J/MNRAS/428/2824 Rapid optical variability in ultracool dwarfs (Koen, 2013)
An extensive search for rapid optical variability in ultracool dwarfs.
Koen C.
<Mon. Not. R. Astron. Soc., 428, 2824-2839 (2013)>
=2013MNRAS.428.2824K 2013MNRAS.428.2824K
ADC_Keywords: Stars, dwarfs ; Photometry
Keywords: techniques: photometric - brown dwarfs - stars: low-mass
Abstract:
In this paper, a summary of optical time-series photometry of 125
ultracool dwarfs is given. The observing strategy was to monitor each
object continuously for 2-3h in order to ascertain whether it was
rapidly variable. Many of the targets were observed at multiple
epochs, to follow up possible short time-scale variability, or to test
for slow brightness changes on longer time-scales. The 353 data sets
obtained contain nearly 22000 individual measurements. Optical (IC)
magnitudes, accurate to roughly 0.1-0.2mag, were derived for 21
objects for which there is no optical photometry in the literature. It
is shown that photometry is affected by variable seeing in a large
percentage of the time-series observations. Since this could give the
appearance of variability intrinsic to the objects, magnitudes are
modelled as functions of both time and seeing. Several ultracool
dwarfs which had not been monitored before are variable, according to
certain model-fitting criteria. A number of objects with multi-epoch
observations appear to be variable on longer time-scales. Since
testing for variability is far from being straightforward, the
time-series data are made available so that interested readers can
perform their own analyses.
Description:
All the photometry reported in this paper was obtained with the South
African Astronomical Observatory (SAAO) CCD camera mounted on the
1.9-m telescope at Sutherland.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 121 125 Observing log and details of the objects observed
table2.dat 110 363 The observing log, and some quantities derived
from the photometry
phot/* . 353 Individual time series of observations
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See also:
J/AJ/126/1526 : IR photometry of ultracool dwarfs (Bouy+, 2003)
J/AJ/126/2421 : 2MASS-Selected sample of ultracool dwarfs (Cruz+, 2003)
J/AJ/126/2449 : Nearby ultracool dwarfs in galactic plane (Reid+, 2003)
J/A+A/446/515 : Distances of nearby ultracool dwarfs (Phan-Bao+, 2006)
J/MNRAS/382/515 : SDSS ultracool and halo WD candidates (Vidrih+, 2007)
J/AJ/133/439 : Luminosity function of M7-L8 ultracool dwarfs (Cruz+, 2007)
J/AJ/133/2258 : Activity and kinematics of ultracool dwarfs (Schmidt+, 2007)
J/AJ/136/1290 : Ultracool dwarfs from the 2MASS (Reid+, 2008)
J/ApJ/723/684 : NIRSPEC ultracool dwarf RV survey (Blake+, 2010)
J/MNRAS/404/1817 : 2MASS/SDSS data for 806 ultracool dwarfs (Zhang+, 2010)
J/A+A/522/A112 : L5 and later dwarf candidates i'z' photometry (Reyle+, 2010)
J/ApJ/746/23 : Radio observations of ultracool dwarf stars (McLean+, 2012)
J/A+A/549/A131 : Radio survey of ultracool dwarfs (Antonova+, 2013)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- SName Short star name (HHMM+DDMM)
11- 39 A29 --- Name Star name
41- 47 A7 --- SpT MK spectral type
49- 53 F5.2 mag Jmag 2MASS J magnitude
55- 59 F5.2 mag Imag ? I(Cousins) magnitude
61- 65 F5.2 mag Rmag ? R(Cousins) magnitude
67- 68 I2 --- r_Rmag ? Reference for Icmag and Rmag (3)
70- 74 F5.2 mag Imag2 ? I(Cousins) magnitude
76- 80 F5.2 mag Rmag2 ? R(Cousins) magnitude
82- 83 I2 --- r_Rmag2 ? Reference for Icmag2 and Rmag2 (3)
85- 89 F5.2 mag Imag3 ? I(Cousins) magnitude
91- 95 F5.2 mag Rmag3 ? R(Cousins) magnitude
97- 98 I2 --- r_Rmag3 ? Reference for Icmag3 and Rmag3 (3)
100 A1 --- l_Sep Limit flag on Sep
101-104 I4 mas Sep ? Angular separation for binaries
105 A1 --- Mult [XO?] Multiplicity code (1)
107-111 A5 --- r_Sep Reference for Sep or Mult (4)
113-121 A9 --- Var Variability code (2)
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Note (1): codes as follows:
X = no information about multiplicity is available in the literature
O = the ultracool dwarf has been checked for multiplicity, with a null result
? = the object is known to be multiple, but the separation between
components is unknown
Note (2): The variability code is in two parts:
* a variability indicator made of 2 or 3 letters, the first for
results from the literature, the second from the present study,
and the third (Z) for longer time-scale variability:
X = no variability information in the literature;
V = detected as variable;
C = no variability detected (constant)
Z = change in nightly mean magnitude (i.e. longer time-scale
variability) in this study.
* the second part of the variability entry summarizes the significance
level of the estimated seeing function fs in model (2), as the quotient
Ns/N, where:
Ns = number of times a significance ≤1% is achieved
N = total number of runs
Note (3): References as follows:
1 = The DENIS Third Release Catalogue (Epchtein et al. 1999, Cat. B/denis)
2 = Liebert & Gizis (2006, Cat. J/PASP/118/659)
3 = Kendall et al. (2007MNRAS.374..445K 2007MNRAS.374..445K)
4 = Phan-Bao et al. (2008, Cat. J/MNRAS/383/831)
5 = Burgasser et al. (2007ApJ...657..494B 2007ApJ...657..494B)
6 = Luhman (2007, Cat. J/ApJS/173/104)
7 = Dahn et al. (2002AJ....124.1170D 2002AJ....124.1170D)
8 = transformed from SDSS i and z photometry
9 = from DENIS photometry of a star in the field of view
10 = from SDSS photometry of a star in the field of view
11 = from zero-point set during the night
Note (4): References for multiplicity data as follows:
1 = Bouy et al. (2003, Cat. J/AJ/126/1526)
2 = Burgasser et al. (2010, Cat. J/ApJ/710, 1142)
3 = Reid et al. (2006ApJ...639.1114R 2006ApJ...639.1114R)
4 = Burgasser et al. (2005ApJ...634L.177B 2005ApJ...634L.177B)
5 = Reid et al. (2006, Cat. J/AJ/132/891)
6 = Burgasser et al. (2010, Cat. J/ApJ/710/1142)
7 = Blake, Charbonneau & White (2010, Cat, J/ApJ/723/684)
8 = Bernat et al. (2010ApJ...715..724B 2010ApJ...715..724B)
9 = Reid et al. (2008AJ....135..580R 2008AJ....135..580R)
10 = Burgasser et al. (2003ApJ...586..512B 2003ApJ...586..512B)
11 = McCaughrean et al. (2004A&A...413.1029M 2004A&A...413.1029M)
12 = Close et al. (2003ApJ...587..407C 2003ApJ...587..407C)
13 = Burgasser & McElwain (2006AJ....131.1007B 2006AJ....131.1007B)
14 = Burgasser et al. (2011ApJ...739...49B 2011ApJ...739...49B)
15 = Gelino & Burgasser (2010AJ....140..110G 2010AJ....140..110G)
16 = Dahn et al. (2008ApJ...686..548D 2008ApJ...686..548D)
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- SName Short name (HHMM-DDMM)
11- 19 F9.4 d HJD Start HJD (HJD-2450000)
21 A1 --- Filt [RIZW] Filter (1)
23- 25 F3.1 h Trun Run length
27- 29 I3 s Texp Exposure time
30 A1 --- --- [-]
31- 33 I3 s Texp2 ? Upper value of exposure time interval
35- 37 I3 --- N Number of useful exposures obtained
39- 40 I2 mmag sigma Standard deviation of the measurements
42- 47 F6.1 mmag/arcsec betas ?=- Slope of the seeing regression (2)
49- 53 F5.3 --- p(s) [0/1]?=- p-value of the seeing regression (2)
55- 59 F5.3 --- p(t) [0/1]?=- p-value of the regression on time (2)
61- 65 F5.3 --- R2 [-0.1/1]?=- Proportion of variance explained
by the fit (2)
67- 69 I3 mmag ZP ?=- Zeropoint (3)
71- 79 A9 --- FileName Name of the file in subdirectory phot
83-110 A28 --- Name Name as in table1
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Note (1): R and I are in Cousins system, while "W " indicates white light
(i.e. no filter)
Note (2): Results of regressing the photometry on seeing (linearly) and time
(an unspecified nonparametric fit). Six datasets were too short (N<10) for
the fit to be attempted. For the 10 datasets with bright outlying points
(see section 4) a second regression solution, excluding the outlier, was
obtained; these results are also provided.
Note (3): Offset of the night's photometry from the brightest mean over all
nights. An "---" indicates that no value could be calculated (i.e. the
particular star/filter combination occurred only once). Further details
are given in the text.
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Byte-by-byte Description of file: phot/*
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Bytes Format Units Label Explanations
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3- 10 F8.6 d Date Fractional part of the Heliocentric Julian Day
15- 20 F6.4 arcsec FWHM Estimated seeing (full width at half-maximum)
24- 30 F7.4 mag UCD PSF photometry of the ultracool dwarf (UCD)
33- 40 F8.4 mag ph1 ? PSF photometry of another star in the field
with brightnesses similar to that of the UCD (1)
43- 50 F8.4 mag ph2 ? PSF photometry of a second star in the field
with brightnesses similar to that of the UCD (1)
53- 60 F8.4 mag ph3 ? PSF photometry of a third star in the field
with brightnesses similar to that of the UCD (1)
63- 70 F8.4 mag ph4 ? PSF photometry of a fourth star in the field
with brightnesses similar to that of the UCD (1)
73- 80 F8.4 mag ph5 ? PSF photometry of a fifth star in the field
with brightnesses similar to that of the UCD (1)
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Note (1): PSF photometry of one or more stars in the field with brightnesses
similar to that of the UCD. Plotting the additional photometry along with
that of the UCD allows the light curve of the latter to be seen in the
proper context. Since the observations were obtained under a variety of
atmospheric conditions, the photometry was routinely standardized with
respect to a few stars in the field of view, i.e. all observations have
been differentially corrected.
There is a single instance, namely the R filter observations of 2M 1300+1912,
in which the field contained only one object aside from the UCD. Since the
former was used for standardization, there are no comparison data for the
UCD photometry.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 06-Feb-2014