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.