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Register a new userThe ionization rates of galactic nuclei and disks from Herschel/HIFI observations of water and its associated ions
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(Abridged) We present Herschel/HIFI spectra of the H2O 1113 GHz and H2O+ 1115 GHz lines toward five nearby prototypical starburst/AGN systems, and OH+ 971 GHz spectra toward three of these. The beam size of 20 corresponds to resolutions between 0.35 and 7 kpc. The observed line profiles range from pure absorption (NGC 4945, M82) to P-Cygni indicating outflow (NGC 253, Arp 220) and inverse P-Cygni indicating infall (Cen A). The similarity of the H2O, OH+, and H2O+ profiles to each other and to HI indicates that diffuse and dense gas phases are well mixed. We estimate column densities assuming negligible excitation (for absorption features) and using a non-LTE model (for emission features), adopting calculated collision data for H2O and OH+, and rough estimates for H2O+. Column densities range from ~10^13 to ~10^15 cm^-2 for each species, and are similar between absorption and emission components, indicating that the nuclear region does not contribute much to the emission in these ground-state lines. The N(H2O)/N(H2O+) ratios of 1.4-5.6 indicate an origin of the lines in diffuse gas, and the N(OH+)/N(H2O+) ratios of 1.6-3.1 indicate a low H2 fraction (~11%) in the gas. Adopting recent Galactic values for the average gas density and the ionization efficiency, we find ionization rates for our sample galaxies of ~3x10^-16 s^-1 which are similar to the value for the Galactic disk, but ~10x below that of the Galactic Center and ~100x below estimates for AGN from excited-state H3O+ lines. We conclude that the ground-state lines of water and its associated ions probe primarily non-nuclear gas in the disks of these centrally active galaxies. Our data thus provide evidence for a decrease in ionization rate by a factor of ~10 from the nuclei to the disks of galaxies, as found before for the Milky Way.
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We present Herschel/HIFI observations of nine transitions of hho and hheo towards six high-mass star-forming regions, obtained as part of the PRISMAS Key Program. Water vapor in translucent clouds is detected in absorption along every sightline. We derive the column density of hho or hheo for the lower energy level of each transition observed. The total water column density is about a few $10^{13} rm{cm^{-2}}$. We find that the abundance of water relative to hydrogen nuclei is $1times10^{-8}$ in agreement with models for oxygen chemistry with high cosmic ray ionization rates. Relative to hh, the abundance of water is remarkably constant at $5times10^{-8}$. The abundance of water in excited levels is at most 15%, implying that the excitation temperature $T_{ex}$ in the ground state transitions is below 10 K. The column densities derived from the two ortho ground state transitions indicates that $T_{ex}simeq5$ K and that the density $n($hh$)$ in the clouds is $le10^4 rm{cm^{-3}}$. For most clouds we derive a water ortho-to-para ratio consistent with the value of 3 expected in thermodynamic equilibrium in the high temperature limit. Two clouds with large column densities exhibit a ratio significantly below 3. This may argue that the history of water molecules includes a cold phase, either when the molecules were formed on cold grains, or when they later become at least partially thermalized with the cold gas ($sim25$ K) in the shielded, low temperature regions of the clouds; evidently, they have not yet fully thermalized with the warmer ($sim50$ K) translucent portions of the clouds.
We model the production of OH+, H2O+, and H3O+ in interstellar clouds, using a steady state photodissociation region code that treats the freeze-out of gas species, grain surface chemistry, and desorption of ices from grains. The code includes PAHs, which have important effects on the chemistry. All three ions generally have two peaks in abundance as a function of depth into the cloud, one at A_V<~1 and one at A_V~3-8, the exact values depending on the ratio of incident ultraviolet flux to gas density. For relatively low values of the incident far ultraviolet flux on the cloud ({chi}<~ 1000; {chi}= 1= local interstellar value), the columns of OH+ and H2O+ scale roughly as the cosmic ray primary ionization rate {zeta}(crp) divided by the hydrogen nucleus density n. The H3O+ column is dominated by the second peak, and we show that if PAHs are present, N(H3O+) ~ 4x10^{13} cm^{-2} independent of {zeta}(crp) or n. If there are no PAHs or very small grains at the second peak, N(H3O+) can attain such columns only if low ionization potential metals are heavily depleted. We also model diffuse and translucent clouds in the interstellar medium, and show how observations of N(OH+)/N(H) and N(OH+)/N(H2O+) can be used to estimate {zeta}(crp)/n, {chi}/n and A_V in them. We compare our models to Herschel observations of these two ions, and estimate {zeta}(crp) ~ 4-6 x 10^-16 (n/100 cm^-3) s^-1 and chi/n = 0.03 cm^3 for diffuse foreground clouds towards W49N.
We derive the dense core structure and the water abundance in four massive star-forming regions which may help understand the earliest stages of massive star formation. We present Herschel-HIFI observations of the para-H2O 1_11-0_00 and 2_02-1_11 and the para-H2-18O 1_11-0_00 transitions. The envelope contribution to the line profiles is separated from contributions by outflows and foreground clouds. The envelope contribution is modelled using Monte-Carlo radiative transfer codes for dust and molecular lines (MC3D and RATRAN), with the water abundance and the turbulent velocity width as free parameters. While the outflows are mostly seen in emission in high-J lines, envelopes are seen in absorption in ground-state lines, which are almost saturated. The derived water abundances range from 5E-10 to 4E-8 in the outer envelopes. We detect cold clouds surrounding the protostar envelope, thanks to the very high quality of the Herschel-HIFI data and the unique ability of water to probe them. Several foreground clouds are also detected along the line of sight. The low H2O abundances in massive dense cores are in accordance with the expectation that high densities and low temperatures lead to freeze-out of water on dust grains. The spread in abundance values is not clearly linked to physical properties of the sources.
(Abridged) We use HIFI maps of the 987 GHz H2O 2(02)-1(11) emission to measure the sizes and shapes of 19 high-mass protostellar envelopes. To identify infall, we use HIFI spectra of the optically thin C18O 9-8 and H2O-18 1(11)-0(00) lines. The high-J C18O line traces the warm central material and redshifted H2O-18 1(11)-0(00) absorption indicates material falling onto the warm core. We probe small-scale chemical differentiation by comparing H2O 752 and 987 GHz spectra with those of H2O-18. Our measured radii of the central part of the H2O 2(02)-1(11) emission are 30-40% larger than the predictions from spherical envelope models, and axis ratios are <2, which we consider good agreement. For 11 of the 19 sources, we find a significant redshift of the H2O-18 1(11)-0(00) line relative to C18O 9-8. The inferred infall velocities are 0.6-3.2 km/s, and estimated mass inflow rates range from 7e-5 to 2e-2 M0/yr, with the highest mass inflow rates occurring toward the sources with the highest masses, and possibly the youngest ages. The other sources show either expanding motions or H2O-18 lines in emission. The H2O-18 1(11)-0(00) line profiles are remarkably similar to the differences between the H2O 2(02)-1(11) and 2(11)-2(02) profiles, suggesting that the H2O-18 line and the H2O 2(02)-1(11) absorption originate just inside the radius where water evaporates from grains, typically 1000-5000 au from the center. In some sources, the H2O-18 line is detectable in the outflow, where no C18O emission is seen. Together, the H2O-18 absorption and C18O emission profiles show that the water abundance around high-mass protostars has at least three levels: low in the cool outer envelope, high within the 100 K radius, and very high in the outflowing gas. Thus, despite the small regions, the combination of lines presented here reveals systematic inflows and chemical information about the outflows.
We report the discovery of water vapour toward the carbon star V Cygni. We have used Herschels HIFI instrument, in dual beam switch mode, to observe the 1(11) - 0(00) para-water transition at 1113.3430 GHz in the upper sideband of the Band 4b receiver. The observed spectral line profile is nearly parabolic, but with a slight asymmetry associated with blueshifted absorption, and the integrated antenna temperature is 1.69 pm 0.17 K km/s. This detection of thermal water vapour emission, carried out as part of a small survey of water in carbon-rich stars, is only the second such detection toward a carbon-rich AGB star, the first having been obtained by the Submillimeter Wave Astronomy Satellite toward IRC+10216. For an assumed ortho-to-para ratio of 3 for water, the observed line intensity implies a water outflow rate ~ (3 - 6) E-5 Earth masses per year and a water abundance relative to H2 of ~ (2-5) E-6. This value is a factor of at least 1E+4 larger than the expected photospheric abundance in a carbon-rich environment, and - as in IRC+10216 - raises the intriguing possibility that the observed water is produced by the vapourisation of orbiting comets or dwarf planets. However, observations of the single line observed to date do not permit us to place strong constraints upon the spatial distribution or origin of the observed water, but future observations of additional transitions will allow us to determine the inner radius of the H2O-emitting zone, and the H2O ortho-to-para ratio, and thereby to place important constraints upon the origin of the observed water emission.
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