrees. These `H2 holes provide valuable, uncontaminated sight lines for extragalactic UV spectroscopy, and a few may be related to the Northern Chimney (low Na I absorption) and Lockman Hole with low N(HI). A comparison of high-latitude H2 with 139 OB-star sight lines surveyed in the Galactic disk suggests that high-latitude and disk H2 clouds may have different rates of heating, cooling, and UV excitation. For rotational states J = 0 and 1, the mean excitation temperature at high latitude, <T_01(high)> = 124 +/- 8 K, is somewhat above that in the Galactic disk, <T_01(disk)> = 86 +/- 20 K. For J = 2-4, the <T_exc> = 498 +/- 28 K, and the column-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate a comparable degree of UV excitation in the disk and low halo for sight lines with N(H2) > 10^18. The distribution of molecular fractions at high latitude shows a transition at lower total hydrogen column density, log N_H = 20.38 +/- 0.13, than in the Galactic disk, log N_H(disk) = 20.7. If the FUV radiation fields are similar in disk and low halo, this suggests an enhanced (dust-catalyzed) H2 formation rate in higher-density, compressed clouds, which could be detectable as high-latitude, sheetlike infrared cirrus.
We present the far-ultraviolet (FUV) fluorescent molecular hydrogen (H_2) emission map of the Milky Way Galaxy obtained with FIMS/SPEAR covering ~76% of the sky. The extinction-corrected intensity of the fluorescent H_2 emission has a strong linear correlation with the well-known tracers of the cold interstellar medium (ISM), including color excess E(B-V), neutral hydrogen column density N(H I), and H_alpha emission. The all-sky H_2 column density map was also obtained using a simple photodissociation region model and interstellar radiation fields derived from UV star catalogs. We estimated the fraction of H2 (f_H2) and the gas-to-dust ratio (GDR) of the diffuse ISM. The f_H2 gradually increases from <1% at optically thin regions where E(B-V) < 0.1 to ~50% for E(B-V) = 3. The estimated GDR is ~5.1 x 10^21 atoms cm^-2 mag^-1, in agreement with the standard value of 5.8 x 10^21 atoms cm^-2 mag^-1.
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