Hubble Source Catalog (HSC) Use Case #2- Time-variable Phenomena

Searching for variable stars in dwarf irregular galaxy IC 1613

Reference:- The ACS LCID project.
                      II. Faint variable stars in the isolated dwarf irregular galaxy IC 1613.
                      Bernard et al. (2010) ApJ, 712, 1259

BACKGROUND:   IC 1613 is one of the Local Group galaxies. It is a highly resolved dwarf of type IB(s)m (de Vaucouleurs et al. 1991). IC 1613 is rich in gas and is actively forming stars; as a consequence it also contains many Classical (Population I) Cepheid variables. Cepheids in IC 1613 were first discovered by Hubble, Mayall, and Baade in the 1930s (as reported by Sandage 1971). It's apparent visual magnitude Mv is ~ -14.7.

Bernard et al. (2010) obtained 24 orbits of ACS observations (see Proposal ID 10505) of a field located about 5' west of the center of IC 1613, in order to reach the oldest main-sequence turnoffs with good photometric accuracy. The data were collected over about 2.4 consecutive days between 2006 August 28 and 30 in F475W and F814W. Each orbit was split into two ~1200s exposures for an optimal sampling of the light curves. They found 259 candidate variables in this field, of which only 13 (all of them bright Cepheids) were previously known, and estimated the distance of IC 1613 to be 760kpc.

Figure 1 (from Bernard et al. 2010 reproduced by permission of the AAS). Shown is the Color-magnitude diagrams of IC 1613 for the ACS (left) and WFPC2 (right) fields. Confirmed and candidate variables stars were plotted, as labeled in the inset panel. The different size error bars in each panel shows the mean photometric error bars at given magnitudes. To show the location of the old main-sequence turnoff, an isochrone from the BaSTI library (Z = 0.0003, 13 Gyr; Pietrinferni et al. 2004) is overlaid in the right panel.

  1. Construct the F475W - F814W vs "composite Vmag" color magnitude diagram (CMD),
  2. Identify variable and non-variable stars,
  3. Use a time-series analysis to determine the period.
Part 1: Download the Data
Use the following criteria:
RA = 01:04:28.39
DEC = 02:09:36.5
Radius = 2.65 arcmin
Mag Type = MagAper2
NumImages > 20
Add to the output Columns A_F475W,  and A_F475W_Sigma, and remove the other non-observed filters from the output.
Make a catalog to download by changing:
                                Maximum Records = 50001
                                Output Format = File: < your choice from the 8 options >
The resulting search returns 9683 rows.
Part 3: Select Variable Stars From the HSC
Of particular interest are the objects that lies between (A_F475W-A_F814W) ~ -0.9 and 0.7 and have VI > 0.1. These are our 210 variable candidates.

Shown above is the Color-magnitude diagrams of IC 1613 for the ACS data, where for the ordinate axis (F475W + F814W)/2 ~V filter combination is used, so that the horizontal branch (HB) appears approximately horizontal. HSC candidate variables are plotted in blue. Bernard et al. (2010) confirmed and candidate variables stars are labeled in the inset panel. The vertical gray dashed lines roughly delimit the instability strip. The magnitudes have been transformed to the VEGAMAG system. Variable star candidates were selected near the instability strip with Variability Index (VI) >=0.1. In total we selected 210 candidate variable stars from the HSC, compared to 259 in the Bernard et al. (2010) paper.

So what is the instability strip?
The instability strip is a narrow, almost vertical region in the HR diagram which contains many different types of variable stars (RR Lyrae, Cepheid variable, W Virginis and ZZ Ceti stars all reside in the instability strip). Most stars more massive than the Sun enter the instability strip and become variable at least once after they have left the main sequence. It is within this region that they suffer instabilities that cause them to pulsate in size and vary in luminosity.

We now need to get the time-resolved data using the HSC Detailed Search Form and the "File Upload Feature". Upload the local saved comma separated values for the candidate variable stars file as follows:-

    Local File Name= File for the 210 candidates identified in Step 4.
    Column Delimiter= ,
    RA/Target/Data ID Column Number= 2
    Dec Column Number= 3
    Resolver= Don't Resolve
    Radius arcmin= 0.018
    Output Columns= add Filter & Wavelength
    Output Format
= File: < your choice from the 8 options >
    Maximum Records per Target= 50

The form will look like this.

Save the resulting file.

  1. Produce finding charts for selected variables in IC 1613. We used the Hubble Legacy Archive website and search around the position (RA & Dec) of each variable star with a search Radius: 0.01 degrees, then select from the Inventory "level  4" to get the color image cutout with window size:10 arcsec (i.e. 200x200 ACS WFC pixels). The finding chart are shown below.
  2. The phased light curves for the candidate variable stars are also included shown below (produced using topcat). To get the obs days, the midMJD was subtracted from 53965.
17759737 17759032 17756492

Non-Variable Candidates over the observed ~2.4 days.
17753613 17764772 17759295

  1. Use the  publicly available Period search service website to determined the period for the light curve. This web page provides two search methods. One is based on Lafler & Kinman (1965) paper. The other is Deeming (1975) Discrete Fourier Transform (DFT) method.
  2. In this example we use the MatchID17753692, F475W light curve which is plotted above. The data can be provided in ASCII format. A minimal and a maximal trial period of 0.1 and 3 days, with an estimate a 0.3 mag max phase shift were used.
  3. The period was found to be 1.3 days, which is equal to the value listed in Bernard et al. (2010) table (4).

  1. We present the results of a new search for variable stars in the isolated dwarf galaxy IC 1613, based on ACS observation for 24 orbits in two filters F475W and F814W. We used the HSC Beta Version 0.2 catalog photometry to search and select variable stars and compare with results from Bernard et al. (2010).
  2. Using HSC Beta Version 0.2 catalog photometry, we select variable and non-variable stars by using the sigma (i.e RMS scatter) from the HSC Summary Form.
  3. We provide finding charts for 6 variable in IC 1613.
  4. We used the publicly available Period search service to solve the light curve for MatchID=17759137, Bernard et al. (2010) ID=V043, the estimated period was found to be 1.3 days, which matches with the value listed in Bernard et al. (2010) table (4).
Things for the HSC to improve:
  1. Provide the users with the individual error estimated for the HLA Source list Sextractor magnitude in the HSC Detailed Form.
  2. Allow the users to submit large search requests by increasing the maximum to more than 50K.
  3. Provide a CasJobs option.
  4. Provide a tool to convert the start/end Time to MJD.
  5. Provide a box search in addition to circular search option.