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Дата изменения: Mon Jun 12 19:04:27 2006 Дата индексирования: Mon Oct 1 21:00:19 2012 Кодировка: Поисковые слова: ori |
Galactic Center, star formation, proper motions, variability
The crowding of stars at the Galactic Center and their heavy obscuration
makes the Galactic Center a very attractive target for NICMOS
observations. From groundbased data, we already know that the Galactic
Center harbors a massive black hole (
)
and
that there are stars with ages significantly less than
years. The NICMOS instrument team has chosen to focus on projects that
take advantage of the extremely stable point spread function which
should permit both very accurate position measurements and very accurate
photometry, even at the shortest wavelengths. The presence of a P
filter is also of interest for studying the ionized gas at the
Galactic Center. Because much of the NICMOS data on the Galactic
Center has been taken recently with several projects still awaiting
completion, this report will focus on describing the observing strategies
and some of the problems that will need to be solved to extract the
maximum return from the NICMOS data.
As originally planned, the NICMOS program to measure proper motions of
Galactic Center stars would have been executed over the projected five-
year lifetime of NICMOS. Because of the thermal short which has reduced
the lifetime to two years, this program has had to change its observing
strategy. The team had hoped to observe with the same stars always hitting
the same pixels, but the desire to acquire a number of repeated
observations has necessitated relaxing this requirement. Groundbased
data suggest that some velocities may be as high as
1000 km/sec,
much higher than the 100 km/sec that the program was designed to
measure and so NICMOS may still contribute to this area in spite of its
shortened lifetime.
The observing strategy is based on a compromise between trying to achieve
the highest possible spatial resolution while still penetrating the dust
that obscures the Center. A filter with a modest width to avoid as
many color problems in the astrometric solutions was also desirable.
The team has chosen the F145M filter with camera 1 which as .042" pixels
as the optimum choice. As a check against possible systematic effects,
a globular cluster, NGC4147, which lies at a distance which results in
similar exposure times as for the Galactic Center, is also being
observed. This cluster has a very small internal velocity dispersion
so no proper motions should be seen. The first epoch data were taken
in June, 1997, as the first NICMOS guaranteed time program to execute.
The stars close to SgrA* have S/N
50:1 in this data set. Four
more sets of data will be taken before NICMOS exhausts its cryogen.
Calibration programs with NICMOS are revealing that it is a very stable
instrument from a photometric point of view with monthly repeats of
standards showing less than 2% spread when flat fielded using NICMOS'
internal lamp (Colina, this conference). The stable point spread function
also will contribute to the ability to make accurate and repeatable
measurements in crowded fields. The NICMOS team is looking for evidence
of stellar variability, possible light flashes or variations due to
variable accretion into SgrA*, or variations due to micro-lensing.
The data that have been obtained to date have not been examined
carefully but do not reveal variations at the level of
30
or
more in exposures 8 minutes in length repeated for nearly an
hour. Much longer timescales will be probed when the completed data
set is available.
This portion of the Galactic Center program is the only part where
data collection is complete, but it is the program requiring the
highest degree of confidence in the flux calibration and the understanding
of the NICMOS photometric system. This project is meant to probe the
history of star formation at the Galactic Center. The presence of M supergiants
such as IRS7 and Wolf-Rayet and/or LBV stars such as the IRS 16 stars
strongly suggests that star formation has occurred recently. The
strong and distributed P
flux also suggest the widespread presence
of hot stars. If NICMOS can be calibrated to the goal of 2
accuracy and
if the interstellar extinction law can also be determined with adequate
precision along this line of sight, then multi-color photometry with
NICMOS can be used to deredden the stellar population and place the
stars on a color magnitude diagram. The bluest stars representing
the tip of the main sequence from the last epoch of star formation
should be apparent - for example, O8-O9 stars should have an apparent
magnitude around K
.
The difficulty will be in proving that
the colors are well enough determined, especially at the shortest
wavelengths where such stars will be about 7 magnitudes fainter in
the F110M filter.
Preliminary analysis of the data reveals a color magnitude diagram with both a clump of blue stars with the correct properties to be the tip of the main sequence and a clump of red stars which are likely to be red giants, another population expected at the Center. The stars closest to SgrA* are all very blue, and only further work will resolve whether they have unusual properties due to being close to a black hole or whether their colors reflect their being ordinary O stars. This preliminary work has also shown that a careful determination of the extinction law in NICMOS filters will be required, with the IRS 16 stars playing a key role in this determination since their spectral types and hence intrinsic colors are now well understood.
The U.S. National Aeronautics and Space Administration is thanked for the funding which permitted the construction and operation of NICMOS.