J/ApJS/211/9 Variability in UV line emission of F-M stars (Loyd+, 2014)
Fluctuations and flares in the ultraviolet line emission of cool stars:
implications for exoplanet transit observations.
Loyd R.O.P., France K.
<Astrophys. J. Suppl. Ser., 211, 9 (2014)>
=2014ApJS..211....9L 2014ApJS..211....9L
ADC_Keywords: Stars, double and multiple ; Spectra, ultraviolet ; Planets ;
Stars, flare
Keywords: planets and satellites: detection; stars: low-mass;
ultraviolet: planetary systems; ultraviolet: stars
Abstract:
Variations in stellar flux can potentially overwhelm the photometric
signal of a transiting planet. Such variability has not previously
been well-characterized in the ultraviolet lines used to probe the
inflated atmospheres surrounding hot Jupiters. Therefore, we surveyed
38 F-M stars for intensity variations in four narrow spectroscopic
bands: two enclosing strong lines from species known to inhabit hot
Jupiter atmospheres, C.II λλ1334, 1335 and
SiIIIλ1206; one enclosing SiIV λλ1393, 1402; and
36.5Å of interspersed continuum. For each star/band combination, we
generated 60s cadence lightcurves from archival Hubble Space Telescope
Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph
time-tagged photon data. Within these lightcurves, we characterized
flares and stochastic fluctuations as separate forms of variability.
Flares: we used a cross-correlation approach to detect 116 flares.
These events occur in the time-series an average of once per 2.5hr,
over 50% last 4 minutes or less, and most produce the strongest
response in SiIV. If the flare occurred during a transit measurement
integrated for 60 minutes, 90/116 would destroy the signal of an
Earth, 27/116 Neptune, and 7/116 Jupiter, with the upward bias in flux
ranging from 1% to 109% of quiescent levels. Fluctuations: photon
noise and underlying stellar fluctuations produce scatter in the
quiescent data. We model the stellar fluctuations as Gaussian white
noise with standard deviation σx. Maximum likelihood values of
σx range from 1% to 41% for 60s measurements. These values
suggest that many cool stars will only permit a transit detection to
high confidence in ultraviolet resonance lines if the radius of the
occulting disk is ≳1RJ. However, for some M dwarfs this limit can
be as low as several R⊕.
Description:
Because the motivation for this work is the characterization of
stellar variability in all potential targets for FUV transit work
(Section 1), we constructed a stellar sample of all F-M stellar
targets with archival HST time-tagged photon data covering the
wavelengths of the CII, SiIV, and occasionally (27/42 data sets) SiIII
lines. These wavelengths are observed with the STIS E140M, COS G130M,
and COS G140L gratings. Thus, we retrieved all public time-tagged
photon data for the sample acquired with these gratings from the
Mikulski Archive for Space Telescopes (MAST). We also obtained some
data still proprietary under program 12464 (France et al.
2013ApJ...763..149F 2013ApJ...763..149F).
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 380 38 Properties of the stars in the sample
table2.dat 132 272 Catalog of observations
table4.dat 102 42 Variability statistics in CII
table5.dat 101 42 Variability statistics in SiIII
table6.dat 102 42 Variability statistics in SiIV
table7.dat 102 42 Variability statistics in Continuum
refs.dat 89 197 References (for table 1)
--------------------------------------------------------------------------------
See also:
B/vsx : AAVSO International Variable Star Index VSX (Watson+, 2006-2014)
V/130 : Geneva-Copenhagen Survey of Solar neighbourhood III (Holmberg+, 2009)
III/244 : Catalog of Stellar Rotational Velocities (Glebocki+ 2005)
II/264 : ASAS Variable Stars in Southern hemisphere (Pojmanski+, 2002-2005)
V/73 : Emission-Line Stars of the Orion Population (Herbig+ 1988)
J/ApJ/754/69 : UV spectroscopy of solar-mass stars (Linsky+, 2012)
J/MNRAS/420/1590 : Abundances of classical Cepheids (Acharova+, 2012)
J/MNRAS/410/190 : Young runaway stars within 3kpc (Tetzlaff+, 2011)
J/A+A/532/A10 : RACE-OC project. II. (Messina+, 2011)
J/A+A/530/A138 : Geneva-Copenhagen survey re-analysis (Casagrande+, 2011)
J/A+A/526/A99 : Metal-poor solar-type stars spectro. & masses (Sousa+, 2011)
J/PASP/123/412 : Exoplanet Orbit Database (Wright+, 2011)
J/ApJ/731/8 : Multiple star formation in Taurus-Auriga (Kraus+, 2011)
J/AZh/88/34 : Long-term (1984-2008) JHKLM phot. (Shenavrin+, 2011)
J/MNRAS/408/475 : HATNet Pleiades Rotation Period Catalogue (Hartman+, 2010)
J/MNRAS/403/1949 : UBV(RcIc)JHK photometry of HIP nearby stars (Koen+, 2010)
J/A+A/520/A15 : RACE-OC project: YSOs within 100pc (Messina+, 2010)
J/ApJ/725/875 : Chromospheric activity for CPS stars (Isaacson+, 2010)
J/ApJ/705/1416 : M7-M9.5 dwarfs <20pc (Reiners+, 2009)
J/ApJ/704/975 : Rotational velocities for M dwarfs (Jenkins+, 2009)
J/ApJ/704/531 : The coevality of young binary systems (Kraus+, 2009)
J/ApJ/699/649 : Young M dwarfs within 25pc. I. (Shkolnik+, 2009)
J/ApJ/694/1085 : Radii of exoplanet host stars (van Belle+, 2009)
J/ApJS/181/62 : Survey of young solar analogs (Metchev+, 2009)
J/other/A+ARV/17.251 : Stars observed with Doppler imaging (Strassmeier, 2009)
J/ApJ/687/1264 : Age estimation for solar-type dwarfs (Mamajek+, 2008)
J/AJ/133/2524 : Spectra of young nearby stars (White+, 2007)
J/ApJS/168/297 : Stellar parameters of nearby cool stars (Takeda+, 2007)
J/MNRAS/374/664 : Stellar parameters in MILES library (Cenarro+, 2007)
J/AJ/132/161 : NStars project: The southern sample. I. (Gray+, 2006)
J/A+A/460/695 : Associations Containing Young stars (Torres+, 2006)
J/ApJS/159/141 : Spectroscopic properties of cool stars. I. (Valenti+, 2005)
J/ApJ/581/1194 : Taurus-Auriga PMS stars ages and Teff (Palla+, 2002)
J/MNRAS/328/45 : Late-type stars members of young groups (Montes+, 2001)
J/A+A/379/976 : Moving groups late-type stars EWs + fluxes (Montes+ 2001)
J/A+AS/144/469 : Delta Scuti stars (Rodriguez+, 2000)
J/A+A/352/555 : Fundamental parameters of stars (Allende Prieto+, 1999)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 12 A12 --- Star Star identifier
14 A1 --- f_Star [j] j: indicates unresolved binary
system; see Section 2.
16- 32 A17 --- 2MASS 2MASS Designation if any (JHHMMSSss+DDMMSSs)
34- 41 A8 --- SpType MK spectral type (1)
43- 45 A3 --- r_SpType Reference(s) for SpType (see refs.dat file)
47- 62 A16 --- OType Other classification type (3)
64- 66 A3 --- r_OType Reference(s) for OType (see refs.dat file)
68 I1 --- Np [1/4]? Number of known planets
70- 88 A19 --- r_Np Reference(s) for Np (see refs.dat file)
90- 96 F7.1 Myr Age [2/10500]? Estimated stellar age
98- 99 A2 --- f_Age [ aefl] Flag(s) on Age (4)
101-104 I4 Myr E_Age ? Upper limit uncertainty in Age
106-111 F6.1 Myr e_Age ? Lower limit uncertainty in Age (5)
113-119 A7 --- r_Age Reference(s) for Age (see refs.dat file)
121 A1 --- l_Prot [≲] Limit or approximation flag on Prot
122-132 F11.7 d Prot [0.4/286]? Stellar rotational period
134-135 A2 --- f_Prot [ bcegh] Flag(s) on Prot (4)
137-146 F10.7 d e_Prot [0.000005/98]? Uncertainty in Prot
148-152 A5 --- r_Prot Reference(s) on Prot (see refs.dat file)
154-159 F6.4 Msun M [0.08/7.7] Stellar mass
161 A1 --- f_M [abm] Flag on M (4)
163-167 F5.3 Msun E_M ? Upper limit uncertainty in M
169-174 F6.4 Msun e_M ? Lower limit uncertainty in M (5)
176-186 A11 --- r_M Reference(s) on M (see refs.dat file)
188-194 F7.4 Rsun R [0.1/71]? Stellar radius
196-197 A2 --- f_R [ abdi] Flag(s) on R (4)
199-205 F7.4 Rsun e_R [0.004/11]? Uncertainty in R
207-217 A11 --- r_R Reference(s) on R (see refs.dat file)
219-222 I4 K Teff [2564/6959] Effective stellar temperature
224 A1 --- f_Teff [a] Flag on Teff (4)
226-228 I3 K E_Teff ? Upper limit uncertainty in Teff
230-232 I3 K e_Teff ? Lower limit uncertainty in Teff (5)
234-247 A14 --- r_Teff Reference(s) on Teff (see refs.dat file)
249-254 F6.4 [cm/s2] log(g) [1.6/5.3]? Log of stellar surface gravity
256-257 A2 --- f_log(g) [ abe] Flag(s) on log(g) (4)
259-264 F6.4 [cm/s2] e_log(g) ? Uncertainty in log(g)
266-314 A49 --- r_log(g) Reference(s) for log(g) (see refs.dat file)
316-321 F6.2 km/s vsini [0.3/162.5]? Projected stellar equatorial
velocity
323-324 A2 --- f_vsini [ abde] Flag(s) on vsini (4)
326-329 F4.2 km/s e_vsini ? Uncertainty in vsini
331-353 A23 --- r_vsini Reference(s) for vsini (see refs.dat file)
355-368 F14.9 Lsun L [0.0003/5291.4] Stellar luminosity
370-376 F7.5 Lsun e_L [0.00024]? Uncertainty in L
378-380 A3 --- r_L Reference for L (see refs.dat file)
--------------------------------------------------------------------------------
Note (1): The default is the spectral type given in the SIMBAD database
(Wenger et al. 2000A&AS..143....9W 2000A&AS..143....9W).
Note (3): The default is the object description given in the SIMBAD database
(Wenger et al. 2000A&AS..143....9W 2000A&AS..143....9W) except for WTTS stars which have
a reference.
Note (4): Flag as follows:
a = Mean of multiple values found in the reference(s) using 1σ2
weighting factors. When uncertainties on a particular value were
asymmetric, we used the average of the two uncertainties as σ.
b = Simple average of multiple values found in the reference(s). When the
literature provided four or more values, we estimated the uncertainty
as the standard deviation of the values.
c = Represents the upper limit, P/sin(i), computed from the vsin(i) and
Radius values. Where possible, we used simple propagation of errors
to estimate uncertainty.
d = Outlier value(s) removed when averaging.
e = Estimates found in the literature were inconsistent. The authors made
a subjective determination of which value(s) seemed most plausible.
f = Age of the cluster within which the star resides or age of nearby
neighbors assumed to be coevolutionary.
g = The Prot value(s) found in the literature is(are) larger than the
largest possible value consistent with the vsin(i) and Radius values
also found in the literature.
h = Assumes the period of photometric variations is the rotation period.
Note that other values may make this assumption, but no statement of
the assumption was found in the reference(s).
i = Radius computed from the Stefan-Boltzmann law using the
luminosity and effective temperature values found in the literature.
l = Measurement quoted as a range in the reference. We give the midpoint
and take the uncertainty to be the difference between the midpoint
and the extrema.
m = Value assumed from stars of similar spectral type.
Note (5): This is the uncertainty when the corresponding "E_*" column is blank.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- Num [1/42] Dataset number (1)
4- 15 A12 --- Star Star identifier
18- 22 A5 --- f_Star For multiple grating observations
24- 27 A4 --- Inst Spectrograph used (COS or STIS)
29- 33 A5 --- Grating Grating used (E140M, G130M or G140L)
35- 54 A20 "datime" Date UT date/time of observation start
56- 63 F8.3 s Texp [208.9/5460.2] Total exposure time (2)
65- 66 I2 --- Nexp [2/47]? Number of exposures taken
68- 79 F12.3 s Tobs ? Total "observed" time (3)
81- 89 F9.3 s ATexp ? Sum of all exposure times
91- 95 F5.2 % PObs ? Percentage observed (4)
97-104 F8.2 s Tmax ? Longest block of time (5)
106-132 A27 --- Bands Spectral bands (6)
--------------------------------------------------------------------------------
Note (1): Corresponding to the numbering of Figure Sets 4 and 9.
Note (2): Not all this time was used: Remainders of less than 60 s at the end
of each exposure were discarded when making lightcurves.
Note (3): Taken as the span of time between the start of the first exposure
and end of the last exposure.
Note (4): Equal to the ratio of sumTexp to Tobs.
Note (5): Where the stellar flux was sampled at least 50% of the time.
Points during flares were discarded.
Note (6): Within which flux was measured for the dataset
(either "CII, SiIII, SiIV, Continuum" or "CII, SiIV, Continuum").
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table[4567].dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 12 A12 --- Star Star identifier
14- 18 A5 --- f_Star Flag on Star (1)
20- 27 F8.1 10-20W/m2 [1/249700]? Mean value of the lightcurve
in 10-17erg/s/cm2
29- 35 F7.1 10-20W/m2 e_ [1/4000]? uncertainty
37 A1 --- l_SigX Limit flag on SigX
39- 45 F7.5 --- SigX [0.006/1]? Mean-normalized excess noise
σx at 60s cadence
47- 52 F6.4 --- E_SigX [0.001/0.2]? SigX positive error
54- 59 F6.4 --- e_SigX [0.002/0.2]? SigX negative error
61 A1 --- l_Rsig Limit flag on Rsig
63- 69 F7.4 Rjup Rsig [0.1/31]? Radius of potential planet (2)
71- 76 F6.4 Rjup E_Rsig [0.009/8]? Rsig positive error
78- 84 F7.4 Rjup e_Rsig [0.009/13]? Rsig negative error
86 A1 --- f_Rsig Flag on Rsig (1)
88- 89 I2 --- Nf [0/28]? Number of flares identified
91- 95 F5.3 --- DC [0/0.4]? Fraction of lightcurve points
encompassed by flares (duty cycle)
97-102 F6.2 s EWp [3/691]? Min photometric equivalent width of a
detectable flare given the lightcurve scatter
--------------------------------------------------------------------------------
Note (1): Flag as follows:
f = Data contain a periodic signal not fully suppressed by the high-pass
filtering.
g = The literature did not provide an uncertainty on the stellar radius.
The uncertainty on Rsig accounts only for uncertainty in σx.
Note (2): Radius of an occulting disk that would produce a transit depth
equivalent to σx projected to 3.5hr
--------------------------------------------------------------------------------
Byte-by-byte Description of file: refs.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- Ref Reference code
5- 23 A19 --- BibCode Bibcode
25- 49 A25 --- Aut ? Author's name(s)
51- 89 A39 --- Comm Comment
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 07-Apr-2014