7376( 4) - 12/14/98 10:56 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7376 Version: 4 Check-in Date: 14-Dec-1998 10:55:22 1.Proposal Title: Understanding Halo Microlensing and Variable Stars in M31: 1) Snapshot Survey of Candidates ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle SNAP 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Arlin Crotts Columbia University CoI: Dr. Austin Tomaney Columbia University N ------------------------------------------------------------------------------------ 5. Abstract We discovered 6 microlensing candidates affecting stars in M31 and presumably due to sub-solar-mass MACHOs, in a survey of the M31 and Galactic halos. HST can test the veracity of these events by independently finding their baseline magnitudes. HST can also determine the luminosity function in our survey sample and allow us to find the contribution of MACHOs to the dark matter in M31 and the Galaxy and produce strong constraints complementary to other MACHO searches in our Galaxy's halo alone. Our survey is unique in allowing lensing to be studied at locations throughout a galaxy, allowing halo spatial distribution models for the halo to be tested. Furthermore, we can reach unprecendentedly small MACHO masses and determine them much more accurately than in competing surveys. Unfortunately, our lensing sample might be confused with short period miras which mock lensing lightcurves when sampled infrequently. HST can show whether our rejection criteria for these events succeed in differentiating miras from lenses. Since our sample involves stars that are crowded in ground-based images, we also need HST to allow us to determine the luminosity function of stars in the field, thereby determining the number of possible lensing sources, and converting our detected event rates into actual MACHO number densities. Simultaneously we can explored the stellar population in the relatively unexplored disk/bulge transition via its variable stars, luminosity function and color- magnitude diagram. ------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 2] Observations Description ------------------------ Table 1 shows the required (quantized) integration times to reach S/N = 50 for the baseline magnitudes in both bands. Our proposed exposure times consist of two integrations which individually are of the duration listed, or 200 s, whichever is longer, for a total of 7920 s integration. Many of these targets are close enough so as not to require new visits. If observations on the same field could be made as pairs of exposures, one in each band, this would be ideal for the study of variable stars. In addition to the six candidates we have included two objects which are only marginally eliminated in our data as microlensing candidates. It will be valuable to determine whether these two objects have colours and magnitudes similar to our microlensing candidates thereby suggesting these are simply a class of variable star. Our secondary aim is to study stars above crowding limit in the various locations in the M31. The crowding limit for WFPC2 images will be ~ 10 stars per arcsec^2 for stellar FWHM of 0.2''. Hodge Etal (1988 ApJ, 324, 172) find the star count and field surface brightness for stars 100' along the major axis of M31. Scaling this to the surface brightness in our target fields and applying the local Galactic disk luminosity function ( c.f., Mihalas and Binney 1981, GA, Table 4-5) at the Hodge V cutoff, predicts that the above crowding limit corresponds to a V magnitude of 25.5, at surface brightness of Mu_R =20 like those found in the center of our field. (This is slight overestimate since the ~ 40\ will be resolved at these limits. The background light of M31 is well matched by a K0 III star Walterbos 1986, PhD Thesis in our field.) The brightest four magnitudes of the RGs in GCs have mean (V- R) and (V-I) colors of +0.7 and +1.0 respectively ( e.g. DaCosta and Armandroff 1990, AJ, 100, 162). Such magnitudes correspond to M_R ~ +0.5 and $M_I ~ 0.0 RGs for our V = 25.5 cutoff in M31. In contrast we will typically reach S/N = 10 at V=23.5 in our fields. Hence these data will be valuable in constructing the luminosity function and color- magnitude diagram for about ~ 10^5 stars with V<23.5. By studying data in the WF CCDs, we can also do the same for ~ 10^5 stars with V<21. We still need, for particularly high amplification events, to reach V = 25.5, which we propose to accomplish in an accompanying proposal. In addition to PSF fitting photometry of all fields, we propose to use our image subtraction technique to study variables. We present an illustration of this technique from our ground based data (Figure 1) which should also apply to PC images in these red bands since they are sufficiently oversampled. We might also study variables in this way with WF data, but only after degrading the resolution slightly to prevent undersampling artifacts. RR Lyraes are too faint to be studied in this way, but eclipsing variables, cepheids, miras and other long- period red variables can be found in this way. Eclipsing variables are particularly interesting, in that these observations can isolate their baseline magnitude. Together with our VATT data these can then be studied with absolutely calibrated lightcurves over their entire cycle. For those eclipsing variables bright enough to be stellar typed (e.g. main sequence supergiants), these lightcurves provide a reliable distance measure to M31. Real Time Justification ----------------------- None, except that we ask that whenever convenient that F675W, F814W pairs of the same field be taken together rather than double integrations with the same filter on the same field in the same visit. We have observed for 40 nights on the VATT 1.8-m and 4 nights on the KPNO 4-m prior to these HST observations, are scheduled for about 40 nights more in 1996 and expect a long- term commitment for more nights in 1997. These new observations may provide even better targets to replace some of those listed. We plan to request an additional sequence of about 20 nights on the VATT concident with the proposed WFPC2 observations in order to correlate to lightcurves of variable stars with the magnitudes observed by HST. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 3] Data Distribution Paper Products: Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 M31-LENS1 STAR RA=0H 42M 55.7S +/- 0.1S, J2000 V = 22+/-2 DEC=+41D 14' 27.0" +/- 1.0" B-V = 1+/-1 Comments: STAR'S BRIGHTNESS IS ESTIMATED FROM MICROLENSING BEHAVIOR. 2 M31-LENS2 STAR RA=0H 42M 42.3S +/- 0.1S, J2000 V = 22+/-2 DEC=+41D 11' 2.0" +/- 1.0" B-V = 1+/-1 Comments: STAR'S BRIGHTNESS IS ESTIMATED FROM MICROLENSING BEHAVIOR. 3 M31-LENS3 STAR RA=0H 42M 54.1S +/- 0.1S, J2000 V = 22+/-2 DEC=+41D 10' 55.0" +/- 1.0" B-V = 1+/-1 Comments: STAR'S BRIGHTNESS IS ESTIMATED FROM MICROLENSING BEHAVIOR. 4 M31-LENS4 STAR RA=0H 43M 22.6S +/- 0.1S, J2000 V = 22+/-2 DEC=+41D 5' 52.0" +/- 1.0" B-V = 1+/-1 Comments: STAR'S BRIGHTNESS IS ESTIMATED FROM MICROLENSING BEHAVIOR. 5 M31-LENS5 STAR RA=0H 43M 49.0S +/- 0.1S, J2000 V = 22+/-2 DEC=+41D 11' 28.0" +/- 1.0" B-V = 1+/-1 Comments: STAR'S BRIGHTNESS IS ESTIMATED FROM MICROLENSING BEHAVIOR. 7376( 4) - 12/14/98 10:56 - [ 5] Visit: 11 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: FIRST HIGHEST PRIORITY VISIT. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 11 M31-LENS1 WFPC2 IMAGE PC1 F675W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 12 M31-LENS1 WFPC2 IMAGE PC1 F814W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 6] Visit: 12 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 21 M31-LENS4 WFPC2 IMAGE PC1 F675W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 22 M31-LENS4 WFPC2 IMAGE PC1 F814W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 7] Visit: 23 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: EIGHTH HIGHEST PRIORITY VISIT. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 31 M31-LENS2 WFPC2 IMAGE PC1 F675W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 32 M31-LENS2 WFPC2 IMAGE PC1 F814W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 8] Visit: 34 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: THIRD HIGHEST PRIORITY VISIT. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 41 M31-LENS3 WFPC2 IMAGE PC1 F675W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 42 M31-LENS3 WFPC2 IMAGE PC1 F814W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 9] Visit: 35 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: FIFTH HIGHEST PRIORITY VISIT. PLEASE DO NOT PERFORM PRIOR TO OR WITHIN 50 DAYS OF Visit_Number 34. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 51 M31-LENS3 WFPC2 IMAGE PC1 F675W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 52 M31-LENS3 WFPC2 IMAGE PC1 F814W CLOCKS=YES 1 400 S ------------------------------------------------------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 10] Visit: 46 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: SEVENTH HIGHEST PRIORITY VISIT. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 61 M31-LENS4 WFPC2 IMAGE PC1 F675W CLOCKS=YES 1 300 S ------------------------------------------------------------------------------------------------------------------------------------ 62 M31-LENS4 WFPC2 IMAGE PC1 F814W CLOCKS=YES 1 300 S ------------------------------------------------------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 11] Visit: 57 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: SECOND HIGHEST PRIORITY VISIT. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 71 M31-LENS5 WFPC2 IMAGE PC1 F675W CLOCKS=YES 1 300 S ------------------------------------------------------------------------------------------------------------------------------------ 72 M31-LENS5 WFPC2 IMAGE PC1 F814W CLOCKS=YES 1 300 S ------------------------------------------------------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 12] Visit: 58 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: SIXTH HIGHEST PRIORITY VISIT. PLEASE DO NOT PERFORM PRIOR TO OR WITHIN 50 DAYS OF Visit_Number 57. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 81 M31-LENS5 WFPC2 IMAGE PC1 F675W CLOCKS=YES 1 300 S ------------------------------------------------------------------------------------------------------------------------------------ 82 M31-LENS5 WFPC2 IMAGE PC1 F814W CLOCKS=YES 1 300 S ------------------------------------------------------------------------------------------------------------------------------------ 7376( 4) - 12/14/98 10:56 - [ 13] Summary Form for Proposal 7376 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CLOCKS=YES ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category SNAP ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F675W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names M31-LENS1 M31-LENS4 M31-LENS2 M31-LENS3 M31-LENS5 ------------------------------------------------------------------------------------------------------------------------------------