Variable star survey in the Magellanic Clouds
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Variable star survey in the Magellanic Clouds -- Ten-years-long NIR monitoring observations --. Yoshifusa Ita & IRSF/SIRIUS variable star survey team email: [email protected] Abstract.

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Variable star survey in the Magellanic Clouds

-- Ten-years-long NIR monitoring observations --

Yoshifusa Ita & IRSF/SIRIUS variable star survey team

email: [email protected]


We started a near-infrared (JHKs) variable star survey in the Large and Small Magellanic Clouds in December 2000. Since then, we keep monitoring an area of 3 square degrees along the bar in the LMC, and also an area of 1 square degree in the central part of the SMC. In the last 10 years, we observed these areas about 80-90 and 100-110 times for LMC and SMC, respectively. As a result, we obtained time series data with more than 3,000 days baseline for both clouds. Our survey is the world's first and only one that provides near-infrared time-series data with such a long baseline.

Why in the NIR?

Our survey is operated in the near-infrared bands, so it can detect infrared very red variables that are difficult to detect in existing optical surveys, such as OGLE (Udalski 2003). Figure 1 shows an infrared variable detected in our survey. It is obvious that the star is faint in the optical, but is very bright in near and mid-infrared wavelengths. The mid-infrared image is taken by AKARI satellite.




Figure 1

A panchromatic image of an infrared variable detected in our survey.

Discovery Spaces / Finding limits

1. What can we discover with our infrared survey?


Figure 2 is the I-K vs. K color-magnitude diagram of variable stars in the LMC. Relevant data is taken from Ita et al. (2004). The horizontal line shows the 10 sigma detection limit for our monitoring survey at K band, which is about 15.5 magnitude. The right diagonal line stands for the 10 sigma detection limit of the OGLE survey at I band, which is about 19.5 magnitude. Also, the left diagonal line shows the saturation limit of the OGLE survey at I band, which is about 13.0 magnitude. Therefore, all of the variable stars in the middle area are not only detected by ours, but also by OGLE optical survey. Our discovery spaces are red and bright part of this diagram. Our survey can discover infrared variables and also bright variables. However, most of the bright variables may have been discovered by the IRAS-oriented studies.

2. Specifications of our survey.

Figure 2

A I-K vs. K color-magnitude diagram of optical variable stars in the LMC showing our discovery spaces.

  • Observations are made with 5 sec * 10 dithering mode

  • 10 sigma detection limit is about 15.5 mag in K-band.

  • How bright/faint variables can our survey detect?

  • Bright detection limit: saturate at 8 mag in K-band

  • Faint detection limit: depends on the amplitudes of variable stars, and also detection limits.

    • +We certainly detected classical cepheids with ΔK of about 0.3 mag that are about 15 mag at K (with pulsation period of a few days).

    • -Very sadly, we could not detect extremely red objects with tau_v ~ 200 (discoveried by Spizer/SAGE survey, Gruendl 2008) whose K band luminosities are fainter than 17 mag.

Tentative results from the SMC survey

We developed a period determination software that uses lomb-scargle periodgram. We calculated

for trial periods (2π/ω) from 2 to 2000 days for each detected variable. We took the 1st, 2nd, and 3rd most powerful signals (P(ω)) if their powers have false alarm probability of less than 1 percent.

Figure 3 is the tentative Period-K magnitude diagram made by using the software. This is the result from fully automated analysis, and we did not eliminate any spurious observational data and so on. The K band magnitudes in the ordinate are calculated by 0.5*(K_min + K_max), and not the time-averaged ones. We will calculate the time-averaged mean magnitudes and that would improve the quality of the P-L relations.

Figure 3

Tentative Period-K magnitude relations for SMC variables detected in all JHK bands with their false alarm plobability is less than 1 percent.

Discovery! I&J-drop variable

in the SMC

Variability in the HR diagram.

Figure 4

CMD of the all source detected in the 1 sq. deg survey area in the SMC(left panel ) and their chance of variability(right panel). Note that red AGB stars are 100% variables.



Figure 4 shows a color magnitude diagram of all sources detected in our 1 square degree survey area in the SMC (left) and their chance of variability (right). As you can see, light variation phenomena is ubiquitous in the HR diagram. The color of the left figure shows the percentage of variability in the 0.05mag x 0.05mag bin. I counted all sources in a bin, and then counted the number of variable stars in that bin.

It is quite interesting to see that much of the luminous and blue sources are variables.

We discovered an infrared variable star that is not detected in the OGLE-I band survey. The above figures show the light curve of the infrared variable. It is detected in H and K-band, and is not detected in J band, probably because of the heavy circumstellar extinction.

There are only 1 star like this in our

1x1 square degree survey area in the SMC. The number of infrared variables will be compared with that in the LMC.

Many interesting variable sources are detected.





We detected all kind of variable sources within the survey areas. The figures shown above are some examples. The upper panel shows the light curve in J(bule), H(green), and K(red) bands, and the lower panel shows the time variation of near-infrared color in J-H(blue), H-K(green), and J-K(red) colors.

We will submit the catalog paper as soon as possible, and the whole data will be published after the acceptance of the paper.