107 research outputs found
Formation of Large-Scale Obscuring Wall and AGN Evolution Regulated by Circumnuclear Starbursts
By considering the radiative force by a circumnuclear starburst as well as an
AGN, we analyze the equilibrium configuration and the stability of dusty gas in
the circumnuclear regions. It is found that the radiative force by an intensive
starburst can support a stable gaseous wall with a scale-height of several
hundred parsecs. Moreover, by taking the simple stellar evolution in the
starburst into account, we find that the covering factor of the wall decreases
on a time-scale of several yr. The large-scale wall, if formed, works to
obscure the nucleus due to the dust opacity. Hence, it is anticipated that the
index of AGN type tends to shift from higher to lower in several yr
according as the circumnuclear starburst becomes dimmer. On the other hand, if
the AGN itself is brighter than the circumnuclear starburst (e.g. quasar case),
no stable large-scale wall forms. In that case, the AGN is highly probably
identified as type 1. The present mechanism may provide a physical explanation
for the putative correlation between AGN type and host properties that Sy2's
are more frequently associated with circumnuclear starbursts than Sy1's,
whereas quasars are mostly observed as type 1 regardless of star-forming
activity in the host galaxies.Comment: 10 pages, 3 figures, ApJ Letters in pres
High-Resolution Near-Infrared Imaging and Polarimetry of Four Proto-Planetary Nebulae
High-resolution near-infrared HST NICMOS (F160W, F222M) images and
polarization (2 um) observations were made of four bipolar proto-planetary
nebulae (PPNs): IRAS 17150-3224, IRAS 17441-2411, IRAS 17245-3951, and IRAS
16594-4656. The first three of these are viewed nearly edge-on, and for the
first time the central stars in them are seen. Color maps reveal a reddened
torus between the bipolar lobes in the edge-on cases, with bluer lobes. The
polarization values are high, with maximum values ranging from 40 to 80%. The
polarization patterns are basically centrosymmetric, with some deviations in
the low polarization equatorial regions. For IRAS 17150-3224, circumstellar
arcs are seen at 1.6 um, along with a newly-discovered loop in the equatorial
region. Bright caps are seen at the end of the lobes, indicating that they are
not open-ended. A distinct point-symmetric pattern is seen in the strengths of
the polarization vectors, especially in IRAS 17150-3224. HST NICMOS
observations provide a valuable complement to the WFPC2 visible images in
deriving the basic structure of bipolar PPNs.Comment: 20 pages, 12 figures, accepted for publication in Astronomical
Journal higher resolution figures are available on
http://katherine.as.arizona.edu/~ksu/cv/su_nicmos7840.ps.g
Absorption-Line Probes of Gas and Dust in Galactic Superwinds
We discuss moderate resolution spectra of the NaD absorption-line in a sample
of 32 far-IR-bright starburst galaxies. In 18 cases, the line is produced
primarily by interstellar gas, and in 12 of these it is blueshifted by over 100
km/s relative to the galaxy systemic velocity. The absorption-line profiles in
these outflow sources span the range from near the galaxy systemic velocity to
a maximum blueshift of 400 to 600 km/s. The outflows occur in galaxies
systematically viewed more nearly face-on than the others. We therefore argue
that the absorbing material consists of ambient interstellar gas accelerated
along the minor axis of the galaxy by a hot starburst-driven superwind. The NaD
lines are optically-thick, but indirect arguments imply total Hydrogen column
densities of N_H = few X 10^{21} cm^{-2}. This implies that the superwind is
expelling matter at a rate comparable to the star-formation rate. This
outflowing material is very dusty: we find a strong correlation between the
depth of the NaD profile and the line-of-sight reddening (E(B-V) = 0.3 to 1
over regions several-to-ten kpc in size). The estimated terminal velocities of
superwinds inferred from these data and extant X-ray data are typically 400 to
800 km/s, are independent of the galaxy rotation speed, and are comparable to
(substantially exceed) the escape velocities for (dwarf) galaxies. The
resulting loss of metals can establish the mass-metallicity relation in
spheroids, produce the observed metallicity in the ICM, and enrich a general
IGM to 10 solar metallicity. If the outflowing dust grains survive their
journey into the IGM, their effect on observations of cosmologically-distant
objects is significant.Comment: 65 pages, including 16 figures. ApJ, in pres
Three quasars from a survey of strong 25-”m emitters
We have carried out a spectroscopic survey of 750 sources that are strong 25-ÎŒm emitters from the IRAS Faint Source data base. Many of these sources are previously unknown active galactic nuclei including new IRAS quasars, three of which we describe here: F21382â2659, Z06367â6845 and Z05558â5008. They are all radio and X-ray quiet, and compared to the known IRAS quasars they have similar 25-ÎŒm luminosities, L(25 ÎŒm), but lower values of L(25 ÎŒm)/L(B). Their F(25 ÎŒm)/F(60 ÎŒm) IRAS colours lie in the range 0.33 to 1.08, indicating the presence of relatively warm dust, presumably in a dusty torus surrounding the central source, and with temperatures similar to those of the known IRAS quasars. The quasar with the warmest dust, F21382â2659, exhibits broad (full width at half-maximum ~4000 km s^(â1)) asymmetric Balmer lines with HÎł having an opposite asymmetry to the other broad lines; also HÎČ (only) is double-peaked. Fe ii is very weak in F21382â2659 but strong in the other two quasars, and the anticorrelation between Fe ii and [O iii] holds as anticipated. Two of the quasars are unpolarized: although F21382â2659 is optically polarized (2.1 per cent at 4950 Ă
), we argue that this provides little insight into the orientation of its dust torus relative to the line of sight
First Observations of the Magnetic Field Geometry in Pre-stellar Cores
We present the first published maps of magnetic fields in pre-stellar cores,
to test theoretical ideas about the way in which the magnetic field geometry
affects the star formation process. The observations are JCMT-SCUBA maps of 850
micron thermal emission from dust. Linear polarizations at typically ten or
more independent positions in each of three objects, L1544, L183 and L43 were
measured, and the geometries of the magnetic fields in the plane of the sky
were mapped from the polarization directions. The observed polarizations in all
three objects appear smooth and fairly uniform. In L1544 and L183 the mean
magnetic fields are at an angle of around 30 degrees to the minor axes of the
cores. The L43 B-field appears to have been influenced in its southern half,
such that it is parallel to the wall of a cavity produced by a CO outflow from
a nearby T Tauri star, whilst in the northern half the field appears less
disturbed and has an angle of 44 degrees to the core minor axis. We briefly
compare our results with published models of magnetized cloud cores and
conclude that no current model can explain these observations simultaneously
with previous ISOCAM data.Comment: 13 pages, 3 figs, to appear in ApJ Letter
Sequence structure emission in The Red Rectangle Bands
We report high resolution (R~37,000) integral field spectroscopy of the
central region (r<14arcsec) of the Red Rectangle nebula surrounding HD44179.
The observations focus on the 5800A emission feature, the bluest of the
yellow/red emission bands in the Red Rectangle. We propose that the emission
feature, widely believed to be a molecular emission band, is not a molecular
rotation contour, but a vibrational contour caused by overlapping sequence
bands from a molecule with an extended chromophore. We model the feature as
arising in a Polycyclic Aromatic Hydrocarbon (PAH) with 45-100 carbon atoms.Comment: 13 pages, 9 figures, accepted for publication in ApJ. A version of
the paper with full resolution figures is available at:
http://www.aao.gov.au/local/www/rgs/Sequence-Structure
Scattered Nuclear Continuum and Broad H-alpha in Cygnus A
We have discovered scattered broad Balmer emission lines in the spectrum of
Cygnus A, using the Keck II telescope. Broad H-alpha appears in polarized flux
from components on either side of the nucleus, and to a lesser extent in the
nucleus. The full-width at half-maximum of broad H-alpha is 26,000 km/s,
comparable to the widest emission lines seen in broad-line radio galaxies.
Scattered AGN light provides a significant contribution to the total flux at
3800 Angstroms (rest) of the western component, where the polarization rises to
16%. The spatially integrated flux of Cygnus A at 5500 Angstroms can be
decomposed into an elliptical galaxy fraction (Fg=0.70), a highly polarized
blue component (FC1=0.15), a less polarized red component (FC=0.09), and a
contribution from the nebular continuum (0.06). Imaging polarimetry shows a
double fan of polarization vectors with circular symmetry which corresponds to
the ionization cone seen in HST images. Our results are consistent with
scattering of light from a hidden quasar of modest luminosity by an extended,
dusty narrow-line region.Comment: 13 pages, 4 figures, Latex, to appear in ApJ Letter
The Circumstellar Disk of the Butterfly Star in Taurus
We present a model of the circumstellar environment of the so-called
``Butterfly Star'' in Taurus (IRAS 04302+2247). The appearance of this young
stellar object is dominated by a large circumstellar disk seen edge-on and the
light scattering lobes above the disk. The model is based on multi-wavelength
continuum observations: Millimeter maps and high-resolution near-infrared
images obtained with HST/NICMOS.
It was found that the disk and envelope parameters are comparable with those
of the circumstellar environment of other young stellar objects. A main result
is that the dust properties must be different in the circumstellar disk and in
the envelope: While a grain size distribution with grain radii up to 100 micron
is required to reproduce the millimeter observations of the disk, the envelope
is dominated by smaller grains similar to those of the interstellar medium.
Preprint with high figure quality available at:
http://spider.ipac.caltech.edu/staff/swolf/homepage/public/preprints/i04302.psComment: 32 pages, 9 figure
The "Twin Jet" Planetary Nebula M2-9
We present a model for the structure, temporal behavior, and evolutionary
status of the bipolar nebula M2-9. According to this model the system consists
of an AGB or post-AGB star and a hot white dwarf companion, with an orbital
period of about 120 years. The white dwarf has undergone a symbiotic nova
eruption about 1200 years ago, followed by a supersoft x-ray source phase. The
positional shift of the bright knots in the inner nebular lobes is explained in
terms of a revolving ionizing source. We show that the interaction between the
slow, AGB star's wind, and a collimated fast wind from the white dwarf clears a
path for the ionizing radiation in one direction, while the radiation is
attenuated in others. This results in the mirror-symmetric (as opposed to the
more common point-symmetric) shift in the knots. We show that M2-9 provides an
important evolutionary link among planetary nebulae with binary central stars,
symbiotic systems, and supersoft x-ray sources.Comment: 13 pages + 2 figures. Submitted to Ap
Spectropolarimetry of the Type IIb Supernova 2001ig
We present spectropolarimetric observations of the Type IIb SN 2001ig in NGC
7424; conducted with the ESO VLT FORS1 on 2001 Dec 16, 2002 Jan 3 and 2002 Aug
16 or 13, 31 and 256 days post-explosion. These observations are at three
different stages of the SN evolution: (1) The hydrogen-rich photospheric phase,
(2) the Type II to Type Ib transitional phase and (3) the nebular phase. At
each of these stages, the observations show remarkably different polarization
properties as a function of wavelength. We show that the degree of interstellar
polarization is 0.17%. The low intrinsic polarization (~0.2%) at the first
epoch is consistent with an almost spherical (<10% deviation from spherical
symmetry) hydrogen dominated ejecta. Similar to SN 1987A and to Type IIP SNe, a
sharp increase in the degree of the polarization (~1%) is observed when the
outer hydrogen layer becomes optically thin by day 31; only at this epoch is
the polarization well described by a ``dominant axis.'' The polarization angle
of the data shows a rotation through ~40 degrees between the first and second
epochs, indicating that the asymmetries of the first epoch were not directly
coupled with those observed at the second epoch. For the most polarized lines,
we observe wavelength-dependent loop structures in addition to the dominant
axis on the Q-U plane. We show that the polarization properties of Type IIb SNe
are roughly similar to one another, but with significant differences arising
due to line blending effects especially with the high velocities observed for
SN 2001ig. This suggests that the geometry of SN 2001ig is related to SN 1993J
and that these events may have arisen from a similar binary progenitor system.Comment: 42 pages, 12 figures (figs. 11 and 12 are both composed of four
subpanels, figs. 6,7,8,11 and 12 are in color, fig. 1 is low res and a high
res version is available at http://www.as.utexas.edu/~jrm/), ApJ Accepte
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