Kapteyn Institute Preprints (1999)
Kapteyn Institute Preprint Service.
Preprints of the Kapteyn Institute.
Paper copies can be ordered at:
Kapteyn Institute
P.O. Box 800
9700 AV Groningen
The Netherlands
All the preprints are compressed using gzip.
2000 Preprints
-
Preprint 319 Atomic Hydrogen at High Redshift
F.H. Briggs
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Preprint 318 Astronomical Constraints on the Cosmic
Evolution of the Fine Structure Constant and Possible Quantum Dimensions C.L.
Carilli, K.M. Menten, J.T.Stocke, E. Perlman, R. Vermeulen, F. Briggs, A.G. de
Bruyn, J.Conway, C.P.Moore
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Preprint 317 21cm Absorption Lines at High Redshift from
Intervening Galaxies F.H. Briggs
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Preprint 316 HI Observations of the starburst galaxy NGC
2146
A. Taramopoulos, H. Payne, F.H. Briggs
-
Preprint 315 Abundances and Morphology in Planetary Nebulae
S.R. Pottasch
- Preprint 314 The central star of the Planetary Nebula
NGC 6537 S.R. Pottasch
- Preprint 313 Abundance in the planetary nebulae
NGC 6537 and He2-111 S.R. Pottasch, D.A. Beintema, and W.A. Feibelman
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Preprint 312
Continuous Fields and Discrete Samples: Reconstruction through
Delaunay Tessellations
W.E. Schaap and R. van de Weygaert
-
Preprint 311
On the viewing angle to broad-lined radio galaxies
Dennett-Thorpe, J., Barthel, P.D. & van Bemmel, I.M.
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Preprint 310
Truncations in stellar disks
P.C. van der Kruit
-
Preprint 309
Removing radio interference from contaminated astronomical
spectra using an independent reference signal and closure relations
F.H. Briggs, J.F. Bell & M.J. Kesteven
-
Preprint 308
Flat radio-spectrum galaxies and BLLacs: Part I: core properties
J. Dennett-Thorpe & M.J. Marcha
-
Preprint 307
HST/NICMOS observations of the host galaxies of powerful radio sources:
Does size matter?
W.H. de Vries, C.P. O'Dea, P.D. Barthel,
C. Fanti, R. Fanti, M.D. Lehnert
-
Preprint 306
The C-C-C bending modes of PAHs: a new emission plateau from 15 to
20 μm,
C. Van Kerckhoven, S. Hony, E. Peeters, A.G.G.M. Tielens, L.J. Allamandola,
D.M. Hudgins, P. Cox, P.R. Roelfsema, R.H.M. Voors, C. Waelkens, L.B.F.M. Waters, P.R. Wesselius
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Preprint 305
Clues to Quasar Broad Line Region Geometry and Kinematics,
M. Vestergaard, B.J. Wilkes, P.D. Barthel
-
Preprint 304
The Nature of Composite LINER/HII Galaxies, As Revealed from
High-Resolution VLA Observations,
Mercedes E. Filho, Peter D. Barthel and Luis C. Ho
-
Preprint 303
ISO observations of 3CR quasars and radio galaxies,
Ilse M. van Bemmel, Peter D. Barthel & Thijs de Graauw
-
Preprint 302
Infrared Observations of Hot Gas and Cold Ice toward the Low
Mass Protostar Elias 29
A.C.A. Boogert, A.G.G.M. Tielens, C. Ceccarelli, A.M.S. Boonman,
E.F. van Dishoeck, J.V. Keane, D.C.B. Whittet, Th. de Graauw
-
Preprint 301
Vertical motions in the disk of NGC 5668 as seen with optical
Fabry-Perot spectroscopy
J. Jimenez-Vicente, E. Battaner
-
Preprint 300
Microlensing and the Physics of Stellar Atmospheres,
Penny D. Sackett
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Preprint 299
The vertical extent and kinematics of the HI in NGC 2403,
W.E. Schaap, R. Sancisi, and R.A. Swaters
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Preprint 298
The evolution of the stellar populations in low surface
brightness galaxies,
L.B. van den Hoek, W.J.G. de Blok, J.M. van der Hulst and T. de
Jong
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Preprint 297
Multiple variations in the radio light-curve of the colliding wind
binary WR 146 (WC6+O): evidence for a third component
Diah Y.A. Setia Gunawan, A. Ger de Bruyn,
Karel A. van der Hucht and Peredur M. Williams
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Preprint 296
The Stellar Content of the Halo of NGC 5907 from Deep HST NICMOS Imaging
Stephen E. Zepf, Michael C. Liu, Francine R. Marleau,
Penny D. Sackett, James R. Graham
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Preprint 295
Using Slitless Spectroscopy to study the
Kinematics of the Planetary Nebula Population in M94
N.G. Douglas, J. Gerssen, K. Kuijken and M.R. Merrifield
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Preprint 294
Limits on Stellar and Planetary Companions in Microlensing Event
OGLE-1998-BUL-14
M.D. Albrow, J.-P. Beaulieu, J.A.R. Caldwell, D.L. DePoy,
M. Dominik, B.S. Gaudi, A. Gould, J. Greenhill, K. Hill,
S. Kane, R. Martin, J. Menzies, R.M. Naber, R.W. Pogge, K.R. Pollard,
P.D. Sackett, K.C. Sahu, P. Vermaak, R. Watson, A. Williams (The PLANET
Collaboration)
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Preprint 293
The Space Density of Primordial Gas Clouds near Galaxies and Groups
and their Relation to Galactic HVCs,
M.A Zwaan, F.H. Briggs
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Preprint 292
Detection of Rotation in a Binary Microlens:
PLANET Photometry of MACHO 97-BLG-41,
M.D. Albrow, J.-P. Beaulieu, J.A.R. Caldwell, M. Dominik, B.S.
Gaudi,
A. Gould, J. Greenhill, K. Hill, S. Kane, R. Martin, J. Menzies, R.M.
Naber,
K.R. Pollard, P.D. Sackett, K.C. Sahu, P. Vermaak, R. Watson, A.
Williams
(The PLANET Collaboration)
and
H.E. Bond, I.M. van Bemmel
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Preprint 291
The 24-Hour Night Shift Astronomy from Microlensing
Monitoring Networks,
Penny D. Sackett
Atomic Hydrogen at High Redshift
F.H. Briggs,
Preprint no. 319
to appear in: Highlights of Astronomy, Vol. 12 (ed. D.J. Wilner), Proc. of the IAU Joint
Discussion 9 on Cold Gas and Dust at High Redshift, 2000.
Abstract
After the production of the ionizing background by the first
generation of stars, neutral gas must be confined to
sufficiently high density to be self-shielding and remain neutral.
Neutral gas is an identifier of the presence of confining gravitational
potentials and a tracer of the kinematics of the potential.
Kinematical studies are being extended to neutral atomic gas
at high redshift.
Astronomical Constraints on the Cosmic Evolution of the Fine
Structure Constant and Possible Quantum Dimensions
C.L. Carilli, K.M. Menten, J.T. Stocke,
E. Perlman, R. Vermeulen, F. Briggs,
A.G. de Bruyn, J. Conway, C.P. Moore
Preprint no. 318
to appear in: Physical Review Letters.
Abstract
We present measurements of absorption by the 21cm hyperfine transition of
neutral hydrogen toward radio sources at substantial look-back times. These
data are used in combination with observations of rotational transitions of
common interstellar molecules, to set limits on the evolution of the fine
structure constant: (dot alpha)/alpha < 3.5e-15/yr to a look-back time of
4.8 Gyr. The neutral hydrogen observations employed Very Long Baseline
Interferometry in order to mitigate the substantial uncertainty arising from
the fact that observations at very different wavelengths may probe different
lines-of-site due to frequency dependent structure of the background source.
We discuss the implication of these results on theories unifying natural
forces based on compact quantum dimensions. In the context of string theory,
the limit on the secular evolution of the scale factor of these compact
dimensions, R, is (dot R)/R < 1e-15/yr. Including terrestrial
and other astronomical measurements places limits (2sigma) on
slow oscillations of R from the present to the epoch of cosmic
nucleosynthesis, just seconds after the big bang, of (Delta R)/R < 1e-5.
21cm Absorption Lines at High Redshift from Intervening Galaxies
F.H. Briggs
Preprint no. 317
to appear in: "The Universe at Low Frequencies", IAU Symposium 199, ASP Conference Series.
Abstract
Radio absorption line observations of neutral hydrogen gas against
extended radio sources offers the means to measure sizes and kinematics
in intervening galaxies at all redshifts up to the maximum redshift
where radio galaxies are detected. Such observations can therefore
trace the evolution of galaxies at redshifts
z greater than 2 where the damped Lyman-alpha statistics indicate
that the mass in neutral gas exceeded the mass in stars.
HI Observations of the starburst galaxy NGC 2146
A. Taramopoulos, H. Payne, F.H. Briggs
Preprint no. 316
Accepted for publication in
Astronomy & Astrophysics.
Abstract
NGC 2146 is a peculiar spiral galaxy which is currently undergoing a major
burst of star formation and is immersed in a extended HI structure that has
morphological and kinematical resemblence to a strong tidal interaction. This
paper reports aperture synthesis observations carried out in the 21cm line with
the Very Large Array (VLA - The National Radio Astronomy Observatory
(NRAO) is operated by Associated Universities, Inc. under cooperative
agreement with the National Science Foundation.) of two fields positioned to
optimally cover the HI streams to the north and south of the galaxy, along
with a 300 ft total power spectral mapping program to recover the low surface
brightness extended emission. The observations reveal elongated streams of
neutral hydrogen towards both the north and the south of the optical galaxy
extending out up to 6 Holmberg radii. The streams are not in the principle plane
of rotation of the galaxy, but instead are suggestive of a tidal interaction
between NGC 2146 and a LSB companion that was destroyed by the encounter and
remains undetected at optical wavelengths. Part of the southern stream is
turning back to fall into the main galaxy, where it will create a long-lived
warp in the HI disk of NGC 2146. Analysis of the trajectory of the outlying
gas suggests that the closest encounter took place about 0.8 billion years
ago and that infall of debris will continue for a similar time span.
Abundances and Morphology in Planetary Nebulae
S.R. Pottasch
Preprint no. 315
To be published in: Asymmetrical Planetary Nebulae II: From Origins to
Microstructures (eds. J.H. Kastner, N. Soker & S. Rappaport), A.S.P.
Conference Series, 2000.
Abstract
The abundances of 16 well studied have been determined. New ISO
measurements have been combined with optical and ultraviolet
data from the literature, in an attempt to obtain accurate values.
Only He, O, C, N, Ne, Ar, and S are considered. High values of N/O are
sometimes, but not always, found in bipolar nebulae. On the other hand,
some bipolar nebulae show low values of N/O, and it is concluded that no
simple relationship between morphology and composition exists.
The central star of the Planetary Nebula NGC 6537
S.R. Pottasch
Preprint no. 314
Accepted for publication in
Astronomy & Astrophysics.
Abstract
The fact that Space Telescope WFPC2 images of the planetary nebula NGC
6537 fails to show the central star is used to derive a limit to its
magnitude; it is fainter than a magnitude of 22.4 in the visible. This
is used to derive a lower limit to the temperature of the star. The
Zanstra temperature is at least 500 000 K. The Energy Balance
temperature is found to be consistent with this value, as is the
ionization state of the nebula. Assuming a reasonable range of distances
for the nebula, the radius of the star can be found. It is consistent
with the mass-radius relation of a white dwarf of 0.9 M_{sun} or
higher.
Abundance in the planetary nebulae NGC 6537 and He2-111
S.R.
Pottasch, D.A. Beintema, and W.A. Feibelman
Preprint no. 313
Accepted for publication in
Astronomy & Astrophysics.
Abstract
The ISO and IUE spectra of the bipolar planetary nebulae NGC~6537 and
He2-111 are presented. These spectra are combined with the spectrum in
the
visual wavelength region from the nebulae to obtain a complete spectrum
that is corrected for extinction. The chemical abundance of the nebulae
is
then determined and compared to previous determinations. The abundance
of the two nebulae is quite similar. A comparison is then made with the
abundance of two other bipolar planetary nebulae whose abundance is
also determined with the help of ISO observations. It is shown that not
all
bipolar nebulae have similar abundance. NGC 6445 has a much lower
nitrogen to oxygen ratio, similar to NGC 7027, but still not as low as
the
Orion nebula.
Continuous Fields and Discrete Samples: Reconstruction through
Delaunay Tessellations
W.E. Schaap and R. van de Weygaert
Preprint no. 312
Accepted for publication in
Astronomy & Astrophysics Letters.
Abstract
Here we introduce the Delaunay Density Estimator Method. Its purpose is
rendering a fully volume-covering reconstruction of a density field from a
set of discrete data points sampling this field. Reconstructing density or
intensity fields from a set of irregularly sampled data is a recurring key
issue in operations on
astronomical data sets, both in an observational
context as well as in the context of numerical simulations. Our technique is
based upon the stochastic geometric concept of the Delaunay
tessellation generated by the point set. We shortly describe the method,
and illustrate its virtues by means of an application to an
N-body simulation of cosmic structure formation. The presented technique
is
a fully adaptive method: automatically it probes high density regions
at maximum possible resolution, while low density regions are recovered as
moderately varying regions devoid of the often irritating shot-noise
effects. Of equal importance is its capability to sharply and undilutedly
recover anisotropic density
features like filaments and walls. The
prominence of such features at a range of resolution levels within a
hierarchical clustering scenario as the example of the standard CDM scenario
is shown to be impressively recovered by our scheme.
On the viewing angle to broad-lined radio galaxies
Dennett-Thorpe J., Barthel P.D. & van Bemmel I.M.
Preprint no. 311
Accepted for publication in
Astronomy & Astrophysics.
Abstract
We address the nature of broad-lined radio galaxies, in particular their
radio axis orientation, using new, matched resolution, dual frequency radio
observations of a sample of twelve nearby broad-lined extragalactic 3C
objects. Radio spectral index and depolarisation asymmetries indicate that
these objects have a preferred orientation with respect to the observer. In
addition, the spectral asymmetries
are suggestive of lower Doppler factors in
the broad-lined radio galaxies when compared to 3C quasars. This is in
agreement with their optical properties, and leads to the conclusion that
some objects are lower powered versions - at similar lines of sight - of the
more distant quasars, whereas others are at larger angles to the line
of sight.
Truncations in stellar disks
P.C. van der Kruit
Preprint no. 310
Invited review to be published in Proceedings of the Conference "Galaxy
Disks and Disk Galaxies" (eds. J.C. Funes S.J. and E.M. Corsini), Vatican/Rome, June 2000.
Abstract
The presence of radial truncations in stellar disks is reviewed. There
is ample evidence that many disk galaxies have relatively shaprt truncations
in their disks. These often are symmetric and independent of the wavelength
band of the observations. The ratio of the truncation radius Rmax to the
disk scalelength h appears often less then 4.5, as expected on a simple model
for the disk collapse. Current samples of galaxies observed may however not
be representative and heavily biased towards disks with the largest
scalelengths. Many spiral galaxies also have HI warps and these generally
start at the truncation radius of the stellar disk. The HI surface density
suddenly becomes much flatter with radius. In some galaxies the start of the
warp and the position of the disk truncation radius is accompanied by a drop
in the rotation velocity. In the regions beyond the disk truncation in the HI
layer some star formation does occur, but the heavy element abundance and the
dust content are very low. All evidence is consistent with the notion that
the outer gas parts of the disks constitute recently accreted material, at
least accreted after formation of what is now the stellar thin disk. Although
various models exist for the origin of the truncations in the stellar disks,
this at present remains unclear.
Removing radio interference from contaminated astronomical
spectra using an independent reference signal and closure
relations
F.H. Briggs, J.F. Bell & M.J. Kesteven
Preprint no. 309
Accepted for publication in the
Astronomical Journal.
Abstract
The growing level of radio frequency interference (RFI) is a
recognized problem for research in radio astronomy. This paper
describes an intuitive but powerful RFI cancellation technique
that is suitable for radio spectroscopy where time-averages are
recorded. An RFI ``reference signal,'' is constructed from the
cross power spectrum of the signals from the two polarizations of
a reference horn pointed at the source of the RFI signal. The
RFI signal paths obey simple phase and amplitude closure
relations, which allows computation of the RFI contamination in
the astronomical data and the corrections to be applied to the
astronomical spectra.
Since the method is immune to the effects of multipath scattering
in both the astronomy and reference signal channels, "clean
copies" of the RFI signal are not required.
The method could be generalized (1) to interferometer arrays, (2)
to correct for scattered solar radiation that causes spectral
"standing waves" in single-dish spectroscopy, and (3) to pulsar
survey and timing applications where a digital correlator plays
an important role in broadband pulse dedispersion.
Future large radio telescopes, such as the proposed LOFAR and SKA
arrays, will require a high degree of RFI suppression and could
implement the technique proposed here with the benefit of faster
electronics, greater digital precision and higher data rates.
Flat radio-spectrum galaxies and BLLacs: Part I: core properties
J. Dennett-Thorpe & M.J. Marcha
Preprint no. 308
Accepted for publication in
Astronomy & Astrophysics.
Abstract
This paper concerns the relationship of BLLacs and flat spectrum weak
emission-line galaxies. We compare the weak emission-line galaxies and the
BLLacs in a sample of 57 flat spectrum objects (Marcha et al. 1996), using
high-frequency radio and non-thermal optical flux densities, spectral
indices and polarization properties. We consider whether objects which are
not 'traditional' BLLacs - due to their larger emission line strengths, and
larger CaII spectral breaks - are simply starlight diluted BLLacs. Their
broad-band spectral properties are consistent with this interpretation,
but their radio polarization may indicate more subtle effects. Comparison
of the weak emission-line galaxies and the BLLacs shows that, on average,
the former have steeper spectra between 8 and 43GHz, and are less
polarized at 8.4GHz. This is consistent with many of the weak-lined
objects being at larger angles to the line of sight than the BLLacs. In
addition to this population, we indicate a number of the weak
emission-line galaxies which may be 'hidden BLLacs': relativistically
boosted objects very close to the line of sight with an apparently weak
AGN.
HST/NICMOS observations of the host galaxies of powerful radio sources:
Does size matter?
W.H. de Vries, C.P. O'Dea, P.D. Barthel,
C. Fanti, R. Fanti, M.D. Lehnert
Preprint no. 307
Accepted for publication in the Astronomical Journal (Nov 2000 issue)
Comments: 21 pages, 9 figures + 19 additional images (jpg).
Abstract
We present near-infrared J and K band imaging of a sample of powerful
radio source host galaxies with the Hubble Space Telescope NICMOS2
camera. These sources have been selected on their double lobed radio
structure, and include a wide range of projected radio source sizes.
The largest projected linear sizes range from the compact Gigahertz
Peaked Spectrum (GPS, <1 kpc) and Compact Steep Spectrum (CSS, <20 kpc)
radio sources, up to the large-scale (>20 kpc) classical doubles (FR II
radio sources). We investigate the dependence of host galaxy properties
(including near-IR surface brightness profiles) on radio source size,
using both our own and published data. The absolute magnitudes and
surface brightness profiles are consistent with the host galaxies being
regular giant elliptical galaxies rather than Brightest Cluster Galaxies
(BCGs). We find that the GPS, CSS, and FR II host galaxies are a
uniform class of objects, consistent with a scenario in which a powerful
radio source evolves along this size sequence.
The C-C-C bending modes of PAHs: a new emission plateau from 15 to
20 µm
C. Van Kerckhoven, S. Hony, E. Peeters, A.G.G.M. Tielens, L.J. Allamandola,
D.M. Hudgins, P. Cox, P.R. Roelfsema, R.H.M. Voors, C. Waelkens, L.B.F.M. Waters, P.R. Wesselius
Preprint no. 306
Accepted for publication in
Astronomy & Astrophysics, 357, 1013-1019, 2000
Abstract
We have obtained 2.5-45 µm spectra of a sample of compact HII regions,
YSOs and evolved stars in order to study the origin and evolution of
interstellar Polycyclic Aromatic Hydrocarbon molecules (PAHs). Besides the
well-known, strong PAH bands at 3.3, 6.2, 7.7, 8.6, and 11.2 µm, these
spectra reveal for the first time, a ubiquitous emission plateau from 15
to 20 µm. While the overall shape of this plateau is very similar in all
sources, the detailed profiles vary from source to source. In particular,
some sources show a distinct emission feature at 16.4 µm. Moreover, the
integrated intensity of this plateau varies relative to the PAH emission
features by a factor 10 in our sample.
We attribute this 15-20 µm plateau to a blend of many emission features
due to the interstellar or circumstellar PAH family present in these
sources. Laboratory studies and quantum chemical calculations show that
PAH molecules invariably possess emission features in this wavelength
region, arising from C-C-C bending modes which cause in- and out-of-plane
distortion of the carbon skeleton. These modes are very sensitive to the
molecular structure of the specific PAHs present and hence different
molecules emit at different wavelengths. Analysis of the available data on
the IR characteristics of PAHs show that a collection of PAHs will give
rise to a broad plateau in this region.
We have analyzed the size distribution of PAHs giving rise to the IR
emission spectra of the sources in our samples. While much of the 15-20 µm
plateau is thought to arise in relatively large PAHs and PAH clusters,
we attribute the 16.4 µm feature to the small end of the interstellar
PAH size distribution. We conclude that the observed increased strength of
the 15-20 µm plateau relative to the shorter wavelength IR emission
features in regions of massive star formation is caused by a preponderance
of larger PAHs and PAH clusters in those sources. Possibly this reflects
the importance of coagulation in the dense molecular cloud environment
from which these stars are formed.
Clues to Quasar Broad Line Region Geometry and Kinematics
M. Vestergaard, B.J. Wilkes, P.D. Barthel
Preprint no. 305
Accepted for publication in
The Astrophysical Journal Letters
Abstract
We present evidence that the high-velocity CIV1549 emission line gas of
radio-loud quasars may originate in a disk-like configuration, in close
proximity to the accretion disk often assumed to emit the low-ionization
lines. For a sample of 36 radio-loud z~2 quasars we find the 20-30%
peak width to show significant inverse correlations with the fractional
radio core-flux density, R, the radio axis inclination indicator.
Highly inclined systems have broader line wings, consistent with a
high-velocity field perpendicular to the radio axis. By contrast, the
narrow line-core shows no such relation with R, so the lowest velocity
CIV-emitting gas has an inclination independent velocity field. We
propose that this low-velocity gas is located at higher disk-altitudes
than the high-velocity gas. A planar origin of the high-velocity
CIV-emission is consistent with the current results and with an
accretion disk-wind emitting the broad lines. A spherical distribution
of randomly orbiting broad-line clouds and a polar high-ionization
outflow are ruled out.
The Nature of Composite LINER/HII Galaxies, As Revealed from
High-Resolution VLA Observations,
Mercedes E. Filho, Peter D. Barthel and Luis C. Ho
Preprint no. 304
Accepted for publication in
The Astrophysical Journal Supplements, July 2000.
Abstract
A sample of 37 nearby galaxies displaying composite LINER/HII and pure
HII spectra was observed with the VLA in an investigation of the nature
of their weak radio emission. The resulting radio contour maps overlaid
on optical galaxy images are presented here, together with an
extensive literature list and discussion of the individual galaxies.
Radio morphological data permit assessment of the "classical AGN"
contribution to the global activity observed in these "transition" LINER
galaxies. One in five of the latter objects display clear AGN
characteristics: these occur exclusively in bulge-dominated hosts.
ISO observations of 3CR quasars and radio galaxies
Ilse M. van Bemmel, Peter D. Barthel & Thijs de Graauw
Preprint no. 303
Accepted for publication in
Astronomy and Astrophysics.
Abstract
In order to check for consistency with the radio-loud AGN unification
scheme, ISOPHOT data obtained for two small sets of intermediate
redshift steep-spectrum 3CR radio galaxies and quasars are being
examined. Supplementary submillimeter and centimeter radio data
for the quasars are also taken into account, in order to assess the
magnitude of any beamed nonthermal radiation.
The fact that we find broad-lined objects to be somewhat more luminous
in their far-infrared output than narrow-lined objects, hints at a
contradiction to the unification scheme. However, as the sample objects
are not particularly well matched, the sample size is small, and the
FIR radiation may still be partly anisotropic, this evidence is, at
the moment, weak.
Infrared Observations of Hot Gas and Cold Ice toward the Low
Mass Protostar Elias 29
A.C.A. Boogert, A.G.G.M. Tielens, C. Ceccarelli, A.M.S. Boonman,
E.F. van Dishoeck, J.V. Keane, D.C.B. Whittet, Th. de Graauw
Preprint no. 302
Accepted for publication in
Astronomy and Astrophysics.
Abstract
We have obtained the full 1-200 um spectrum of the low luminosity (36
Lsun) Class I protostar Elias 29 in the Rho Ophiuchi molecular
cloud. It provides a unique opportunity to study the origin and
evolution of interstellar ice and the interrelationship of
interstellar ice and hot core gases around low mass protostars. We see
abundant hot CO and H2O gas, as well as the absorption bands of CO,
CO2, H2O and "6.85 um" ices. We compare the abundances and physical
conditions of the gas and ices toward Elias 29 with the conditions
around several well studied luminous, high mass protostars. The high
gas temperature and gas/solid ratios resemble those of relatively
evolved high mass objects (e.g. GL 2591). However, none of the ice
band profiles shows evidence for significant thermal processing, and
in this respect Elias 29 resembles the least evolved luminous
protostars, such as NGC 7538 : IRS9. Thus we conclude that the
heating of the envelope of the low mass object Elias 29 is
qualitatively different from that of high mass protostars. This is
possibly related to a different density gradient of the envelope or
shielding of the ices in a circumstellar disk. This result is
important for our understanding of the evolution of interstellar ices,
and their relation to cometary ices.
Vertical motions in the disk of NGC 5668 as seen with optical
Fabry-Perot spectroscopy
J. Jimenez-Vicente, E. Battaner
Preprint no. 301
Accepted for publication in
Astronomy and Astrophysics.
Abstract
We have observed the nearly face-on spiral galaxy NGC 5668 with the
TAURUS II
Fabry-Perot interferometer at the William Herschel Telescope using the
Halpha line to study the kinematics of the ionized gas. From the
extracted
data cube we construct intensity, velocity and velocity dispersion maps.
We
calculate the rotation curve in the innermost 2 arcmin and we use the
residual
velocity field to look for regions with important vertical motions. By
comparing the geometry of these regions in the residual velocity field
with the
geometry in the intensity and velocity dispersion maps we are able to
select
some regions which are very likely to be shells or chimneys in the disk.
The
geometry and size of these regions are very similar to the shells or
chimneys
detected in other galaxies by different means. Moreover, it is worth
noting
than this galaxy has been reported to have a population of neutral
hydrogen
high velocity clouds (Schulman et al. 1996) which, according to these
observations, could have been originated by chimneys similar to those
reported
in this paper.
Microlensing and the Physics of Stellar Atmospheres
Penny D. Sackett
Preprint no. 300
To appear in the ASP conference proceedings:
Microlensing 2000: A New Era of Microlensing Astrophysics,
eds. J.W. Menzies and P.D. Sackett
Abstract
The simple physics of microlensing provides a well-understood tool with
which to probe the atmospheres of distant stars in the Galaxy and Local
Group
with high magnification and resolution. Recent results in measuring
stellar
surface structure through broad band photometry and spectroscopy of
high
amplification microlensing events are reviewed, with emphasis on the
dramatic
expectations for future contributions of microlensing to the field of
stellar atmospheres.
The vertical extent and kinematics of the HI in NGC 2403
W.E. Schaap, R. Sancisi, and R.A. Swaters
Preprint no. 299
Accepted for publication in
Astronomy and Astrophysics Letters.
Abstract
The neutral hydrogen line profiles along the major axis of the
nearby spiral galaxy NGC 2403 show a wing towards the systemic
velocity. This asymmetry can be explained with the presence of an
abnormally thick HI disk (FWHM ~ 5 kpc) or with a two-component
structure: a thin disk and a slowly rotating, thicker (1-3 kpc) HI
layer. The latter model gives a better representation of the
observations. These results throw a new light on the disk-halo
connection in spiral galaxies. In particular, the decrease of
rotational velocity with height above the plane may be the result of
a galactic fountain flow. A vertically extended, slowly rotating HI
layer may be common among spiral galaxies with high levels of star
formation.
The evolution of the stellar populations in low surface
brightness galaxies,
L.B. van den Hoek, W.J.G. de Blok, J.M. van der Hulst and T. de
Jong
Preprint no. 298
Accepted for publication in
Astronomy and Astrophysics.
Abstract
We investigate the star formation history and chemical evolution of low
surface brightness (LSB) disk galaxies by modelling their observed
spectro-photometric and chemical properties using a galactic chemical
{and photometric} evolution model incorporating a detailed metallicity
dependent set of stellar input data.
For a large fraction of the LSB galaxies in our sample, observed
properties are best explained by models incorporating an exponentially
decreasing global star formation rate (SFR) ending at a present-day gas
fraction M(gas)/[M(gas)+M(stars)] = 0.5 for a galaxy age of 14 Gyr. For
some galaxies small amplitude star formation bursts are required to
explain the contribution of the young (5-50 Myr old) stellar population
to the galaxy integrated luminosity. This suggests that star formation
has proceeded in a stochastic manner.
The presence of an old stellar population in many late-type LSB galaxies
suggests that LSB galaxies roughly follow the same evolutionary history
as HSB galaxies, { except at a much lower rate}. In particular, our
results imply that LSB galaxies do not form late, nor have a delayed
onset of star formation, but simply evolve slowly.
Multiple variations in the radio light-curve of the colliding wind
binary WR 146 (WC6+O): evidence for a third component
Diah Y.A. Setia Gunawan, A. Ger de Bruyn,
Karel A. van der Hucht and Peredur M. Williams
Preprint no. 297
Accepted for publication in
Astronomy and Astrophysics.
Abstract
The Wolf-Rayet star WR 146 (HM19-3, WC6+O) is the brightest WR star at
radio wavelengths. We have been monitoring this system with the
Westerbork Synthesis Radio Telescope (WSRT) at 1.4 and 5 GHz (21
and 6 cm) since 1989. The time-averaged spectral index alpha
5-1.4 GHz \simeq -0.62 clearly points to a domination by
non-thermal radiation, which we associate with colliding winds in this
binary system. The non-thermal radio flux distribution shows a turn-over
at low frequency, which we suggest to be due to free-free absorption of
the synchrotron emission from the colliding wind region by plasma around
the system.
In the period 1989-1997 the average 1.4-GHz flux density increased
from ~61 to ~73 mJy; in the the period 1989-1999 the
average 5-GHz flux density increased from ~29 to ~37 mJy.
The light-curves show three different kinds of variations:
(i) a slow linear rise in a time-span of a decade;
(ii) a 3.38 yr periodic variation; and,
(iii) rapid non-periodic variations on a time-scale of weeks.
We examine whether the slow rise of the flux density could be explained
by decreasing free-free absorption in the line-of-sight through the
radiophotosphere of the O component, while moving in an eccentric orbit
around the WR component. However, the similarity of the amplitudes
(~22% in 10 yr) of the rises at 1.4 and 5 GHz argues against
a change in free-free absorption, expected to be strongly wavelength
dependent. This points to an intrinsic flux-density variation, possibly
due to modulation of the magnetic field strength resulting from orbital
motion in a very-long-period eccentric binary system. The relation
between the flux-density increase and orbital motion is supported by
positional measurements of the 5-GHz data.
We detect a possible motion of the shock zone relative to one of the
control sources (Control A) of ~0{\stackrel{''}{_{\cdot}}}05
in the 10 yr observing span. At a distance of 1250 pc this motion
corresponds to a projected tangential velocity of about 30 km s-1,
which is a plausible orbital velocity for a system like WR 146.
Superimposed on the 1.4-GHz slow rise, we find a sinusoidal variation
with a period P = 3.38 ± 0.02 yr and a semi-amplitude of
4.3 ± 0.2 mJy. Adopting a distance of 1250 pc to the system and
a 162 mas WR+O separation, we consider the observed 3.38 yr period too
short to be the WR+O binary period by at least two orders of magnitude.
We suggest that the periodic variability is caused by a third, low-mass
object, modulating the mass flow and/or the magnetic-field of the O
component. Unfortunately, our 5-GHz data are far too few and not
adequately spread over the whole phase to confirm that they consistently
follow the 3.38 yr period found in the 1.4-GHz data.
The erratic `micro'-variation in the 1.4-GHz light-curve is about
4 sigma of the typical 0.5 mJy observational uncertainty, on
a time-scale of weeks to months. When irregularities in the mass flow
(clumps, inhomogeneities and/or turbulence in the O and/or WR star
winds) reach the wind collision region, variation in the non-thermal
emission can be expected. Such irregularities can also affect the
free-free line-of-sight absorption at the lowest observing frequencies.
The Stellar Content of the Halo of NGC 5907 from Deep HST NICMOS Imaging
Stephen E. Zepf, Michael C. Liu, Francine R. Marleau,
Penny D. Sackett, James R. Graham
Preprint no. 296
Accepted for publication in
the Astronomical Journal.
Abstract
We present H-band images obtained with NICMOS of a field 75" (5 kpc)
above the plane of the disk of the edge-on spiral galaxy NGC 5907.
Ground-based observations have shown that NGC 5907 has a luminous
halo with a shallow radial profile between 4 and 8 kpc that roughly
traces the dark matter distribution of the galaxy deduced from its
rotation curve. Our NICMOS observations were designed
to resolve bright giants in the halo of NGC 5907 to constrain its
stellar composition with the goal of understanding its nature and
origin.
More than 100 stars are expected in the NICMOS images if the
dwarf-to-giant
ratio in the halo of NGC 5907 is consistent with that expected from
standard stellar initial mass functions, and if ground-based estimates
of the
distance to NGC 5907 and the integrated colors of its halo are correct.
Instead we observe only one candidate giant star. This apparent
discrepancy
can be resolved by assuming either a significantly larger distance than
suggested by several studies, or a halo metallicity much lower than
suggested by ground-based colors and as low as metal-poor Galactic
globular
clusters. If previous distance and halo color estimates for NGC 5907
are
correct, our NICMOS results suggest that its extended light is composed
of stars that formed with an initial mass function different than that
observed locally, leading to a much higher ratio of dwarfs to giants.
We describe how these three possible explanations for the absence of
bright giants in our NICMOS images of the halo of NGC 5907 might be
constrained by future observations.
Using Slitless Spectroscopy to study the
Kinematics of the Planetary Nebula Population in M94
N.G. Douglas, J. Gerssen, K. Kuijken and M.R. Merrifield
Preprint no. 295
Accepted for publication in
Monthly Notices of the Royal Astronomical Society.
Abstract
The planetary nebula populations of relatively nearby galaxies can be
easily observed and provide both a distance estimate and a tool with
which dynamical information can be obtained. Usually the requisite
radial velocities are obtained by multi-object spectroscopy once the
planetary nebulae have been located by direct imaging. Here we report
on a technique for measuring planetary nebula kinematics using the
double-beam ISIS spectrograph at the William Herschel Telescope in a
novel slitless mode, which enables the detection and radial velocity
measurements to be combined into a single step. The results on our
first target, the Sab galaxy NGC 4736, allow the velocity dispersion of
the stellar population in a disk galaxy to be traced out to four scale
lengths for the first time and are consistent with a simple isothermal
sheet model.
Limits on Stellar and Planetary Companions in Microlensing Event
OGLE-1998-BUL-14
M.D. Albrow, J.-P. Beaulieu, J.A.R. Caldwell, D.L. DePoy,
M. Dominik, B.S. Gaudi, A. Gould, J. Greenhill, K. Hill,
S. Kane, R. Martin, J. Menzies, R.M. Naber, R.W. Pogge, K.R. Pollard,
P.D. Sackett, K.C. Sahu, P. Vermaak, R. Watson, A. Williams (The PLANET
Collaboration)
Preprint no. 294
Accepted for publication in The Astrophysical Journal.
Abstract
We present the PLANET photometric data set for OGLE-1998-BUL-14,
a high magnification (A_max ~ 16) event alerted by the OGLE
collaboration
toward the Galactic bulge in 1998. The PLANET data set consists a total
of
461 I-band and 139 V-band points, the majority of which was taken over a
three month period. The median sampling interval during this period is
about 1 hour, and the 1-sigma scatter over the peak of the event is
1.5%.
The excellent data quality and high maximum magnification of this event
make it a prime candidate to search for the short duration, low
amplitude
perturbations that are signatures of a planetary companion orbiting the
primary lens. The observed light curve for OGLE-1998-BUL-14 is
consistent
with a single lens (no companion) within photometric uncertainties. We
calculate the detection efficiency of the light curve to lensing
companions as a function of the mass ratio and angular separation of the
two components. We find that companions of mass ratio >= 0.01 are ruled
out at the 95% significance level for projected separations between
0.4-2.4 r_E, where r_E is the Einstein ring radius of the primary lens.
Assuming that the primary is a G-dwarf with r_E ~ 3 AU our detection
efficiency for this event is ~60% for a companion with the mass and
separation of Jupiter and ~5% for a companion with the mass and
separation of Saturn. Our efficiencies for planets like those around
Upsilon And and 14 Her are >75%.
The Space Density of Primordial Gas Clouds near Galaxies and Groups
and their Relation to Galactic HVCs
M.A Zwaan, F.H. Briggs
Preprint no. 293
Accepted for publication in The Astrophysical Journal Letters.
Abstract
The Arecibo HI Strip Survey probed the halos of ~300 cataloged galaxies
and the environments of ~14 groups with sensitivity to neutral hydrogen
masses > 107 Msun. The survey detected no objects
with properties resembling the High Velocity Clouds (HVCs) associated
with the Milky Way or Local Group. If the HVCs were typically
MHI=107.5 Msun objects distributed
throughout
groups and galaxy halos at distances of 1 Mpc, the survey should have
made ~70 HVC detections in groups and ~250 detections around galaxies.
The null detection implies that HVCs are deployed at typical distances
of <200 kpc from the galaxies or group barycenters. If the clouds are
in virial equilibrium, their average dark matter fraction must be 98% or
higher.
Detection of Rotation in a Binary Microlens:
PLANET Photometry of MACHO 97-BLG-41
M.D. Albrow, J.-P. Beaulieu, J.A.R. Caldwell, M. Dominik, B.S.
Gaudi,
A. Gould, J. Greenhill, K. Hill, S. Kane, R. Martin, J. Menzies, R.M.
Naber,
K.R. Pollard, P.D. Sackett, K.C. Sahu, P. Vermaak, R. Watson, A.
Williams
(The PLANET Collaboration)
and
H.E. Bond, I.M. van Bemmel
Preprint no. 292
Accepted for publication in vol 534 of the May 2000 Astrophysical Journal.
Abstract
We analyze PLANET collaboration data for MACHO 97-BLG-41, the only
microlensing event observed to date in which the source transits two
disjoint
caustics. The PLANET data, consisting of 46 V-band and 325 I-band
observations
from five southern observatories, span a period from the initial alert
until
the end of the event. Our data are incompatible with a static binary
lens, but
are well fit by a rotating binary lens of mass ratio q=0.34 and angular
separation d ~ 0.5 (in units of the Einstein ring radius) in which the
binary
separation changes in size by delta d = -0.070 +/- 0.009 and in
orientation by
delta theta = (5.61 +/- 0.36) degrees during the 35.17 days between the
separate caustic transits. We use this measurement combined with other
observational constraints to derive the first kinematic estimate of the
mass,
distance, and period of a binary microlens. The relative probability
distributions for these parameters peak at a total lens mass M ~ 0.3
solar
masses (M-dwarf binary system), lens distance D_L ~ 5.5 kpc, and binary
period
P ~ 1.5 yr. The robustness of our model is demonstrated by its striking
agreement with MACHO/GMAN data that cover several sharp features in the
light
curve not probed by the PLANET observations, and which did not enter our
modeling procedure in any way. Available data sets thus indicate that
the light
curve of MACHO 97-BLG-41 can be modeled as a source crossing two
caustics of a
physically-realistic rotating binary so that, contrary to a recent
suggestion,
the additional effects of a postulated planetary companion to the binary
lens
are not required.
The 24-Hour Night Shift
Astronomy from Microlensing Monitoring Networks
Penny D. Sackett
Preprint no. 291
Invited Target Talk at
Gravitational Lensing: Recent Progress and Future Goals on
28 July 1999, Boston, Massachusetts.
To appear in the ASP Conference Series, eds. T.G. Brainerd and C.S. Kochanek.
Abstract
Scores of on-going microlensing events are now announced yearly by the
microlensing discovery teams OGLE, MACHO and EROS.
These early warning systems have allowed other international
microlensing networks to focus considerable resources on intense
photometric --- and occasionally spectroscopic --- monitoring of
microlensing events. Early results include:
metallicity measurements of main sequence Galactic bulge stars;
limb darkening determinations for stars in the Bulge and
Small Magellanic Cloud;
proper motion measurements that constrain microlens identity; and
constraints on Jovian-mass planets orbiting (presumably stellar) lenses.
These results and auxiliary science such as variable star studies
and optical identification of gamma ray bursts are reviewed.