Search Publications
Solar EUV and UV Emission Line Observations Above a Polar Coronal Hole
Wilhelm, K.; Bodmer, R.
The roll manoeuvre of SOHO on September 3, 1997 provided the opportunity to study the northern coronal hole with SUMER slits in east-west orientation, so that polar plumes and inter-plume lanes could be observed simultaneously. A preliminary analysis of the observations shows that lines emitted by ions with the lowest formation temperatures (with …
The Structure of Cometary Knots in the Helix Nebula
O'Dell, C. R.; Burkert, A.
The Hα surface brightness distribution of cometary knots in the Helix Nebula is investigated. The brightness profiles of their photoionized surfaces (cusps) drop exponentially with increasing distance r from the center of the knots: Σ(r) ~ exp (-r/h). An r-3 profile, which would be expected for a freely expanding ionization region, in g…
The acoustic cut-off frequency of roAp stars
Provost, J.; Morel, P.; Weiss, W. W. +2 more
Some of the rapidly oscillating (roAp) stars, have frequencies which are larger than the acoustic cut-off frequency determined from published stellar models which usually assume a grey atmosphere. As the cut-off frequency depends on the T(tau ) relation, we have computed models and adiabatic frequencies for pulsating Ap stars with more realistic a…
Unexpected stellar velocity distribution in the warped Galactic disk
Lattanzi, M. G.; Drimmel, R.; Smart, R. L. +1 more
It is now over 40 years since radio observations of neutral hydrogen revealed the gaseous disk of our Galaxy to be warped. Subsequently, the warp has been detected in the distribution of Galactic dust, molecular clouds, and luminous stars,. Roughly half of all spiral galaxies have similarly warped disks, which suggests that warps are a common and …
A Color-Magnitude Diagram for a Globular Cluster in the Giant Elliptical Galaxy NGC 5128
Harris, William E.; Harris, Gretchen L. H.; Poole, G. B.
The Hubble Space Telescope has been used to obtain WFPC2 V, I photometry for a large sample of stars in the outer halo of the giant elliptical NGC 5128 (d ~= 4 Mpc). The target region is located at a projected distance 21 kpc from the center of NGC 5128. The globular cluster N5128-C44, at the center of the Planetary Camera field, is well enough re…
A double-frequency dwarf nova oscillation in OY CAR
Horne, Keith; Marsh, T. R.
We have detected coherent oscillations (`dwarf nova oscillations') in Hubble Space Telescope spectra of the dwarf nova OY Car. The oscillations were seen towards the end of a superoutburst of OY Car. The oscillations are extraordinary compared with the many other examples in the literature for two reasons. First, their amplitude is large, with a p…
Acoustic Signatures of Subphotospheric Structure Underlying Sunspots
Braun, D. C.; Lindsey, C.
Helioseismic holography of active regions at frequencies in the range of 5-6 mHz renders acoustic signatures that we think signify acoustic perturbations several megameters beneath the photosphere. The application of holographic diagnostics at 5-6 mHz gives us images with considerably finer horizontal spatial resolution, and likewise much finer de…
The symbiotic system AG Dra: an unexpected photometric period
Bastian, U.
A photometric period of about 380 days is found in 6810 visual magnitude estimates of AG Dra. This is unexpected, since the orbital period of the system is about 552 days. The period is confirmed by Hipparcos photometry. The new periodicity provides an additional constraint on physical models for AG Dra. Detailed observations before the end of the…
ISO Detection of CO(+) toward the protostar IRAS 16293-2422
White, G. J.; Ceccarelli, C.; Caux, E. +4 more
In this letter we report the detection of eight high-N rotational transitions of CO(+) towards a low mass protostar, IRAS 16293-2422. The source was observed with the Long Wavelength Spectrometer on board the Infrared Space Observatory. This is the first time that CO(+) has been detected in a low luminosity source and the first time that high-N li…
Hubble Space Telescope WFPC2 Imaging of the Seyfert Galaxy NGC 3516
Mulchaey, John S.; Ferruit, Pierre; Wilson, Andrew S.
We have used the WFPC2 camera aboard the Hubble Space Telescope to obtain images of the Seyfert galaxy NGC 3516 in the [O III] λ5007 and [N II] λλ6548, 6583 + Hα emission lines as well as in their adjacent continua. The emission-line images reveal a close morphological association between an arclike structure 1.7" north of the nucleus and radio em…