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
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
Preprint 317 21cm Absorption Lines at High Redshift from Intervening Galaxies F.H. Briggs
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
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.
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
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
Preprint 299 The vertical extent and kinematics of the HI in NGC 2403, W.E. Schaap, R. Sancisi, and R.A. Swaters
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
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
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
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
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)
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
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
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.

### 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)

### 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.

 Maintained by:  Gineke Alberts