Hubble Source Catalog (HSC) Use Case #2- Time-variable Phenomena
Searching for variable stars in dwarf irregular galaxy IC 1613
The ACS LCID project.
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.
II. Faint variable stars in the isolated dwarf irregular galaxy IC 1613.
Bernard et al. (2010) ApJ, 712, 1259
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.
USE CASE GOAL:
Part 1: Download the Data
- Construct the F475W - F814W vs "composite Vmag" color magnitude
- Identify variable and non-variable stars,
- Use a time-series analysis to determine the period.
- STEP 2- Download
the HSC catalog for the dwarf galaxy IC1613 field (i.e., similar to
step 10 of the "HSC Walkthrough") using the Summary Search form.
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.
- Make Aperture Corrections
- Use the ACS aperture correction for a 3 pixel radius (Sirianni et al. 2005, Table 3, i.e. 2.5log[Encircled Energy])
- for A_F475W add 2.5log[0.801] ... i.e add -0.241
- for A_F814W add 2.5log[0.764] ... i.e add -0.292
Part 3: Select Variable Stars From the HSC
- STEP 3- Download the Bernard et al. (2010) data set from CDS.
Note that Bernard et al. (2010) used
DAOPHOT/ALLFRAME (Stetson 1994
obtain the instrumental photometry of the stars on the individual,
non-drizzled images, so they have better sampling over the light
curve. In total they used 24 detections for each filter. The HSC
have a binned phase covering of 10 points in F475W and 11 in F814W,
since the HSC use the visit-based combined drizzled images.
- STEP 4- Use the downloaded HSC Summary table:
- Calculate the "composite V magnitude" Vmag as ~ 0.5*(A_F475W+A_F814W).
- Calculate the Vmag "Variability Index (VI)" [i.e. VI= sqrt(A_F475Wsigma**2 +A_F814Wsigma**2)]
- Plot the V mag [AB] vs the VI [AB] mag. A large fraction of the stars have a VI
<0.025 mag, which will be needed further for selecting non-variable
- Plot the F475W-F814W in [AB] vs the VI [AB mag].
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.
To select variable and non-variable stars we required that a star be
detected in all 21 frames to be checked for variability. We also
excluded all stars in crowded region by rejecting candidates with a
companion contributing more than 50% of total light within a two pixel
A large fraction of the stars have a VI [AB] mag <0.025, to select non-variable stars we required that VI [AB] mag <0.01.
- Transform the magnitudes from the HSC in the AB mag system to the VEGA mag system using the Bernard et al. 2009 transformation equation. This allow us to make a direct comparison with Bernard et al. (2010) .
- STEP 5- Plot the Color Magnitude Diagram.
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.
- STEP 6- Download the HSC data using the HSC Detailed Search Form.
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.
- Make Aperture Correction (as in step 2 above).
- If you plan to compare your result to others reported
in the literature you may need to convert to the BVI system (e.g. using the Bernard et al. 2009 transformation equation).
Note that the Bernard et al. (2009) equations are limited to the
variable stars confined in the instability strip, as their
intermediate color and narrow temperature range minimized the
difficulties related to extreme temperatures.
- Convert the observed start and end time to Julian Date (JD) or Modified JD (MJD), via e.g. (WCSTools, python PyAstronomy, pyephem, idl, topcat, etc.).
- Estimate the midMJD (or midJD)
- STEP 7- Visual inspection (optional)
- 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.
- 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.
Non-Variable Candidates over the observed ~2.4 days.
- Step 8- Determining the period for MatchID=17753692 (Also know as V043, Bernard et al. 2010 with a period of 1.3 days)
- 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.
- 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.
- The period was found to be 1.3 days, which is equal to the value listed in Bernard et al. (2010) table (4).
- 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).
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
- We provide finding charts for 6 variable in IC 1613.
- 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:
- Provide the users with the
individual error estimated for the HLA Source list Sextractor magnitude in the HSC Detailed Form.
the users to submit large search requests by
increasing the maximum to more than 50K.
- Provide a CasJobs option.
- Provide a tool to convert the start/end Time to MJD.
- Provide a box search in addition to circular search option.