J/AJ/150/6 Solar neighborhood. XXXVI. VRI variability of M dwarfs (Hosey+, 2015)
The solar neighborhood. XXXVI. The long-term photometric variability of nearby
red dwarfs in the VRI optical bands.
Hosey A.D., Henry T.J., Jao W.-C., Dieterich S.B., Winters J.G.,
Lurie J.C., Riedel A.R., Subasavage J.P.
<Astron. J., 150, 6 (2015)>
=2015AJ....150....6H 2015AJ....150....6H
ADC_Keywords: Stars, dwarfs ; Stars, nearby ; Stars, M-type ; Photometry, VRI ;
Parallaxes, trigonometric
Keywords: stars: low-mass - stars: statistics - stars: variables: general -
planetary systems - techniques: photometric
Abstract:
We present an analysis of long-term photometric variability for nearby
red dwarf stars at optical wavelengths. The sample consists of 264 M
dwarfs south of decl.=+30 with V-K=3.96-9.16 and MV≈10-20,
corresponding to spectral types M2V-M8V, most of which are within
25pc. The stars have been observed in the VRI filters for ∼4-14yr at
the CTIO/SMARTS 0.9m telescope. Of the 238 red dwarfs within 25pc, we
find that only ∼8% are photometrically variable by at least 20mmag
(∼2%) in the VRI bands. Only four stars have been found to vary by
more than 50mmag, including GJ 1207 at 8.6pc, which experienced a
single extraordinary flare, and GJ 2006 A, TWA 8 A, and TWA 8 B, which
are all young stars beyond 25pc linked to moving groups. We find that
high variability at optical wavelengths over the long term can in fact
be used to identify young stars. Overall, however, the fluxes of most
red dwarfs at optical wavelengths are steady to a few percent over the
long term. The low overall rate of photometric variability for red
dwarfs is consistent with results found in previous work on similar
stars on shorter timescales, with the body of work indicating that
most red dwarfs are only mildly variable. As expected, we find that
the degree of photometric variability is greater in the V band than in
the R or I bands, but we do not find any obvious trends in variability
over the long term with red dwarf luminosity or temperature. We
highlight 17 stars that show long-term changes in brightness,
sometimes because of flaring activity or spots, and sometimes because
of stellar cycles similar to our Sun's solar cycle. Remarkably, two
targets show brightnesses that monotonically increase (G 169-029) or
decrease (WT 460AB) by several percent over a decade. We also provide
long-term variability measurements for seven M dwarfs within 25pc that
host exoplanets, none of which vary by more than 20mmag. Both as a
population, and for the specific red dwarfs with exoplanets observed
here, photometric variability is therefore often not a concern for
planetary environments, at least at the optical wavelengths where they
emit much of their light.
Description:
We present an analysis of long-term photometric variability for nearby
red dwarf stars at optical wavelengths (Table1). The sample consists
of 264 M dwarfs south of decl.=+30 with V-K=3.96-9.16 and MV≈10-20,
corresponding to spectral types M2V-M8V, most of which are within
25pc.
Our 264 dwarf stars have been observed in the VRI filters over the
past 14yr (with a median duration in the coverage of 7.9yr). The
REsearch Consortium On Nearby Stars (RECONS; www.recons.org) has been
using the Cerro Tololo Inter-American Observatory/Small & Moderate
Aperture Research Telescope System (CTIO/SMARTS) 0.9m telescope for
astrometric and photometric observations since 1999, first as an
National Optical Astronomy Observatory (NOAO) Surveys Program, and
since 2003 under the auspices of the SMARTS Consortium. The telescope
is equipped with a 2048*2048 Tektronix CCD camera. Images taken during
the program are used here to investigate the photometric variability
of the nearby M dwarfs that have been targeted for parallax and proper
motion measurements. Observations are made using the central quarter
of the chip, which provides a 6.8' square field of view and pixels
401mas in size. Parallax frames are taken in the VJ, RKC, and
IKC filters with magnitudes ranging from 9 to 20. The central
wavelengths for the VJ, RKC, and IKC filters used in this study
are 5438/5475, 6425, and 8075Å, respectively. The subscript "J"
indicates Johnson, "KC" indicates Kron-Cousins (usually known as
Cousins). VRI photometry from our program is given for the sample
stars in Table1. Details of the photometry observations and reductions
can be found in Jao et al. (2005AJ....129.1954J 2005AJ....129.1954J) and Winters et al.
2011 (cat. J/AJ/141/21).
For astrometry, five images of each star are typically taken per
night, usually within 30 minutes of transit. The target star is
positioned in the field so that 5-10 reference stars, normally fainter
by 1-4mag, surround the target. These stars constitute a reference
grid for the astrometric reductions, and are also used for the
photometric variability study described here. Additional details of
the observations can be found in Jao et al. (2005AJ....129.1954J 2005AJ....129.1954J).
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 127 264 Red dwarfs studied for long-term variability
refs.dat 77 16 References
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See also:
J/AJ/149/5 : Solar neighborhood. XXXV. M dwarfs distances (Winters+, 2015)
J/AJ/147/94 : Solar neighborhood. XXXII. L and M dwarfs (Dieterich+, 2014)
J/AJ/147/85 : Solar neighborhood. XXXIII. 45 M dwarfs (Riedel+, 2014)
J/AJ/141/21 : CCD distance estimates of SCR targets (Winters+, 2011)
J/AJ/130/1658 : New high proper motion stars (-47<DE< -00) (Subasavage+, 2005)
J/AJ/129/413 : New high proper motion stars (-90<DE< -47) (Subasavage+, 2005)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 16 A16 --- Name Name of the red dwarf (1)
18 A1 --- n_Name 'Y' indicates star confirmed to be young in
Riedel et al. 2014 (cat. J/AJ/147/85)
20- 21 I2 h RAh Hour of Right Ascension (J2000)
23- 24 I2 min RAm Minute of Right Ascension (J2000)
26- 27 I2 s RAs Second of Right Ascension (J2000)
29 A1 --- DE- Sign of the Declination (J2000)
30- 31 I2 deg DEd Degree of Declination (J2000)
33- 34 I2 arcmin DEm Arcminute of Declination (J2000)
36- 37 I2 arcsec DEs Arcsecond of Declination (J2000)
39- 43 F5.2 mag Vmag [9.6/21.4] Johnson V-band magnitude (2)
45- 49 F5.2 mag Rmag [8.7/18.9] Kron-Cousins R-band magnitude (2)
51- 55 F5.2 mag Imag [7.3/16.5] Kron-Cousins I-band magnitude (2)
57- 61 A5 --- r_Imag Reference for VRI magnitudes (in refs.dat file)
63- 67 F5.2 mag Kmag [4.3/12.3] The 2MASS Ks band magnitude
69- 72 F4.2 mag V-K [3.9/9.2] The (V-Ks) color index
74 A1 --- Flt [VRI] Filter used in long-term photometry
(VJ, RKC, or IKC)
76- 80 F5.1 mmag Amp [6/197] Photometric variability in Flt (3)
82- 88 F7.2 yr Date0 [1999/2010] First epoch in the data serie
89 A1 --- --- [-]
90- 96 F7.2 yr Date1 [2003/2014] Last epoch in the data serie
98-102 F5.2 yr Ny [3.8/14.1] Number of years (from the first to
last epoch)
104-105 I2 --- Nn [7/63] Number of observational nights
107-109 I3 --- Nf [29/374] Total number of frames (typically ∼5
frames per night)
111-116 F6.2 mas plx [20/769]? Trigonometric parallax πtrig (4)
118-121 F4.2 mas e_plx [0.4/4.1]? 1σ uncertainty in plx
123-127 A5 --- r_plx Reference for plx (in refs.dat file)
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Note (1): Known multiples with separations less than 1'' are treated as single
sources and noted with component letters, e.g., AB, or ABC. Multiples
separated by more than 3'' can be measured separately for photometry and
are given individual entries.
Note (2): From our REsearch Consortium On Nearby Stars (RECONS; www.recons.org)
program. The RECONS team has gathered photometry in the VJRKCIKC
optical bands for more than 1000 nearby red dwarfs since 1999 (Winters et
al. 2011, cat. J/AJ/141/21; 2015, cat. J/AJ/149/5). The central wavelengths
for the VJ, RKC, and IKC filters used in this study are 5438/5475,
6425, and 8075Å, respectively.
Note (3): Here we define photometric variability to be the standard deviation of
a star's flux, measured in milli-magnitudes (mmag), when compared to a set
of reference stars. The computing methods used are described in Section 4.
Of the 238 red dwarfs within 25pc, we find that only ∼8% are
photometrically variable by at least 20mmag (∼2%) in the VRI bands. Only
four stars have been found to vary by more than 50mmag, including GJ 1207
at 8.6pc, and GJ 2006 A, TWA 8 A, and TWA 8 B, which are all young stars
beyond 25pc linked to moving groups.
Note (4): For stars in our sample for which we have published parallaxes to
date.
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Byte-by-byte Description of file: refs.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 5 A5 --- Ref Reference identifier
7- 25 A19 --- BibCode Bibliographic code
27- 43 A17 --- Aut Author's name
45- 77 A33 --- Com Comment
--------------------------------------------------------------------------------
History:
From electronic version of the journal
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(End) Sylvain Guehenneux [CDS] 23-Jul-2015