Predicting Gaia Observations from a Star-Count Model

Baltic Astronomy, vol. 8, 171-179, 1999.

PREDICTING GAIA OBSERVATIONS FROM A STAR-COUNT MODEL

J. Torra1'2, B. Chen1, F. Figueras1, C. Jordi1 and X. Luri1 1 Departament d'Astronomia i Meteorologia, Universität de Barcelona, Avda. Diagonal 64 7, E-08028 Barcelona, Spain 2 Institut de Estudis .Espacials de Catalunya, Edifici Nexus 104, Gran Capita 2-4, E-08034 Barcelona, Spain Received January 25, 1999.

Abstract. Our star-count Galaxy model (Chen 1997) has been updated and used to estimate the number of stars of different galactic populations (disk, thick disc, halo) that can be observed by Gaia, and to assess the observing conditions (background and crowding). Statistical distributions are presented, and the important role of interstellar extinction is discussed. We use the procedure of Lindegren (1998a) to compute the Gaia astrometric precision of parallaxes and proper motions as a function of position and magnitude of the star. Then we can estimate the distribution of the expected errors on the measured distances and tangential velocities. Key words: Galaxy: stellar content - astrometry - orbiting obser- vatories: Gaia

1. INTRODUCTION

The models of the Galaxy from the less complex, the star-count models, to the complete evolutionary models aim to give a portrait of the Galaxy, and a comparison of the predicted and the observed star distributions allows determining parameters describing its structure. Star-count models are the "static" models that provide the expected number of stars of a given magnitude in a given direction via the integration of the fundamental equation of stellar statistics. To do this, the density law and the luminosity function for each component together with the interstellar extinction law are needed. 172 J. Torra, B. Chen, F. Figueras, C. Jordi, X. Luri

The two component model (disk and halo) of Bahcall & Soneira (1980) is a good and widely used example of this kind of models. It was modified by Gilmore (1984) to include a new component, the thick disk, to explain the observed discrepancies. A consider- ation of the kinematic properties of each population in the Galaxy model (Ratnatunga et al. 1989, Mendez & Van Altena 1996, Chen 1997), leads to prediction of the distribution of proper motions and radial velocities. Population synthesis models result in a complete simulation of the galactic evolution, then, luminosities and colors are decoupled from the age-density distribution. Among others, the models of Robin & Creze (1986), Haywood (1994) and the Padova group (Ng 1995) belong to this category. With the aim to evaluate: (1) the number of stars involved and/or observable in the Gaia mission, (2) the composition of the observed samples, (3) the influence of error distribution on the kinematic parameters and (4) the overlapping, crowding and background effects, we have updated the Chen (1997) star-count and kinematic model. In Section 2 we give a short description of the model. The all-sky figures and the results for some particular directions are presented in Section 3, where our predictions are compared with observational data and the results from other star-count models. The statistics of the precision of parallaxes and tangential velocities is presented in Section 4.

2. STAR-COUNT GALAXY MODEL INPUTS

Our star-count Galaxy model contains three main components: disk, thick disk and halo, whose structural parameters and the sources of luminosity functions are given in Table 1. A detailed treatment of the halo and disk white dwarfs, described by Figueras et al. (1999, this volume), is also included. Our computations can provide the number of stars as a function of V or I magnitudes as well as of B-V or V-I colors. The V-I color index has been derived from My after a careful search and comparison of different calibra- tions available. Using V and V-I, the photometric transformation established by Lindegren (1998b) allows us to obtain the number of stars as a function of G, the apparent Gaia magnitude. Three different extinction laws have been considered: first, we used the Arenou et al. (1992) law, modeled from the Hipparcos Input Catalogue contents, designed to work up to distances ~2 kpc. Later on, we have included the more recent model of Hakkila et al. Predicting Gaia observations from star-count model 173

Table 1. Main structural parameters of our Galaxy model.

Disk Population Luminosity Function Wielen et al. (1983) Density law Exponential Scale length 2250 pc Scale height a function of MV Thick Disk Population: Luminosity Function Wielen et al. (1983) Density law Exponential Scale length 3250 pc Scale height 1070 pc Local normalization 2% Halo Population: Luminosity Function M3- Density law de Vaucouleurs (1977) Axial ration 0.85 Local normalization 0.125%

(1997), based on combination of the previous models (including the model of Arenou et al.) and incorporating statistical corrections for large distances. However, as it was shown by Chen et al. (1999a), the model is unreliable at distances larger than 5 kpc. So, trying to overcome the observed discrepancies, we have developed a new model, Chen et al. (1999a), based on the Cobe/Iras all-sky reddening map. A comparison of the predicted star counts with the real data shows that this new model gives reliable reddening estimates even at large distances. Unfortunately, its inclusion into the Galaxy model is not finished, and we can use it only in some specific directions.

3. NUMBER OF STARS OBSERVED BY GAIA

In Table 2 the number of stars predicted to be observable by Gaia using Hakkila et al. (1997) and Arenou et al. (1992) extinction laws is presented. The impressive total number of ~1.5 billion stars (about 1-2 % of total stellar content of the Galaxy), observed during a five year mission, is one of the demanding figures to be retained. As Table 2 shows, the extinction is one of the most critical points in 174 J. Torra, B. Chen, F. Figueras, C. Jordi, X. Luri

Table 2. Total number of stars observable down to G = 20 mag.

Hakkila et al. (1997) Arenou et al. (1992) extinction law extinction law

Disk MS 1125.9 X106 1297.3 X106 Disk giants 96.9 X106 102.2 X106 Thick disk 146.4 X106 161.7 X106 Halo 89.4 X106 95.4 X106

Total 1458.5 X106 1656.7 X106

Table 3. Comparison of star counts in some dense regions (N is the number of stars per square degree brighter than magnitude i); NQ: towards the galactic center; N^W- in Baade's Window.

I log No log NBW log N0 log NBW

Gilmore (1984) Paczynski et al. Our model model (1994) model

15 4.06 4.67 4.19 4.71 16 4.38 5.08 4.57 5.07 17 4.67 5.29 4.93 5.40 18 4.99 5.50 5.26 5.73 19 5.34 5.98 5.58 6.20 20 5.73 6.45 5.87 6.63

the computation of the numbers of observable stars. The differences of the estimates reach about 10-15%, i.e. about 200 million stars. We find, as expected, that the variation of parameters involving the thick disk (i.e. changing the scale length or the scale height, combined with the local normalization factor) gives smaller effect on the results than the ambiguity of the extinction. Fig. 1 shows the number of stars as a function of the observed V-I color index. As expected, ~53% of the stars have V-I >1.5 mag. In this context, we recognize our poor knowledge of the faint end of the thin disk luminosity function as a drawback in the Predicting Gaia observations from star-count model 175

V-I

Fig. 1. Distribution of the total number of stars with G< 20 mag as a function of the observed V-I color index. Bins of 0.1 mag in V-I and Hakkila et al. (1997) extinction law have been considered.

Fig. 2. Stars per square degree and per 0.5 visual apparent magnitude bin in the directions: (£,b) = (0°,2°) (left) and (£,b) = (90°,2°) (right).

computation of the number of cool stars to be observed by the satellite. Given the huge amount of faint main sequence stars to be observed, Gaia will definitely set the in situ full luminosity function for all galactic populations. 176 J. Torra, B. Chen, F. Figueras, C. Jordi, X. Luri

In this paragraph we use the I apparent magnitude to compare our model predictions with observational data and the Gilmore (1984) star-count model. The comparison is performed in two areas, Baade's Window and the galactic center, complicated both for the satellite operation and for model predictions. Table 3 shows that the observational results of Paczynski et al. (1994) for Baade's Window are well fitted by our model using the Hakkila et al. extinction law. According to the present definition of the mission and the detection system, Gaia can manage these huge numbers of stars without diffi- culties. In the direction of the galactic center, Gilmore's model and our model give quite similar predictions although a more detailed treatment is needed. The new Cobe/Iras reddening map (Schlegel et al. 1998), with a high spatial resolution, allows the improvement of the extinction estimates. Chen et al. (1999a,b) have shown that this is true if the region with | b |< 2° is avoided. In Fig. 2 we compare the simulated star counts for two regions at b = 2°. The differences are large enough to require a new extinction law to be included in our Galaxy model for future predictions.

4. PRECISION OF PARALLAXES AND TANGENTIAL VELOCITIES

Due to the scanning law of the Gaia satellite, very similar to that used by Hipparcos (ESA SP-1200), the errors of the astrometric parameters depend on the G magnitude of the star and on its ecliptic coordinates. Lindegren (1998a) gives the error in parallax and proper motion as a function of these variables. From these estimates we can derive the error distribution in parallaxes, proper motions and tangential velocities for the stars simulated by our model. The errors of tangential velocities have been computed by propagating both parallax and proper motion errors. From Fig. 3, where three representative galactic directions are plotted, we can conclude that for all directions the stars brighter than V = 14.5 mag will have relative parallax error <10%. The fraction of stars satisfying this condition decreases to 20-50 % at V = 17.5 mag, depending on the galactic latitude. This means that at the end of the Gaia mission, astronomers will have at their disposal millions of stars (all types and populations) with distance errors less than 10%. Predicting Gaia observations from star-count model 177

Fig. 3. Percentage of stars with G < 20 mag with relative error in parallax smaller than 10 % as a function of V magnitude for three different galactic directions. Solid line: (£,b) = (0°,2°), dotted line: (¿,b) = (0°,30°), dashed line: (£,6) = (0°,90°).

Fig. 4. Cumulative number of stars with G <15 mag (left) and G< 20 mag (right) as a function of the standard error of tangential velocities. 178 J. Torra, B. Chen, F. Figueras, C. Jordi, X. Luri

The results for tangential velocities are presented in Fig. 4 for two limiting Gaia magnitudes. For G < 15 more than 85% of stars will have errors <5 km/s, and 75 % of them will have errors <2 km/s. When a limiting magnitude of G = 20 is reached, for the galactic latitudes higher than 5-10°, 40 % of stars observed by Gaia will have tangential velocities precise to <10 km/s. These figures are a challenge for the measurement of radial velocities by the Spectro telescope within the Gaia instrument package. If we want to achieve the same precision both in radial velocities and in tangential velocities (necessary for kinematic and dynamical studies), then the Spectro instrument should be able to give radial velocities with the precision better than 5 km/s at least down to G = 15 mag.

5. CONCLUSIONS

We have used our galactic structure and kinematic model to analyze the samples of stars which the Gaia mission will be able to observe. We have estimated the number of stars belonging to the three galactic populations (disk, thick disk and halo). At the present level of definition, the satellite can accomplish the required observations down to G = 20 mag, thus the main scientific goal, a complete census of the Galaxy, can be achieved. We find that the extinction is the most limiting parameter for our predictions. To overcome this problem, we have proposed a new extinction model (Chen et al. 1999a). In the present paper we use this new model in some specific galactic directions. The error distribution derived in the present work for distances and tangential .velocities of the stars accessible to Gaia, together with the possibility of determining the radial velocity component by the Spectro instrument, make Gaia the first mission providing a very accurate map of the Galaxy in a six-dimensional phase space. Moreover, as Gaia will also provide information on metallicities and ages, our knowledge on the structure, formation and evolution of the Galaxy will be drastically improved.

ACKNOWLEDGMENT. This work has been supported by the CICYT under contract ESP97-1803. Predicting Gaia observations from star-count model 179

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