UVCS Stars, Comets, Planets Working Group Report

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This working group covers a range of topics including stellar observations for calibration or for astrophysical purposes, Sun-grazing comets, and studies of the F-Corona and interplanetary Ly . A handy reference to get an idea of existing UV spectra of these objects is "Exploring the Universe with the IUE Satellite" edited by Y. Kondo.

Membership:

J. Raymond, chair

A. Ciaravella, P. L. Smith, D. Dobrzycka, C. St. Cyr, R. O'Neal, J.Michels, D. Mullan, M. Huber, M. Romoli, S. Fineschi, C. Benna, G. Naletto, L. Gardner, G. Tondello, M. Landini, S. Cranmer

Calibration:

Several people attending this working group are primarily interested in stars for UVCS calibration. That topic is mainly the responsibility of the calibration working group, for whom stars provide checks on radiometric calibration, spatial resolution and co-registration with other instruments, as well as detector flatfields. Eight calibration stars have been observed so far, and a comparable number will be observed over the next month. After that, it will be important to catch Feige 110, the only hot white dwarf which passes within our reach, and to reobserve the stars observed over the last year to check for changes of sensitivity with time. Rai Wu has gotten some of the reprocessed IUE spectra already, and we will use it to reanalyze our spectra. There is a chance that the ORFEUS telescope will observe Leo, a star used by SUMER and UVCS for radiometric calibration, or Tau. Unfortunately, Leo is probably too bright for OFREUS to observe. The UBC spectrometer on ORFEUS can cover the 912-1176 range at about 70 km/s resolution.

Because the passage of a star is time-critical, stellar calibration observations require scheduling priority.

Stellar Physics:

Most UV spectroscopy of stars is limited to wavelengths above the cutoff (e.g. IUE, HST). The shorter wavelengths accessible to UVCS include the OVI doublet, generally the highest ionization state observable in the optical-UV range. They also include the bands arising from the lowest vibrational level and strong lines of some elements not easily detectable at longer wavelengths. The competition in this wavelength range is limited to Voyager (18 resolution, limited to fairly bright stars), the Hopkins Ultraviolet Telescope (3 resolution, 35 effective area; 2 shuttle missions carried out, none planned for the future), Copernicus (the high spectral resolution satellite of the early 70's, limited to bright stars, mostly uncalibrated spectra were published), ORFEUS (one shuttle mission completed and one more underway; three spectrographs, one with resolution comparable to UVCS), and FUSE (a satellite planned for 2 years from now with bigger collecting area and better spectral resolution, designed for interstellar absorption line studies). The unique advantages of UVCS, aside from the fact of being in orbit, are our ability to observed uninterrupted for 10 hours and our sensitivity to diffuse emission. Our disadvantages (aside from that fact that stars are not our first priority) are the observing window of a few days for each star and the relatively small effective area (0.1 ). We cannot really compete with IUE and HST in studies at longer wavelengths, except possibly for very extended objects or when our ability to observe close to the Sun could be crucial, such as a nova declining from outburst as it approaches the Sun.

The easiest stars to observe are the relatively bright O and B stars. The most obvious feature of interest is the O VI P Cygni profile formed in the winds of O stars and some early B stars. The unanticipated presence of O VI ("superionization") is generally attributed to photoionization by X-rays, but shock heating may also be important. In particular, the P Cygni profiles in the C IV line observed with IUE show absorption features which form and shift to higher velocities over time ("Discrete Absorption Components"), and these may be associated with shocks, wind material modulated by strong nonradial stellar pulsations, or density waves in circumstellar disks. The 5 days that a star stays within our field of view are enough to search for velocity shifts. A candidate for such study must be chosen carefully based on Copernicus spectra of stars of similar spectral type, avoiding those with deeply saturated P Cygni profiles. It may also be worthwhile to observe Be stars and other peculiar B stars. Some of the old Copernicus papers should be looked at to see what new things we might add. Copernicus did observe quite a few stars at these wavelengths, but the theoretical picture has changed a lot since then.

Cataclysmic variables are fainter, but there are at least 5 which come within 10 solar radii, one of which (TT Ari) we briefly observed. HUT has observed about 10 at 3 resolution, ORFEUS has observed 3 and will probably observe several more. CV orbital periods of 2-5 hours make our continuous observations vastly preferable to those with low-Earth satellites, but our small effective area limits our time resolution. Major questions are the strength of the O VI emission line in low mass transfer rate () systems, the strength of O VI P Cygni profiles from accretion disk winds in high systems, the temperatures of the hot white dwarfs in low systems, and the unexpected flattening of the accretion disk spectrum at wavelengths corresponding to our OVI channel. The Intermediate Polar FO Aqr is a good target for looking for the 20 minute white dwarf spin period. U Gem is the best-known CV available to UVCS. It has been observed with HUT, but higher resolution spectra would be valuable for getting the white dwarf parameters.

Cool stars have been observed extensively with IUE. Again the O VI line which bridges the enormous gap between N V (200,000 K) and X-rays (above 1,000,000 K) is probably the most interesting feature. One might also use the C III 1176/977 density diagnostic or look for the [Fe XVIII] line at 974 . It may be difficult to find a cool star with bright enough emission lines to be worthwhile. An interesting variant is V471 Tau, an eclipsing binary composed of a moderately hot white dwarf and an active K star. Near eclipse, material apparently trapped in magnetic regions above the K star surface creates absorption lines in the WD spectrum. Cool giants and supergiants generally do not show high excitation emission lines, but may show fluorescence.

At least 4 planetary nebulae pass within the UVCS field. They may be interesting as diffuse sources of narrow (20 km/s) emission lines for calibration. It is also expected that those with very hot central stars will produce O VI. It is important to find out more about the candidate PN.

Interplanetary Ly:

SWAN is observing the intensity of solar Ly photons back-scattered from the local interstellar gas. We have evidence that the line is broader than expected, and recent HST observations and theory suggest that this may be the "wall" where the interstellar gas encounters the heliosphere, possibly forming a shock. We can get a profile of this emission, but it takes a while. The profile should vary with angle between the line-of-sight and the direction of solar system motion through the ISM. We should get several observations through the year.

F-Corona:

If we can observe cool chromospheric emission lines scattered by dust in the F-Corona, we can derive the size distribution of the dust particles by comparison with the optical scattering. LASCO has seen a difference in the ratio of North and South polar F-Corona brightness depending on whether SOHO is above or below the ecliptic plane. It is important to sort this out in order to understand the F-Corona contribution and to separate it from instrumental scattered light. Observations above the poles with detector masks covering C III 977, O I 1300 and Si III 1206 will allow us to figure this out if the N-S contrast is high enough and if the UVCS pointing is known accurately enough. Late December is a propitious time in that SOHO is near its maximum distance below the ecliptic plane.

Interstellar or Coronal Absorption Lines:

FUSE is being built largely to study interstellar absorption lines in the OVI channel range. Our spectral resolution is not quite good enough for some ISM projects, but measurements of the O VI absorption lines and bands offer some exciting possibilities. We should keep in mind that FUSE will do far better a few years down the road, but if there is unique science we should do it. Before proposing something, we should look carefully at the Copernicus ISM papers.

In principle we might detect O VI absorption in the Solar Corona. Optical depths predicted in the Yellow Book Stellar Observations page are only 0.02. Higher optical depths may be found in streamers, but it is hard to get as much exposure time if the star does not pass over the pole. This will be a great observation if we can do it, but it will be very hard. The O VI column density drops quickly with height, and our occulter cuts down the count rates severely at small heights.

Sun-Grazing Comets:

LASCO has already discovered several Sun-grazing comets. They can give us about 1.5 day warning, but we have to find something moving very fast, and we only have about a day as it plunges in. A Sun-grazer could be spectacular in our wavelength range. Molecules boil off, dissociate into atoms, then ionize, producing all kinds of remarkable spectral features. Lacking other knowledge, we might expect , CO, O I, C I and lots of other things. Given our ignorance of the spectrum, we ought to try a full detector mask, binned spatially. This program is not only time-critical but also unpredictable. We need to establish its priority and an idea how bright a Sun-Grazer is worth a shot.

Supernova Remnants:

Who could resist? The Crab Nebula comes by in June. Our large area on the sky compensates for our small effective area. IC443 also comes close enough.

Planets:

UVCS observed Venus when it passed close to the Sun in June, but basically saw only a dark spot as it occulted the corona. In principle, we should be able to see fluorescent emission from the illuminated atmosphere, but only a very small sliver is visible to SOHO. The night side is quite faint. HUT has observed planets in the UVCS wavelength range, but not near conjunction, and only at 3 resolution. Planets may be useful for calibration of scattered light. Peter Smith and Peter Bochsler are the contact people

RECOMMENDATIONS:

Stellar calibration observations are necessary to the program. They have been accommodated so far by assigning A. Ciaravella or J. Raymond as Lead Operations Scientist for weeks when important stars come by. As we begin to make 2nd epoch observations of stars previously observed (to look for sensitivity changes with time) we may want only a couple hours, so stellar observations will be slipped in with other observing programs. The Calibration Working Group should provide a schedule of important stars which have priority over 'normal' coronal observations, along with a set of recommended grating positions and detector masks.

For observations of stars for their own sake, proposers should 1) provide enough lead time to allow scheduling, preferably 2 months, 2) provide flux estimates adequate for assessing the feasibility and exposure times, and 3) explain the unique scientific results to be gained by a UVCS observation. As an aid for this, the accompanying file is a list of stars observed by IUE through 1991 which pass within about 10 solar radii. Absurdly faint objects were left out, and it may not be complete even for reasonably bright objects. The dates of closest approach are good to about 2 days. IUE spectra from the archive will give fluxes down to about 1160 .

For Sun-grazing comets, this working group recommends that if LASCO reports an object brighter than 3rd magnitude at 20-25 , it be given priority over regular UVCS coronal observations. At that level we expect a few targets per year. Of course, the Lead Operations Scientist has the option to observe fainter objects. If a JOP is in progress, it will be necessary to assess the importance of UVCS participation in the JOP relative to the comet. The working group will prepare an observing sequence which can be implemented in a hurry when the occasion arises.