123 research outputs found
Interstellar H2 toward HD 37903
We present an analysis of interstellar H2 toward HD 37903, which is a hot, B
1.5 V star located in the NGC 2023 reflection nebula. Meyer et al. (2001) have
used a rich spectrum of vibrationally excited H2 observed by the HST to
calculate a model of the interstellar cloud toward HD 37903. We extend Mayer's
analysis by including the v"=0 vibrational level observed by the FUSE
satellite.
The T01 temperature should not be interpreted as a rotational temperature,
but rather as a temperature of thermal equilibrium between the ortho and para
H2. The ortho to para H2 ratio is lower for collisionally populated levels than
for the levels populated by fluorescence.
The PDR model of the cloud located in front of HD 37903 points to a gas
temperature Tkin=110-377 K, hydrogen density nH=1874-544 cm^-3 and the
star-cloud distance of 0.45 pc
Turbulent Flow through Idealized Emergent Vegetation
This paper presents results of several large-eddy simulations (LES) of turbulent flow in an open channel through staggered arrays of rigid, emergent cylinders, which can be regarded as idealized vegetation. In this study, two cylinder Reynolds numbers, RD=1,340 and RD=500, and three vegetation densities are considered. The LES of the lowest density and at RD=1,340 corresponds to a recently completed laboratory experiment, the data of which is used to validate the simulations. Fairly good agreement between calculated and measured first- and second-order statistics along measurement profiles is found, confirming the accuracy of the simulations. The high resolution of the simulations enables an explicit calculation of drag forces, decomposed into pressure and friction drag, that are exerted on the cylinders. The effect of the cylinder Reynolds number and the cylinder density on the drag and hence on the flow resistance is quantified and in agreement with previous experimental studies. Turbulence structures are visualized through instantaneous pressure fluctuations, isosurfaces of the Q-criterion and contours of vertical vorticity in horizontal planes. Analysis of velocity time signals and distributions of drag and lift forces over time reveals that flow and turbulence are more influenced by the vegetation density than by the cylinder Reynolds number
The End of the Lines for OX 169: No Binary Broad-Line Region
We show that unusual Balmer emission line profiles of the quasar OX 169,
frequently described as either self-absorbed or double peaked, are actually
neither. The effect is an illusion resulting from two coincidences. First, the
forbidden lines are quite strong and broad. Consequently, the [N II]6583 line
and the associated narrow-line component of H-alpha present the appearance of
twin H-alpha peaks. Second, the redshift of 0.2110 brings H-beta into
coincidence with Na I D at zero redshift, and ISM absorption in Na I D divides
the H-beta emission line. In spectra obtained over the past decade, we see no
substantial change in the character of the line profiles, and no indication of
intrinsic double-peaked structure. The H-gamma, Mg II, and Ly-alpha emission
lines are single peaked, and all of the emission-line redshifts are consistent
once they are correctly attributed to their permitted and forbidden-line
identifications. A systematic shift of up to 700 km/s between broad and narrow
lines is seen, but such differences are common, and could be due to
gravitational and transverse redshift in a low-inclination disk. Stockton &
Farnham (1991) had called attention to an apparent tidal tail in the host
galaxy of OX 169, and speculated that a recent merger had supplied the nucleus
with a coalescing pair of black holes which was now revealing its existence in
the form of two physically distinct broad-line regions. Although there is no
longer any evidence for two broad emission-line regions in OX 169, binary black
holes should form frequently in galaxy mergers, and it is still worthwhile to
monitor the radial velocities of emission lines which could supply evidence of
their existence in certain objects.Comment: 19 pages, 5 figures, accepted for publication in Ap.
Densities and filling factors of the DIG in the Solar neighbourhood
For the first time we have combined dispersion measures and emission measures
towards 38 pulsars at KNOWN distances from which we derived the mean electron
density in clouds, N_c, and their volume filling factor, F_v, averaged along
the line of sight. The emission measures were corrected for absorption by dust
and contributions from beyond the pulsar distance. Results: The scale height of
the electron layer for our sample is 0.93+/-0.13 kpc and the midplane electron
density is 0.023+/-0.004 cm^-3, in agreement with earlier results. The average
density along the line of sight is = 0.018+/-0.002 cm^-3 and nearly
constant. Since = F_v N_c, an inverse relationship between F_v and N_c is
expected. We find F_v(N_c) = (0.011+/-0.003) N_c^{-1.20+/-0.13}, which holds
for the ranges N_c = 0.05-1 cm^-3 and F_v = 0.4-0.01. Near the Galactic plane
the dependence of F_v on N_c is significantly stronger than away from the
plane. F_v does not systematically change along or perpendicular to the
Galactic plane, but the spread about the mean value of 0.08+/-0.02 is
considerable. Conclusions: The inverse F_v-N_c relation is consistent with a
hierarchical, fractal density distribution in the diffuse ionized gas (DIG)
caused by turbulence. The observed near constancy of then is a signature
of fractal structure in the ionized medium, which is most pronounced outside
the thin disk.Comment: 9 pages, 9 figures. Accepted for publication in A&
The UV Properties of the Narrow Line Quasar I Zwicky 1
I Zw 1 is the prototype narrow line quasar. We report here the results of our
study of the UV emission of I Zw 1 using a high S/N (50-120) spectrum obtained
with the HST FOS. The following main new results are obtained: 1. The Mg II and
Al III doublets are partially/fully resolved. The measured doublet ratios
verify theoretical predictions that the lines are thermalized in the BLR. 2. A
weak associated UV absorption system is detected in N~V, and possibly also in C
IV and Lya, suggesting an outflow with a velocity of 1870 km/s and velocity
dispersion <300 km/s. 3. Lines from ions of increasing ionization level show
increasing excess blue wing flux, and an increasing line peak velocity shift,
reaching a maximum blueshift of about 2000 km/s for He II 1640. This may
indicate an out-flowing component in the BLR, where the ionization level
increases with velocity, and which is visible only in the approaching
direction. The highest velocity part of this outflow may produce the associated
UV absorption system. 4. The small C III] 1909 EW, and the small C III]
1909/Lya and C III] 1909/Si III] 1892 flux ratios indicate a typical BLR
density of 10^11, i.e. about an order of magnitude larger than implied by C
III] 1909 in most quasars. A BLR component of a higher density is implied by
the EW and doublet ratio of the Al III 1857 doublet. 5. Prominent Fe II UV 191
emission is seen, together with weaker line emission at 1294 and 1871 A. These
three features have been proposed as evidence for significant Lya pumping of
the 8-10 eV levels of Fe II. 6. Significant Fe III emission is present. The Fe
III UV 34 and UV 48 multiplets are clearly resolved, and Fe III UV 1, UV 47, UV
50, and UV 68 may also be present. (Shortened version)Comment: 28 pages, 1 table and 7 figures included. Uses aas2pp4.sty. Scheduled
for the Astrophysical Journal November 10, 1997 issue, Vol. 48
Exploring the dust content of SDSS DR7 damped Lyman alpha systems at 2.155.2
We have studied a sample of 1084 intervening absorption systems with 2.155.2, having log(N) 20.0 in the spectra of QSOs in
Sloan Digital Sky Survey (SDSS) data release 7 (DR7), with the aim of
understanding the nature and abundance of the dust and the chemical abundances
in the DLA absorbers. Composite spectra were constructed for the full sample
and several subsamples, chosen on the basis of absorber and QSO properties.
Average extinction curves were obtained for the samples by comparing their
geometric mean composite spectra with those of two samples of QSOs, matching in
z and i magnitude with the DLA sample, one sample without any absorbers
along their lines of sight and the other without any DLAs along their lines of
sight irrespective of the presence of other absorption systems. While the
average reddening in the DLA sample is small, we find definite evidence for the
presence of dust in subsamples based on absorber properties, in particular the
strength of metal absorption lines. DLAs along lines of sight to QSOs which are
not colour selected are found to be more dusty compared to those along the
lines of sight to the more numerous colour selected QSOs. From these studies
and from the strengths of absorption lines in the composite spectra, we
conclude that 10% of the DLAs in SDSS DR7 cause significant reddening,
have stronger absorption lines and have higher abundances as compared to the
rest of the sample. The rest of the sample shows little reddening. Due to the
dominant color selection method used to target QSOs in the SDSS DR7, this
fraction of 10% likely represents a lower limit for the global fraction of
dusty DLAs at high-z.Comment: 12 pages, 9 figures. To appear in MNRA
Old stellar Galactic disc in near-plane regions according to 2MASS: scales, cut-off, flare and warp
We have pursued two different methods to analyze the old stellar population
near the Galactic plane, using data from the 2MASS survey. The first method is
based on the isolation of the red clump giant population in the color-magnitude
diagrams and the inversion of its star counts to obtain directly the density
distribution along the line of sight. The second method fits the parameters of
a disc model to the star counts in 820 regions. Results from both independent
methods are consistent with each other. The qualitative conclusions are that
the disc is well fitted by an exponential distribution in both the
galactocentric distance and height. There is not an abrupt cut-off in the
stellar disc (at least within R<15 kpc). There is a strong flare (i.e. an
increase of scale-height towards the outer Galaxy) which begins well inside the
solar circle, and hence there is a decrease of the scale-height towards the
inner Galaxy. Another notable feature is the existence of a warp in the old
stellar population whose amplitude is coincident with the amplitude of the gas
warp.
It is shown for low latitude stars (mean height: |z|~300 pc) in the outer
disc (galactocentric radius R>6 kpc) that: the scale-height in the solar circle
is h_z(R_sun)=3.6e-2 R_sun, the scale-length of the surface density is h_R=0.42
R_sun and the scale-length of the space density in the plane (i.e. including
the effect of the flare) is H=0.25 R_sun. The variation of the scale-height due
to the flare follows roughly a law h_z(R) =~ h_z(R_sun) exp
[(R-R_\odot)/([12-0.6R(kpc)] kpc)] (for R<~15 kpc; R_sun=7.9 kpc). The warp
moves the mean position of the disc to a height z_w=1.2e-3 R(kpc)^5.25
sin(phi+(5 deg.)) pc (for R<~13 kpc; R_sun=7.9 kpc).Comment: LaTEX, 20 pages, 23 figures, accepted to be published in A&
Global properties of the HI distribution in the outer Milky Way
Aims: We derive the 3-D HI volume density distribution for the Galactic disk
out to R = 60 kpc. Methods: Our analysis is based on parameters for the warp
and rotation curve derived previously. The data are taken from the
Leiden/Argentine/Bonn all sky 21-cm line survey. Results: The Milky Way HI disk
is significantly warped but shows a coherent structure out to R = 35 kpc. The
radial surface density distribution, the densities in the middle of the warped
plane, and the HI scale heights all follow exponential relations. The radial
scale length for the surface density distribution of the HI disk is 3.75 kpc.
Gas at the outskirts for 40 < R < 60 kpc is described best by a distribution
with an exponential radial scale length of 7.5 kpc and a velocity dispersion of
74 km/s. Such a highly turbulent medium fits also well with the average shape
of the high velocity profile wings observed at high latitudes. The turbulent
pressure gradient of such extra-planar gas is on average in balance with the
gravitational forces. About 10% of the Milky Way HI gas is in this state. The
large scale HI distribution is lopsided; for R < 15 kpc there is more gas in
the south. The HI flaring indicates that this asymmetry is caused by a dark
matter wake, located at R = 25 kpc in direction of the Magellanic System.
Conclusions: The HI disk is made up of two major components. Most prominent is
the normal HI disk which can be traced to R = 35 kpc. This is surrounded by a
patchy distribution of highly turbulent gas reaching large scale heights but
also large radial distances. At the position of the Sun the exponential scale
height in the z direction is 3.9 kpc. This component resembles the anomalous
gas discovered previously in some galaxies.Comment: to be published in A&
Electron density and carriers of the diffuse interstellar bands
We have used the ionisation equilibrium equation to derive the electron
density in interstellar clouds in the direction to 13 stars. A linear relation
was found, that allows the determination of the electron density from the Mg I
and Mg II column densities in diffuse clouds.
The comparison of normalised equivalent width of 12 DIBs with the electron
density shows that the DIBs equivalent width do not change with electron
density varying in the range ne=0.01-2.5 cm^-3. Therefore the DIBs carriers (1)
can be observed only in one ionisation stage, or (2) the DIBs are arising in
cloud regions (eg. cores or cloud coronas) for which we can not determine the
electron density
- …