TOWARD A CENSUS

OF VARIABLE STARS IN

NORTHERN LOCAL GROUP DWARF IRREGULAR GALAXIES

J M. S1, C A. G2, U H1,2

(1) M P I E P, G; (2) U M

{, , } @ .-.

A:

Dwarf galaxies in the local group provide a unique astrophysical laboratory. Especially, they allow us to probe pulsating (and other) variable stars in low-metallicity environments with abun-dances below that of the SMC.

Our observing program, described in detail by another contribution (C. Gössl), yields a large number of intrinsically bright variables which can serve as probes of the stellar population and star formation history of these galaxies. Most promi-nent are pulsation variables like Miras and delta Cep stars, but we also find other type of variable stars, e.g. RV Tauri stars, irregular red variables etc.

We present a first census for the three galaxies DDO 216, Leo A and GR8.

T G S:

We selected a sample of six local group irregular dwarf galaxies which are visible with the 0.8 m telescope of our institute at Mt. Wendelstein.

Three color composite of the Leo A dwarf galaxy, © C. A. Gössl.

The observations so far were carried out in R and B-Band sparsely sampling a three year period starting with test observation in 1999. This part of the data set consists of approximately 80 indi-vidual epochs and is sensitive for long period variable stars with periods up to ∼〜～500 days. Ad-ditional observation in R, B and I-Band were ob-tained during 3 observing campaigns at the 1.23 m telescope on Calar Alto densely sampling three two week long periods. These observations pro-vide a ground for a search for variable stars with shorter periods ranging from ∼〜～1.5 days up to ∼〜～10 days. The depth of an individual epoch is roughly around 22 - 23 mag in the R-Band.

I R:

The acquired data was bias subtracted, flat-fielded and cosmic rejected, at the same time propagating the error of each pixel. After that the images from one night were astrometrically aligned to a common reference frame and com-bined with individual weights proportional to the S/N. For each epoch, consisting of the stacked images from a night, a difference image against a common deep reference frame was created using an implementation (Gössl & Riffeser, 2002) of the Alard algorithm (Alard & Lupton, 1998), still propagating the individual pixel error. These dif-ference images were, in a final step, convolved with a stellar PSF. The short period variables were detected using an implementation of the Lomb algorithm (Scargle, 1982). For the LPVs the Lafler-Kinman (Lafler & Kinman, 1965) sta-tistic was applied.

C S:

To obtain a measure, of the completeness of the resulting catalogue of variable sources, we carried out an extensive set of simulations, covering the complete set of relevant parameters, magnitude, period and amplitude.

1 0.8 0.6 0.4 0.2 0

18 19 20 21 22 23 24 0 50 100 150 200 250 300 350 400 450

1 0.8 0.6 0.4 0.2

0

R [mag] period [days]

Completeness simulation for variables with a cosine shaped light-curve and an amplitude of 1 mag, using the Lafler-Kinman statis-tic. The break-in for periods of about one year, stems from the visibility of only half the light-curve due to our half-year observing window.

The tests were conducted using a sample of nearly 900 artificial sources. As shape of the light-curve, both a cosine and a sawtooth were used, to test both a ideal, and a worst case.

1 0.8 0.6 0.4 0.2 0

18 19 20 21 22 23 24 0 20 40 60 80 100 120

1 0.8 0.6 0.4 0.2

0

R [mag] period [days]

Completeness simulation for variables with a cosine shaped light-curve with an amplitude of one mag, using the Lomb algorithm.

The simulations show, with exception of the limi-tation for one year periods, no notable deficiency.

P V C:

So far, we finished analyzing three galaxies from our sample. Table 1 gives a short overview of the amount of detected variables.

20.4

20.5

20.6

20.7

20.8

20.9

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

R [m

ag]

phase

WSTCA

Light-curve for a δ Cep variable in Leo A with a period of 6.49 days. The blue points are observations from Mt. Wendelstein, the red points from Calar Alto.

δ Cep LPVs

Leo A 14 16

DDO 216 10 47

GR 8 1 1Preliminary census of the detected variables. The Cepheids were detected using the Lomb algorithm, the LPVs using the Lafler-Kinman statistic. Sources detected by both algorithms, were used for cross-checking.

18.45

18.5

18.55

18.6

18.65

18.7

18.75 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

R [m

ag]

phase

WSTCA

Light-curve for a LPV in Leo A with a period of 74.9 days.

O:

The next step will be the final reduction and evaluation of the remaining dwarf galaxies, and a scientific analysis of possible SFHs of these, us-ing the found LPVs and the completeness simu-lations.

21

21.5

22

22.5

23

23.5

24

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

R [m

ag]

phase

WST

Light-curve for a LPV with 255 days period in the pegasus dwarf galaxy (DDO 216).

R:Alard & Lupton 1998, ApJ, 503, 325

Gössl & Riffeser 2002, A&A, 381, 1005

Lafler & Kinman 1965, ApJS, 11, 216

Scargle 1982, ApJ, 263, 835

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