J/AJ/151/166 Stellar flares and variables from 2009-2010 CSTAR (Oelkers+, 2016)
Stellar variability and flare rates from Dome A, Antarctica, using 2009 and
2010 CSTAR observations.
Oelkers R.J., Macri L.M., Wang L., Ashley M.C.B., Cui X., Feng L.-L.,
Gong X., Lawrence J.S., Qiang L., Luong-van D., Pennypacker C.R., Yuan X.,
York D.G., Zhou X., Zhu Z.
<Astron. J., 151, 166-166 (2016)>
=2016AJ....151..166O 2016AJ....151..166O (SIMBAD/NED BibCode)
ADC_Keywords: Stars, variable ; Photometry, ugriz ; Spectral types ;
Stars, flare
Keywords: methods: data analysis - stars: flare - stars: variables: general
Abstract:
The Chinese Small Telescope Array (CSTAR) carried out high-cadence
time-series observations of ∼20.1 square degrees centered on the South
Celestial Pole during the 2008, 2009, and 2010 winter seasons from
Dome A in Antarctica. The nearly continuous six months of dark
conditions during each observing season allowed for >106 images to
be collected through gri and clear filters, resulting in the detection
of >104 sources over the course of three years of operation. The
nearly space-like conditions in the Antarctic plateau are an ideal
testbed for the suitability of very small-aperture (<20cm) telescopes
to detect transient events, variable stars, and stellar flares. We
present the results of a robust search for such objects using
difference image analysis of the data obtained during the 2009 and
2010 winter seasons. While no transients were found, we detected 29
flaring events and find a normalized flaring rate of
5±4*10-7flare/hr for late-K dwarfs, 1±1*10-6flare/hr for M
dwarfs and 7±1*10-7flare/hr for all other stars in our sample. We
suggest future small-aperture telescopes planned for deployment at
Dome A would benefit from a tracking mechanism, to help alleviate
effects from ghosting, and a finer pixel scale, to increase the
telescope's sensitivity to faint objects. We find that the light
curves of non-transient sources have excellent photometric qualities
once corrected for systematics, and are limited only by photon noise
and atmospheric scintillation.
Description:
The Chinese Small Telescope Array (CSTAR) was deployed to Dome A in
early 2008 and carried out observations during three Antarctic winter
seasons supported by the Plateau Observatory (PLATO) observatory
before being returned to China for comprehensive upgrades in early
2011; the following description applies to the original version of the
system. It is composed of four Schmidt-Cassegrain wide-field
telescopes, each with a 145mm aperture and a Field Of View (FOV)
4.5° on a side. The focal planes contain ANDOR DV435 1K*1K
frame-transfer CCDs with a pixel size of 13µm, equivalent to a
plate scale of 15''/pix. Filters are mounted at the top of the optical
tubes, with a 10W electric current run through a coating of indium tin
oxide. Three of the filters are standard SDSS gri while the remaining
one is a clear filter. The observations described in this work took
place during the Antarctic winters of 2009 and 2010.
During the 2009 winter season, observations were carried out by the
telescopes equipped with g, clear, and r filters. The remaining
telescope, equipped with an i filter, failed to return data. The
observations used in our analysis span MJD 54935-85 in g and clear and
MJD 54955-5143 in r. Exposure times varied between 5 and 20s and
∼8*105 scientifically useful images were acquired over 125 days.
During the 2010 winter season, observations were only carried out with
the telescope equipped with the i filter since the other telescopes
failed to return data. The observations used in our analysis spanned
MJD 55317-5460 with exposures times of 20-40s.
The final star lists consist of 6179, 12801, and 10681 stars in gri,
respectively, for a total of 15496 stars (see Table1), with 10094
sources in at least 2 bands.
We identified ten, ten, and nine flare events in i, g, and r,
respectively, leading to a total of 29 flares throughout the nearly
3000 combined hours of observations between 2009 and 2010. Details for
each flare event are shown in Table2.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 101 15496 Difference image stellar library
table2.dat 77 29 Identified candidate stellar flares
--------------------------------------------------------------------------------
See also:
J/ApJS/218/20 : Photometric variability search in CSTAR field (Wang+, 2015)
J/AJ/149/50 : Photometry of 105 variable stars from CSTAR (Oelkers+, 2015)
J/AJ/146/139 : 2010 photometry of variable stars from Dome A (Wang+, 2013)
J/AJ/142/155 : Variable stars photometry from Dome A (Wang+, 2011)
J/PASP/122/347 : Chinese Small Telescope ARray (CSTAR) (Zhou+, 2010)
J/ApJ/675/1254 : M37 variable stars (Hartman+, 2008)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 5 A5 --- --- [CSTAR]
6- 22 A17 --- CSTAR Chinese Small Telescope Array (CSTAR)
identification (JHHMMSS.ss+DDMMSS)
24- 25 I2 h RAh Hour of Right Ascension from CSTAR (J2000)
27- 28 I2 min RAm Minute of Right Ascension from CSTAR (J2000)
30- 34 F5.2 s RAs Second of Right Ascension from CSTAR (J2000)
36 A1 --- DE- Sign of the Declination from CSTAR (J2000)
37- 38 I2 deg DEd Degree of Declination from CSTAR (J2000)
40- 41 I2 arcmin DEm Arcminute of Declination from CSTAR (J2000)
43- 44 I2 arcsec DEs Arcsecond of Declination from CSTAR (J2000)
46- 51 F6.3 mag [5.7/14]? Mean SDSS g band magnitude
53- 58 F6.3 mag [5.7/15]? Mean SDSS r band magnitude
60- 65 F6.3 mag [6.5/15.9]? Mean SDSS i band magnitude
67 I1 --- V [0/4] Variability score (0-4) (1)
69 I1 --- LS [0/4] Periodicity Lomb-Scargle (LS) score
(0-4) (2)
71 I1 --- BLS [0/3] Periodicity Box-Least-Squares (BLS)
algorithm score (0-3) (3)
74- 75 A2 --- Cl Tentative classification of variable (star with
a variability or periodicity (LS or BLS) score
of three or more, signifying a high likelihood
of variability; EB, IR, LT, MP, PR, or
RR) (4)
77- 86 F10.6 d Per-LS [0.07/75.3]? Main Lomb-Scargle (LS) period
88- 97 F10.6 d Per-BLS [0.02/4.31]? Main Box-Least-Squares (BLS)
algorithm period
99 I1 --- Crowd [0/1] Possibly increased magnitude from
crowding (0=normal, or 1=possible
contamination) (5)
101 I1 --- M [0/1] Possible match to another star in field
(0=no multi-match, or 1=multi-match) (6)
--------------------------------------------------------------------------------
Note (1): Previous studies of CSTAR data (Wang et al. 2011, cat. J/AJ/142/155;
Wang et al. 2013, cat. J/AJ/146/139; Oelkers et al. 2015,
cat. J/AJ/149/50; Yang et al. 2015ApJS..217...28Y 2015ApJS..217...28Y) have generated lists of
variables by applying a binary classification (i.e., an object is either
variable or not, based on a set of criteria: rms, Δ90, J). We
considered a star to be variable if the star passed all three of the above
tests in either g, r, or i. See Section 6.1 for more details about these
tests. In this work, we present the likelihood of variability and/or
periodicity for every object, computed as follows. Stars meeting the
variability criteria in g or r received one point per band, while two
points were awarded for i because those images were in focus and well
sampled.
Note (2): We searched each light curve for periodic signals using a LS
periodogram (Lomb, 1976Ap&SS..39..447L 1976Ap&SS..39..447L; Scargle, 1982ApJ...263..835S 1982ApJ...263..835S) as
implemented in VARTOOLS (Hartman et al. 2008, cat. J/ApJ/675/125). See
Section 6.2 for additional details. Stars exhibiting a significant
periodicity received two points in i and 1 in g and r.
Note (3): We searched for transit-like events using the Box-Least-Squares (BLS)
algorithm (Kovacs et al. 2002A&A...391..369K 2002A&A...391..369K; BLS). See Section 6.2 for
more details. Stars exhibiting a significant periodicity received two
points in i and 1 in g and r.
Note (4): Estimated types for variables with a score of 3+ in either V, LS, or
BLS defined as follows:
EB = Eclipsing Binary;
LT = Long-Term variation;
PR = Unclassified periodic;
RR = RR Lyrae;
IR = IRregular;
TR = Transit-like eclipse;
MP = Multiple Periods recovered.
Note (5): Some stars in g and r show magnitudes significantly brighter than
would be expected for typical g-r or r-i colors. This is probably because
the aperture magnitudes in the 2009 reference frames likely caused an
increase in flux in both bands for some crowded stars.
Note (6): We found 292 stars in g and 1959 stars in r to match to multiple stars
in i because of the defocused Point-Spread Function (PSF) in the 2009 data
set. We kept each match and flagged the appropriate stars in the stellar
library described in Section 7.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 5 A5 --- --- [CSTAR]
6- 22 A17 --- CSTAR Chinese Small Telescope Array (CSTAR)
identification (JHHMMSS.ss+DDMMSS)
24- 25 I2 h RAh Hour of Right Ascension from CSTAR (J2000)
27- 28 I2 min RAm Minute of Right Ascension from CSTAR (J2000)
30- 34 F5.2 s RAs Second of Right Ascension from CSTAR (J2000)
36 A1 --- DE- Sign of the Declination from CSTAR (J2000)
37- 38 I2 deg DEd Degree of Declination from CSTAR (J2000)
40- 41 I2 arcmin DEm Arcminute of Declination from CSTAR (J2000)
43- 44 I2 arcsec DEs Arcsecond of Declination from CSTAR (J2000)
46 A1 --- Band [gri] Band where flare was identified
(g, r, or i)
48- 50 A3 --- SpT Estimated Spectral Type (K/M dwarf)
52- 61 F10.6 d MJD [449.685/887.182] Modified Julian Date
(MJD-2454500)
63- 67 F5.3 mag Amp [0.022/0.777] Flare amplitude
69- 73 F5.3 d Length [0.014/0.779] Length of flare
75- 77 A3 --- Com Information about counterpart flare
(only the i band was observed in 2010 and
would not have any counterpart bands. The
only observed bands in 2009 in our reduction
were g and r) (1)
--------------------------------------------------------------------------------
Note (1): The five flags are defined as follows:
0 = Only band observed that season;
1 = Flare seen in the other band;
2 = No star in other band;
3 = Outside observing window for other band during flare;
4 = Data drop out in the other band (r band) during flare.
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Prepared by [AAS]; Sylvain Guehenneux [CDS] 21-Jul-2016