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
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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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