NICMOS Observes the Galactic Center

The NICMOS team has devised a comprehensive program of observations of the Galactic Center. The suite of observations include those aimed at measuring proper motions, at determining the spectral type at the tip of the main sequence from the last episode of star formation, at searching for variability near SgrA^*, and at looking at the distribution of ionized gas. Data in hand give a first look at variability and a detailed look at the search for the tip of the main sequence will be discussed where NICMOS has demonstrated adequate sensitivity and photometric precision to identify likely O8-B1 stars at the Galactic Center.

Introduction

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 ( $\sim 2 \times 10^6 M_\odot$ ) and that there are stars with ages significantly less than $50 \times 10^6$ 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 $\alpha$ 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.

Proper Motion Studies

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 $\sim$ 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 $\sim$ 50:1 in this data set. Four more sets of data will be taken before NICMOS exhausts its cryogen.

Variability Studies

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 $\sim$ 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.

Observations of P $\alpha$

Search for the Tip of the Main Sequence

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 $\alpha$ 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 $\sim 13-14$ . 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.