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The Ultraviolet Radiation Environment around M Dwarf Exoplanet Host Stars

TL;DR: The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres using observations from the Hubble Space Telescope as discussed by the authors.
Abstract: The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. Ultraviolet photons influence the atmospheric temperature profiles and production of potential biomarkers on Earth-like planets around these stars. At present, little observational or theoretical basis exists for understanding the ultraviolet spectra of M dwarfs, despite their critical importance to predicting and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades. Using observations from the Hubble Space Telescope, we present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both far-UV (FUV) and near-UV (NUV) wavelengths. The combined FUV+NUV spectra are publicly available in machine-readable format. We find that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition region UV emission. No "UV-quiet" M dwarfs are observed. The bright stellar Lyα emission lines are reconstructed, and we find that the Lyα line fluxes comprise ~37%-75% of the total 1150-3100 A flux from most M dwarfs; ≳10^3 times the solar value. We develop an empirical scaling relation between Lyα and Mg II emission, to be used when interstellar H I attenuation precludes the direct observation of Lyα. The intrinsic unreddened flux ratio is F(Lyα)/F(Mg II) = 10 ± 3. The F(FUV)/F(NUV) flux ratio, a driver for abiotic production of the suggested biomarkers O_2 and O_3, is shown to be ~0.5-3 for all M dwarfs in our sample, >10^3 times the solar ratio. For the four stars with moderate signal-to-noise Cosmic Origins Spectrograph time-resolved spectra, we find UV emission line variability with amplitudes of 50%-500% on 10^2-10^3 s timescales. This effect should be taken into account in future UV transiting planet studies, including searches for O_3 on Earth-like planets. Finally, we observe relatively bright H_2 fluorescent emission from four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832). Additional modeling work is needed to differentiate between a stellar photospheric or possible exoplanetary origin for the hot (T(H_2) ≈ 2000-4000 K) molecular gas observed in these objects.

Summary (3 min read)

1 Introduction

  • Numerical tools for spray combustion help engineers to design more efficient and less pollutant aeronautical engines.
  • These techniques seem also attractive for spray combustion and they have already been used to perform DNS (Direct Numerical Simulation) and LES of turbulent spray flames [1,8–10].
  • Results are also compared to those of the 24-species chemistry to assess the accuracy of the tabulated method.
  • The experimental configuration as well as the numerical setup are presented in Section 4.
  • Then, the turbulent reactive two-phase flow is characterized in Section 5.

2 Chemical description

  • In the following, the two different chemical descriptions considered, i.e. a multi-species kinetics and the FPI look-up table technique, are presented.
  • 1 Multi-species chemistry A 24-species mechanism developed to perform DNS of n-dodecane spray flames [11,16] is considered here.
  • The mixture fraction, defined as Yz = WF WCnCF Nspec∑ k=1 Yk nCkWC Wk (2) is often retained as a parameter of a look-up table method, where WC is the element weight of carbon atom, Yk, Wk and nCk are the mass fraction, the molar weight and the number of carbon atoms of the kth species, respectively.
  • Any thermo-chemical quantity ϕ is then stored in a 2-D look-up table ϕ = ϕFPI[Yc, Yz], where ϕFPI is obtained from laminar premixed flames.
  • 30], the global behavior of laminar spray flames is correctly reproduced by the FPI method.

3 LES system of equations

  • LES of the MERCATO spray flame configuration is performed with the AVBP solver [31– 33] using an Euler-Euler approach under the assumption of monodisperse-monokinetic liquid phase.
  • This assumption may affect the accuracy of the spray description but significantly reduces the simulation CPU cost.
  • For the subgrid unclosed term τsgsij , a viscosity-type closure is used: τsgsij = 2µ tS̃ij − 1 3 τsgskk δij ; , (10) and the turbulent viscosity µt is evaluated using the WALE model of Nicoud et al. [36].
  • The thickening factor F increases the molecular diffusion and decreases the reaction rate to thicken the flame front while preserving the laminar flame speed.

3.1.2 Thermodynamic and transport properties of the gaseous mixture

  • In the multi-species description, all thermodynamic quantities are derived from enthalpy and entropy information for each species based on the JANAF tables [39].
  • Concerning the transport properties, a simplified model based on constant and equal Schmidt (Sc) and Prandtl (Pr) numbers,4 i.e. unity Lewis number for all species, is considered here in order to guarantee consistency with the tabulated chemistry.
  • It has to be noted that the flow temperature T obtained from Eq. (3) may differ from the tabulated value T FPI .
  • While being negligible in incompressible flow, this subgrid pressure may become predominant in highly compressible flows, such as the Eulerian disperse phase.
  • The retained formulation for the thermodynamic and transport properties at the droplet surface in the case of a multi-species chemistry is provided in the following section together with its extension to the tabulated approach.

3.2.1 Transport and thermodynamic properties in the droplet vicinity

  • In analogy with the gaseous mixture treatment discussed in Section 3.1.1, Devap and λevap are calculated with both multi-species and tabulated approaches by using constant Schmidt and Prandtl numbers assumptions: ρDevap = µevapSc−1 and λevap = µevapcevapp Pr−1.
  • Results on the finer 20 million-cell mesh using the tabulation approach are also added in red dotted lines to verify the grid convergence.
  • In [47], the experimental droplet size distribution varies between 2 µm and 150 µm whereas the mean diameter is 44 µm.

5 Flame characterization

  • The flame structure and the complex dynamics of the swirled spray flame MERCATO were not investigated experimentally.
  • The presence of a secondary reaction zone is clearly identified by looking to the CO formation (positive in blue) and destruction (negative in red) zones in Fig. 6(c).
  • Close to the injection, the IRZ is characterized by a moderate temperature (Fig.7(a)right) so that the evaporation is relatively slow and the mixture equivalence ratio slowly increases (Fig.7(a)-left).
  • This complex flame structure is due to an intricate coupling between evaporation, mixing and combustion governing the stabilization location of the flame front.
  • The liquid volume fraction presents an oscillating behaviour at the same frequency as the PVC, but with a phase shift due to its high Stokes number (StPV C ≈ 10).

6 Evaluation of the FPI tabulation method for swirled spray flames

  • In Section 5, it has been discussed that the coupling between evaporation, mixing and combustion governs the flame dynamics, stabilisation and structure.
  • Time-averaged results for the flame structure are presented in Fig. 10 for both the 24- species (left) and the FPI look-up table technique .
  • The presence of an high CO concentration region is correctly reproduced by the FPI approach.
  • This species can be an indication of the ability of the FPI model to describe the complex nature of the chemical processes leading to intermediates, minor species and radicals.
  • It should be reminded that in the context of soot prediction a correct description of the acetylene and of the precur- sors, which are strongly sensitive to strain rate [51], is essential [52].

7 Conclusion

  • LES of the MERCATO experimental benchmark has been performed using a detailed chemical description accounting for 24-species to study the behavior of an industrial swirled twophase injection system.
  • Numerical results have been compared to the experimental flow in terms of mean and fluctuations of axial velocity of both gas and liquid phases as well as droplet diameter profiles.
  • It has been shown that tabulated chemistry methods allow the numerical investigation of swirled industrial spray flames, at least for the prediction of global flame behavior, with a cost which is eight times smaller than the considered multi-species description.
  • The first one is due to the spray monodisperse assumption, which cannot accurately predict equivalence ratio stratification.

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The Astrophysical Journal, 763:149 (14pp), 2013 February 1 doi:10.1088/0004-637X/763/2/149
C
2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE ULTRAVIOLET RADIATION ENVIRONMENT AROUND M DWARF EXOPLANET HOST STARS
Kevin France
1
, Cynthia S. Froning
1
, Jeffrey L. Linsky
2
, Aki Roberge
3
, John T. Stocke
1
, Feng Tian
4
,
Rachel Bushinsky
1
, Jean-Michel D
´
esert
5
, Pablo Mauas
6
, Mariela Vieytes
6
, and Lucianne M. Walkowicz
7
1
Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, CO 80309, USA; kevin.france@colorado.edu
2
JILA, University of Colorado and NIST, 440 UCB, Boulder, CO 80309, USA
3
Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
4
Center for Earth System Sciences, Tsinghua University, Beijing 100084, China
5
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
6
Instituto de Astronomsica del Espacio (CONICET-UBA), C.C. 67 Sucursal 28, 1428 Buenos Aires, Argentina
7
Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
Received 2012 October 10; accepted 2012 December 17; published 2013 January 17
ABSTRACT
The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution
of planetary atmospheres. Ultraviolet photons influence the atmospheric temperature profiles and production
of potential biomarkers on Earth-like planets around these stars. At present, little observational or theoretical
basis exists for understanding the ultraviolet spectra of M dwarfs, despite their critical importance to predicting
and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades. Using
observations from the Hubble Space Telescope, we present a study of the UV radiation fields around nearby
M dwarf planet hosts that covers both far-UV (FUV) and near-UV (NUV) wavelengths. The combined FUV+NUV
spectra are publicly available in machine-readable format. We find that all six exoplanet host stars in our sample
(GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition region
UV emission. No “UV-quiet” M dwarfs are observed. The bright stellar Lyα emission lines are reconstructed, and we
find that the Lyα line fluxescomprise 37%–75% of the total 1150–3100 Å flux from most M dwarfs; 10
3
times the
solar value. We develop an empirical scaling relation between Lyα and Mg ii emission, to be used when interstellar
H i attenuation precludes the direct observation of Lyα. The intrinsic unreddened flux ratio is F(Lyα)/F(Mg ii) =
10 ± 3. The F(FUV)/F(NUV) flux ratio, a driver for abiotic production of the suggested biomarkers O
2
and O
3
,is
shown to be 0.5–3 for all M dwarfs in our sample, >10
3
times the solar ratio. For the four stars with moderate
signal-to-noise Cosmic Origins Spectrograph time-resolved spectra, we find UV emission line variability with
amplitudes of 50%–500% on 10
2
–10
3
s timescales. This effect should be taken into account in future UV transiting
planet studies, including searches for O
3
on Earth-like planets. Finally, we observe relatively bright H
2
fluorescent
emission from four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832). Additional
modeling work is needed to differentiate between a stellar photospheric or possible exoplanetary origin for the hot
(T(H
2
) 2000–4000 K) molecular gas observed in these objects.
Key words: planetary systems stars: activity stars: individual (GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C,
GJ 1214) stars: low-mass ultraviolet: stars
Online-only material: color figures
1. INTRODUCTION
Stellar ultraviolet (UV) photons play a crucial role in the dy-
namics and chemistry of all types of planetary atmospheres.
Extreme-UV (EUV; 200 λ 911 Å) photons capable of
ionizing hydrogen heat the atmosphere and are the primary
drivers of atmospheric escape in both the solar system and ex-
oplanetary systems. For short-period gas giant planets, EUV
irradiation can heat the atmosphere to 10
4
K (Yelle 2004;
Murray-Clay et al. 2009), resulting in atmospheric mass loss
(Sanz-Forcada et al. 2011) and the inflated exospheres ob-
served in H i Lyα (Vidal-Madjar et al. 2003; Lecavelier des
Etangs et al. 2012) and metal absorption line (Vidal-Madjar
et al. 2004; Linsky et al. 2010; Fossati et al. 2010) transit spec-
tra. For terrestrial atmospheres, an increase in the EUV flux
can elevate the temperature of the thermosphere by a factor
Based on observations made with the NASA/ESA Hubble Space Telescope,
obtained from the data archive at the Space Telescope Science Institute. STScI
is operated by the Association of Universities for Research in Astronomy, Inc.,
under NASA contract NAS 5-26555.
of 10 (Tian et al. 2008), potentially causing significant and
rapid atmospheric mass loss.
Far-UV (FUV; 912 λ 1700 Å) and near-UV (NUV;
1700 λ 4000 Å) photons from the host star are the
drivers of atmospheric chemistry through their effects on the
photoexcitation and photodissociation rates of many abundant
molecular species. Solar chromospheric emission (specifically
H i Lyβ)excitesH
2
fluorescence in gas giant planets, producing
rovibrationally excited H
2
that catalyzes chemistry in the Jovian
atmosphere (Cravens 1987;Kim&Fox1994). In exo-Jovian
atmospheres heated to T > 1000 K, Lyα fluorescence of both
H
2
and CO may become an important excitation mechanism
(Wolven et al. 1997); this process may be an observable
diagnostic for gas giant atmospheres outside the solar system
(Yelle 2004; France et al. 2010b). H
2
O, CH
4
, and CO
2
are
sensitive to FUV radiation, in particular the bright H i Lyα
line which has spectral coincidences with these species. UV
radiation can be important for the long-term habitability of
Earth-like planets (Buccino et al. 2006), and the combination
of FUV and NUV photons can influence the molecular oxygen
chemistry. For example, the O
2
abundance can be significantly
1

The Astrophysical Journal, 763:149 (14pp), 2013 February 1 France et al.
enhanced through the photodissociation of CO
2
and H
2
O. The
subsequent formation of O
3
depends sensitively on the spectral
and temporal behavior of the FUV and NUV radiation fields of
the host star (Tian et al. 2012; Domagal-Goldman et al. 2012).
The UV (FUV + NUV) spectra of solar-type stars have
been studied extensively, both observationally and theoretically
(e.g., Woods et al. 2009; Fontenla et al. 2011; Linsky et al.
2012a). By studying solar analogs of various ages, we also
have an understanding of the evolution of G star spectra
(Ayres 1997; Ribas et al. 2005; Linsky et al. 2012b). By
contrast, the observational and theoretical literature on the UV
spectra of M dwarfs is sparse. The photospheric UV continuum
of M dwarfs is very low relative to solar-type stars due to
their lower effective temperature. Bright chromospheric and
transition region emission lines dominate the UV spectrum of
M dwarfs, comprising a much larger fraction of the stellar
luminosity than for solar-type stars. Outside of a few well-
studied flare stars (e.g., AU Mic, AD Leo, EV Lac, Proxima
Cen), M dwarfs have largely been ignored by UV observers
because most investigations were aimed at understanding the
origin and nature of energetic events on low-mass stars. There
are very few UV observations of the older, quiescent M dwarfs
that are most likely to harbor potentially habitable planets, and
at present no theoretical atmosphere models exist that self-
consistently predict the UV emission from an M dwarf.
UV variability of M dwarf exoplanet host stars is also essen-
tially unconstrained by observation at present. M dwarfs display
variability on many timescales and flare activity occurs with
greater frequency and amplitude than on main-sequence G and
K dwarfs (Hawley et al. 1996;Westetal.2004; Welsh et al. 2007;
Walkowicz et al. 2011). Because most flare activity is thought to
be related to magnetic energy deposition in the corona, transition
region, and chromosphere (Haisch et al. 1991), UV flare activity
has an amplitude comparable to or greater than that observed
in optical light curves. UV flare behavior on active M dwarfs is
characterized by strong blue/NUV continuum emission and the
enhancement of chromospheric and transition region emission
lines (e.g., Hawley & Pettersen 1991; Hawley et al. 2003). FUV
flares may also be signpost for enhanced soft X-ray, EUV, and
energetic particle arrival rates on planets orbiting these stars,
all of which can play an important role for the chemistry and
evolution of exoplanetary atmospheres (Segura et al. 2010) and
the long-term habitability of these worlds (Buccino et al. 2007).
Given the paucity of existing observational data and lack of
theoretical models of the UV spectra of M dwarfs, the most
reliable incident radiation field for photochemical models of
extrasolar planetary atmospheres is one created from direct
observations of the host star. Toward the goal of a comprehensive
library of UV radiation fields for use as inputs in models of
exoplanet atmospheres, we have carried out a pilot program
to observe the spectral and temporal behavior of M dwarf
exoplanet host stars, Measurements of the Ultraviolet Spectral
Characteristics of Low-mass Exoplanet host Stars (MUSCLES).
In this paper, we present observational results from the
MUSCLES pilot program, spectrally and temporally resolved
UV data for six M dwarf exoplanet host stars (GJ 581, GJ 876,
GJ 436, GJ 832, GJ 667C, and GJ 1214) observed with the
Hubble Space Telescope (HST) Cosmic Origins Spectrograph
(COS) and Space Telescope Imaging Spectrograph (STIS). We
describe the targets and their planetary systems in Section 2.
Sections 3 and 4 describe the observations, the spectral recon-
struction of the important stellar Lyα emission line, and the
creation of light curves for chromospheric and transition region
emission lines. In Sections 5 and 6, we describe the spectral
and temporal characteristics, respectively, of our target sample.
Section
7 places these results in context of models of terrestrial
planetary atmospheres and discusses implications for UV transit
studies. We summarize our results in Section 8.
2. TARGETS
In this section, we briefly summarize the stellar and planetary
system properties of our M dwarf targets. Optical spectra of
most of our targets have been published in the literature, and
while we do not make a detailed comparison of the UV and vis-
ible emission lines here, it is valuable to place the MUSCLES
targets in the context of traditional optical activity indicators.
The stars in our sample would traditionally be considered “opti-
cally inactive, based on their Hα absorption spectra (Gizis et al.
2002). However, all of our stars with measured Ca ii H and K
profiles show weak but detectable emission (equivalent widths,
EW(Ca ii) > 0), indicating that at least a low level of chro-
mospheric activity is present in these stars (Rauscher & Marcy
2006; Walkowicz & Hawley 2009). Our stars show Hα in ab-
sorption with equivalent widths in the range 0.4 < EW(Hα) <
0.2 Å and Ca ii emission with equivalent widths in the range
0.2 < EW(Ca ii) < 0.5 Å. Adopting the M dwarf classification
from Walkowicz & Hawley (2009), these stars would be referred
to as possessing intermediate chromospheres, or weakly active
M dwarfs.
GJ 581. GJ 581 is an M2.5 dwarf at a distance of 6.3 pc.
It is estimated to have an age of 8 ± 1 Gyr (Selsis et al.
2007) and a somewhat subsolar metallicity, [Fe/H] =−0.10 to
0.02 (Johnson & Apps 2009; Rojas-Ayala et al. 2010). GJ 581
is not detected in X-ray surveys (log
10
L
X
< 26.89 erg s
1
;
Poppenhaeger et al. 2010) and its optical spectrum displays
Hα in absorption, therefore chromospheric and coronal activity
are thought to be low for this target. GJ 581 has one of the richest
known planetary systems, with possibly up to six planets (four
confirmed) including several of Earth/super-Earth mass (Mayor
et al. 2009; Tuomi 2011). GJ 581d is a super-Earth (M
P
6 M
)
that resides on the outer edge of the habitable zone (HZ; a
P
=
0.22 AU; Wordsworth et al. 2011; von Braun et al. 2011).
GJ 876. GJ 876 is an M4 dwarf at 4.7 pc, and is the only
planet-hosting M dwarf with a well-characterized UV spectrum
prior to the present work (Walkowicz et al. 2008; France et al.
2012a). GJ 876 has super-solar metallicity ([Fe/H] = 0.37–0.43;
Johnson & Apps 2009; Rojas-Ayala et al. 2010), and differing
estimates on the stellar rotation period (40 days P
97 days)
result in large uncertainties in the age estimate for this system,
0.1–5 Gyr (Rivera et al. 2005, 2010; Correia et al. 2010). While
the star would be characterized as weakly active based on
its Hα absorption spectrum, UV and X-ray observations have
shown the presence of an active upper atmosphere (Walkowicz
et al. 2008; France et al. 2012a;log
10
L
X
= 26.48 erg s
1
;
Poppenhaeger et al. 2010). GJ 876 has a rich planetary system,
with four planets ranging from a super-Earth (GJ 876d, M
P
6.6 M
) in a short-period orbit (a
P
= 0.02 AU; Rivera et al.
2010) to two Jovian-mass planets in the HZ (GJ 876b, M
P
2.27 M
Jup
, a
P
= 0.21 AU; GJ 876c, M
P
0.72 M
Jup
, a
P
=
0.13 AU; Rivera et al. 2010).
GJ 436. GJ 436 is an M3 dwarf star located at a distance
of 10.3 pc. It has a 45 day rotation period, a relatively old age
(6
+4
5
Gyr; Torres 2007), and may have a super-solar metallicity
([Fe/H] = 0.00–0.25; Johnson & Apps 2009; Rojas-Ayala et al.
2010). GJ 436 does show signs of an active corona (log
10
L
X
=
27.16 erg s
1
; Poppenhaeger et al. 2010), and its chromospheric
2

The Astrophysical Journal, 763:149 (14pp), 2013 February 1 France et al.
Lyα emission has been observed by Ehrenreich et al. (2011).
GJ 436 is notable for its well-studied transiting Neptune-mass
planet (Butler et al. 2004; Pont et al. 2009), orbiting at a
semimajor axis of 0.03 AU, interior to its HZ (0.16–0.31 AU;
von Braun et al. 2012). Additional low-mass planets may also
be present in this system (Stevenson et al. 2012).
GJ 832. GJ 832 is an M1 dwarf at d = 4.9 pc. GJ 832 is
not as well characterized as other targets in our sample; an
age determination for this star is not available. Coronal X-rays
have been detected from GJ 832, log
10
L
X
= 26.77 erg s
1
(Poppenhaeger et al. 2010). This subsolar metallicity star
([Fe/H] =−0.12; Johnson & Apps 2009) hosts a 0.64 M
Jup
mass planet in a 9.4 year orbit (a
P
= 3.4 AU; Bailey et al. 2009).
GJ 667C. GJ 667C (M1.5V) is a member of a triple star
system (GJ 667AB is a K3V + K5V binary) at a distance of
6.9 pc. This 2–10 Gyr M dwarf (Anglada-Escud
´
eetal.2012)
is metal-poor ([Fe/H] =−0.59 ± 0.10, based on an analysis
of GJ 667AB; Perrin et al. 1988) and may host as many as
three planets, including a super-Earth mass planet (GJ 667Cc,
M
P
4.5 M
, a
P
= 0.12 AU) orbiting in the HZ (0.11–0.23 AU;
Anglada-Escud
´
eetal.2012).
GJ 1214. GJ 1214 is a late M dwarf (M6V) at 13 pc, making
it the coolest and most distant of the targets in the MUSCLES
pilot study. It has an age of 6 ± 3 Gyr (Charbonneau et al. 2009),
a super-solar metallicity ([Fe/H] = +0.39 ± 0.15; Berta et al.
2011; Rojas-Ayala et al. 2010), and shows signs of optical flare
activity (Kundurthy et al. 2011). GJ 1214b is a transiting super-
Earth (M
P
6.5 M
, a
P
= 0.014 AU; Charbonneau et al. 2009),
possibly harboring a dense, water-rich atmosphere (Bean et al.
2010;D
´
esert et al. 2011).
3. OBSERVATIONS
The MUSCLES observing plan uses the two primary ultravi-
olet spectrographs on HST to create quasi-continuous M dwarf
exoplanet host star data from 1150 to 3140 Å. Data for the
MUSCLES program were obtained as part of HST GTO and
GO programs 12034, 12035, and 12464, acquired between 2011
June and 2012 August. COS has a factor of >10 times more
effective area than the medium resolution modes of STIS in
the FUV, and its low detector background and grating scat-
ter make it factors of 50 times more efficient for the study
of faint far-UV chromospheric and transition region emission
from M dwarfs (Green et al. 2012). We used COS, in its time-tag
(TTAG) acquisition mode, to observe the FUV spectra of our
target stars and study time variability on scales from minutes
to hours (Section 4.2 and Section 6). Multiple central wave-
lengths and focal-plane offset (FP-POSs) settings with the COS
G130M and G160M modes were used to create a continuous
FUV spectrum from 1145 to 1795 Å. These modes provide a
point-source resolution of Δv 17 km s
1
with 7 pixels per
resolution element (Osterman et al. 2011).
Because COS is a slitless spectrograph, observations at
H i Lyα are heavily contaminated by geocoronal emission.
We therefore observed the stellar Lyα profile with STIS, using
either the G140M/cenwave 1222 mode (Δv 30 km s
1
)
through the 52

× 0.

1 slit, or the E140M mode (Δv
7.5 km s
1
) through the 0.

2 × 0.

2 slit for the brightest targets. In
order to include the important Lyα emission line for GJ 436 and
GJ 1214, we downloaded STIS G140M observations of these
stars from the MAST archive (program IDs 11817 and 12165,
respectively). The Lyα profile of GJ 436 has been described in
Ehrenreich et al. (2011). In order to maximize the combination
of wavelength coverage and sensitivity, near-UV spectra were
Table 1
MUSCLES HST Observations
Target Date Instrument, Mode PID T
exp
(s)
GJ 581 2012 Apr 18 STIS, G140M 12034 1107
GJ 581 2012 Apr 18 STIS, G230L 12034 265
GJ 581 2011 Jul 20 COS, G130M 12034 1478
GJ 581 2011 Jul 20 COS, G160M 12034 900
GJ 876 2011 Nov 12 STIS, G140M 12464 1138
GJ 876 2011 Nov 12 STIS, G230L 12464 267
GJ 876 2012 Jan 5 COS, G130M 12464 2017
GJ 876 2012 Jan 5 COS, G160M 12464 2779
GJ 436 2010 Jan 5 STIS, G140M 11817 1762
GJ 436 2012 May 10 STIS, G230L 12464 1665
GJ 436 2012 Jun 23 COS, G130M 12464 3372
GJ 436 2012 Jun 23 COS, G160M 12464 4413
GJ 832 2011 Jun 9 STIS, E140M 12035 2572
GJ 832 2012 Jun 10 STIS, E230H 12035 3135
GJ 832 2012 Apr 10 STIS, G230L 12464 917
GJ 832 2012 Jul 28 COS, G130M 12464 2163
GJ 832 2012 Jul 28 COS, G160M 12464 2925
GJ 667C 2011 Sep 4 STIS, E140M 12035 2396
GJ 667C 2011 Sep 4 STIS, E230H 12035 3023
GJ 1214 2011 Apr 27 STIS, G140M 12165 7282
GJ 1214 2012 Aug 12 STIS, G230L 12464 1620
GJ 1214 2012 Aug 4 COS, G130M 12464 3289
GJ 1214 2012 Aug 4 COS, G160M 12464 4368
observed with the STIS G230L/cenwave 2376 mode (Δv
600 km s
1
) through the 52

× 0.

1 slit. In order to constrain the
effects of interstellar absorption on the stellar Mg ii emission
profile, we also acquired Mg ii spectra with the STIS E230H
mode (Δv 2.6 km s
1
) through the 0.

2 × 0.

2 slit for GJ 832
and GJ 667C.
Exposure times were generally short due to the pilot-study
nature of this project; considerably longer integrations will be
essential for future observations to measure UV flare frequencies
in exoplanet host stars and to produce high-quality line profiles
of important chromospheric and transition region tracers such
as C ii λ1335, N v λ1240, and Si iv λ1400. A complete list of the
M dwarf observations used in this work is presented in Table 1,
and complete spectra for the four targets in which Lyα,FUV,
and NUV emission is observed are displayed in Figure 1.
In order to place the M dwarf exoplanet host stars in context
with other well-studied cool stars, we assembled archival spectra
of AD Leo (M3.5Ve), HD 189733 (K1V), and the Sun (G2V).
AD Leo is one of the best-studied flare stars, and despite its
extreme activity levels is has been the object of choice for
models of extrasolar planets orbiting M dwarfs, mainly because
of the lack of reasonable alternatives. We created a complete UV
spectrum of AD Leo by combining STIS E140M monitoring
observations (Hawley et al. 2003) with an “average” NUV
spectrum from IUE.TheIUE spectrum is the average of flare and
quiescent states, scaled to the FUV flux from STIS. HD189733
is an active K dwarf hosting the prototypical transiting hot
Jupiter. While high-quality COS spectra of HD 189733 exist
in the archive (e.g., Linsky et al. 2012b; Haswell et al. 2012),
we elected to use the combined NUV+FUV (STIS E140M +
E230M) spectra of a surrogate star taken from the STARCAT
library (Ayres 2010) to eliminate uncertainties about temporal
variability between different observations. We chose HD37394
(K1V variable star; V = 6.23) because it is a good match in
stellar mass and activity level to HD 189733, and because the
FUV spectra of the two stars are qualitatively similar. The UV
3

The Astrophysical Journal, 763:149 (14pp), 2013 February 1 France et al.
1500 2000 2500 3000
Wavelength (Å)
−16
−14
−12
−10
log
10
Flux + Offset
(ergs cm
−2
s
−1
Å
−1
)
GJ581
GJ876
GJ436
GJ832
COS/STIS
Gap
CIII
SiIII
Lyα
CII
SiIV
CIV
HeII
CI
AlII
FeII
FeII
MgII
Figure 1. Observed 1150–3140Å fluxes from the M dwarf exoplanet host stars
in the MUSCLES sample with complete (Lyα + FUV + NUV) data sets. The
1760–2100 Å emission has been omitted because the flux levels are below the
STIS G230L detection level. For display purposes, the data have been convolved
with a 3 Å FWHM Gaussian kernel and offset as follows: GJ 581, no offset;
GJ 876, log
10
F
λ
+ 1.2; GJ 436, log
10
F
λ
+ 2.4; GJ 832, log
10
F
λ
+3.6.
(A color version of this figure is available in the online journal.)
spectrum of HD37394 was then scaled to the V-magnitude of
HD 189733 (V = 7.77). The spectrum of the quiet Sun was
taken from Woods et al. (2009), and serves as the prototype
main-sequence G-type star.
4. INTRINSIC Lyα PROFILE RECONSTRUCTION AND
TIME VARIABILITY ANALYSIS
4.1. Lyα Reconstruction
The stellar Lyα emission line dominates the UV output
of M dwarfs, containing approximately as much energy as
the rest of the FUV+NUV spectrum combined (France et al.
2012a). Therefore, measurements of Lyα emission provide an
important constraint on the source term for calculations of
the heating and photochemistry of exoplanetary atmospheres
(Linsky et al. 2012a). Due to resonant scattering of neutral
hydrogen and deuterium in the interstellar medium (ISM), the
intrinsic Lyα radiation field cannot be directly measured, even
in the nearest stars (log
10
N(H i) 18.5). In order to produce
an accurate estimate of the local Lyα flux incident on the
planets around our target stars, the Lyα profiles observed by
STIS must be reconstituted. We do this using a technique that
simultaneously fits the observed Lyα emission line profile and
the ISM component, using the MPFIT routine to minimize χ
2
between the fit and data (Markwardt 2009).
The intrinsic Lyα emission line is approximated as a two-
component Gaussian, as suggested for transition region emis-
sion lines from late-type stars by Wood et al. (1997). This ap-
proximation is further justified by the reconstructed Lyα profiles
for M dwarfs, which show approximately Gaussian line shapes
and little evidence for self-reversal compared to F, G, and
K dwarfs (Wood et al. 2005). The Lyα emission components
are characterized by an amplitude, FWHM, and velocity cen-
troid. Interstellar absorption is parameterized by the column
density of neutral hydrogen (N(H i)), Doppler b parameter, D/H
ratio, and the velocity of the interstellar absorbers. We assumed
afixedD/H ratio (D/H = 1.5 × 10
5
; Linsky et al. 1995, 2006).
Degeneracy between the two emission components makes errors
difficult to define for the individual fit parameters. We estimate
the uncertainty on the integrated Lyα fluxes from our technique
by comparing the reconstructed Lyα flux for a range of initial
guesses on the parameters. The uncertainty on the integrated
intrinsic flux is 10%–20% for the stars observed with E140M
(GJ 832 and GJ 667C) and 15%–30% for the stars observed
with G140M (GJ 581, GJ 876, GJ 436). In order to test our
methodology, we fitted the STIS E140M Lyα profile of AU Mic.
Our total integrated flux agrees with that presented in Wood et al.
(2005)to5%, with an identical derivation of the interstellar
atomic hydrogen column on the line of sight (log
10
N(H i) =
18.36 ± 0.01).
Employing our iterative least-squares reconstruction tech-
nique, we also derived the intrinsic Lyα spectrum of HD 189733.
Using the 2010 April 6 STIS G140M spectra (Lecavelier des
Etangs et al. 2012), we find an intrinsic flux of F(Lyα) = 7.5 ×
10
13
erg cm
2
s
1
and an interstellar hydrogen column density
log
10
N(H i) = 18.45. We also refitted the intrinsic Lyα emission
from AD Leo, finding F(Lyα) = 7.5 × 10
12
erg cm
2
s
1
and
an interstellar hydrogen column density log
10
N(H i) = 18.47 ±
0.02. This value agrees with the Lyα reconstruction from Wood
et al. (2005)to30%, which gives us confidence in the iterative
technique, even for sightlines with multiple interstellar velocity
components.
We present the individual Lyα spectra in Figure 2, and
narrow/broad emission component ratios (F
narrow
/F
broad
), spec-
tral line widths, H i column densities, and Doppler b-values in
Table 2. We find F
narrow
/F
broad
to be in the range 5–12 for the
MUSCLES stars, again independent of the resolution of the
observations on which the reconstructions are based. For com-
parison, we find F
narrow
/F
broad
3 for the more active M dwarf
(AD Leo) and more massive star (HD 189733). Reconstructed
Lyα fluxes are given in Tables 3, 4, and 5. GJ 1214 has no
Lyα emission detected, and we discuss this in greater detail in
Section 5.1.1.
4.2. Timing Analysis
In our initial study of GJ 876, we described the large
chromospheric/transition region flare observed in several UV
emission lines during our COS observations (France et al.
2012a). We have carried out a similar analysis on the entire
MUSCLES sample to constrain the heretofore unknown level
of UV variability in optically quiet M dwarf exoplanet host stars.
Light curves in several chromospheric and transition region
lines were extracted from the calibrated three-dimensional data
by exploiting the time-tag capability of the COS microchannel
plate detector (France et al. 2010a). We extract a [λ
i
,y
i
,t
i
] photon
list (where λ is the wavelength of the photon, y is the cross-
dispersion location, and t is the photon arrival time) from each
exposure i and combine these to create a master [λ,y,t] photon
list. The total number of counts in a [Δλ,Δy] box is integrated
over a time step Δt. We use a flux-dependent time step of Δt =
40–120 s for the MUSCLES targets. The instrument background
level is computed in a similar manner, with the background
integrated over the same wavelength interval as the emission
lines, but physically offset below the science region in the cross-
dispersion direction. The signal-to-noise ratio (S/N) was not
high enough to temporally resolve the GJ 1214 data.
5. STELLAR UV EMISSION LINES AND
THE FUV/NUV RATIO
5.1. Chromospheric and Transition Region Emission
Chromospheric and transition region emission lines are
observed in all of the MUSCLES spectra, suggesting that
4

The Astrophysical Journal, 763:149 (14pp), 2013 February 1 France et al.
Table 2
Stellar Lyα Emission Line Widths and Interstellar Atomic Hydrogen
a
Target F
narrow
/F
broad
FWHM
narrow
FWHM
broad
log
10
N(H i) b
H i
(km s
1
)(kms
1
)(cm
2
)(kms
1
)
GJ 581
b
8.4 110 228 18.35 ± 0.06 10.1
GJ 876
b
5.0 132 303 18.06 ± 0.03 8.0
GJ 436
b
11.8 109 294 18.19 ± 0.04 7.4
GJ 832
c
7.6 96 163 18.47 ± 0.02 9.7
GJ 667C
c
8.9 85 233 18.07 ± 0.03 12.7
AD Leo
c
2.9 166 412 18.47 ± 0.01 9.0
HD 189733
b,d
3.5 170 480 18.45 10
Notes.
a
Lyα profiles reconstructed using the iterative least-squares technique described in Section 4.1.
b
STIS G140M observations.
c
STIS E140M observations.
d
HD 189733 reconstruction failed to converge, the presented profile parameters provide a fit to
the observed line profile that is qualitatively similar to the formal fits for the M dwarf spectra.
Table 3
M dwarf Broadband UV and Reconstructed Lyα Fluxes (erg cm
2
s
1
)
a
Target d F(Lyα)
b
F(FUV)
c
F(NUV)
c
(pc) (Δλ
d
= 1210–1222 Å) (Δλ = 1160–1690 Å) (Δλ = 2300–3050 Å)
GJ 581 6.3 3.0 × 10
13
3.6 × 10
14
2.3 × 10
13
GJ 876 4.7 4.4 × 10
13
1.7 × 10
13
3.9 × 10
13
GJ 436 10.2 3.5 × 10
13
3.3 × 10
14
2.8 × 10
13
GJ 832 4.9 5.0 × 10
12
9.8 × 10
14
2.2 × 10
12
GJ 667C 6.9 7.6 × 10
13
... ...
GJ 1214 13.0 <2.4 × 10
15
1.0 × 10
14
2.0 × 10
14
AD Leo 4.7 7.5 × 10
12
3.4 × 10
12
9.3 × 10
12
HD 189733
e
19.5 7.5 × 10
13
4.1 × 10
13
1.0 × 10
10
Sun 1 AU 5.9 × 10
0
5.9 × 10
0
1.6 × 10
4
Notes.
a
Flux measurements are averaged over all exposure times.
b
Uncertainty on the reconstructed Lyα flux is estimated to be between 10%–20% for the stars observed
with E140M (GJ 832, GJ 667C, and AD Leo) and 15%–30% for stars observed with G140M (GJ 581,
GJ 876, GJ 436, and HD 189733).
c
F(FUV) refers to the integrated 1160–1690 Å flux excluding Lyα. Flux uncertainties for the broad
band measurements are dominated by instrumental calibrations and target acquisition errors for faint
sources. The flux errors for F(FUV) and F(NUV) are estimated to be 10% and 5%, respectively.
d
Δ λ is the bandpass over which the flux is integrated.
e
HD 189733 Lyα reconstruction made from direct STIS G140M observations. F(FUV) and F(NUV)
were measured using a proxy star (HD 37394) scaled to the V-magnitude of HD 189733.
Table 4
M dwarf Broadband UV Luminosity (erg s
1
)
a
Target d log
10
L(Lyα)log
10
L(FUV)
b
log
10
L(NUV)
(pc) (Δλ
c
= 1210–1222 Å) (Δλ = 1160–1690 Å) (Δλ = 2300–3050 Å)
GJ 581 6.3 27.16 26.23 27.03
GJ 876 4.7 27.07 26.65 27.01
GJ 436 10.2 27.65 26.62 27.55
GJ 832 4.9 28.15 26.44 27.80
GJ 1214 13.0 <25.69 26.31 26.61
log
10
L(Lyα)/L
bol
d
log
10
L(FUV)/L
bol
log
10
L(NUV)/L
bol
GJ 581 6.3 4.11 5.04 4.24
GJ 876 4.7 4.26 4.68 4.32
GJ 436 10.2 4.07 5.10 4.17
GJ 832 4.9 3.50 5.21 3.85
GJ 1214 13.0 <6.24 5.62 5.31
Notes.
a
Flux measurements are averaged over all exposure times.
b
L(FUV) refers to the integrated 1160–1690 Å flux excluding Lyα.
c
Δ λ is the bandpass over which the flux is integrated.
d
L
bol
is the integral of the X-ray through IR flux of each star (Section 5.2).
5

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TL;DR: A stellar spectral flux library of wide spectral coverage and an example of its application are presented in this paper, which consists of 131 flux-calibrated spectra, encompassing all normal spectral types and luminosity classes at solar abundance, and metal-weak and metalrich F-K dwarf and G-K giant components.
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846 citations


"The Ultraviolet Radiation Environme..." refers background in this paper

  • ...By studying solar analogs of various ages, we also have an understanding of the evolution of G star spectra (Ayres 1997; Ribas et al. 2005; Linsky et al. 2012b)....

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Using observations from the Hubble Space Telescope, the authors present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both far-UV ( FUV ) and near-UV ( NUV ) wavelengths. The intrinsic unreddened flux ratio is F ( Lyα ) /F ( Mg ii ) = 10 ± 3. The F ( FUV ) /F ( NUV ) flux ratio, a driver for abiotic production of the suggested biomarkers O2 and O3, is shown to be ∼0. 

Continuous HST observing campaigns, spanning 6–10 orbits each, of a larger sample of exoplanet host stars will be essential to better constraining the frequency, duration, and amplitude of the variable UV radiation environment of planets around M dwarfs. 

In the case of no UV flux, the short-wavelength spectral cutoff is determined by the photosphere of a theoretical star with Teff 3500 K, typically having negligible flux below 2500 Å and no chromospheric emission features. 

The replenishment term from the escaping atmospheres of their short-period planets is Φloss = Ṁlossm−1H2 F(H2/H i) V −1emit, where Ṁloss is the planetary atmosphere mass-loss rate, F(H2/H i) is the fraction of the total cloud in molecular form, and Vemit is the volume of the emitting area (Vemit = πr2H2zH2). 

Using an iterative least-squares approach, the authors reconstruct the intrinsic Lyα radiation field strengths assuming a two-component Gaussian emission line. 

Lyα is the brightest line in the UV spectrum of low-mass stars, although resonant scattering in the ISM makes direct line profiles inaccessible, even for the nearest stars (Wood et al. 2000, 2005). 

Light curves in several chromospheric and transition region lines were extracted from the calibrated three-dimensional data by exploiting the time-tag capability of the COS microchannel plate detector (France et al. 2010a). 

Until such time that stellar models of M-type stars can reliably predict the observed UV-through-IR spectrum, a direct UV observation will be the most reliable means for determining the radiation field incident on planets orbiting M dwarfs. 

Keeping the crudeness of this approach in mind, the authors find that the H2 disk can be sustained for mass-loss rates Ṁloss 6 × 1010 g s−1, well within the range of mass-loss estimates of short-period planets around G-, K-, and M-type stars (Vidal-Madjar et al. 

Outside of a few wellstudied flare stars (e.g., AU Mic, AD Leo, EV Lac, Proxima Cen), M dwarfs have largely been ignored by UV observers because most investigations were aimed at understanding the origin and nature of energetic events on low-mass stars. 

A final possibility is that the observed H2 fluorescence originates in a circumstellar gas envelope that is being replenished by atmospheric mass loss from escaping planetary atmospheres. 

A more complete understanding of UV variability on weakly active and inactive M dwarfs, in combination with better constrained flare frequencies for stars of a variety of activity strengths, would allow one to use the wealth of data from current and upcoming large optical surveys to inform planetary atmosphere models. 

Factoring in uncertainties on the interstellar correction and the Lyα reconstruction, the authors estimate that this scaling relation is good to ∼30%, or the ISM-corrected intrinsic F(Lyα)/ F(Mg ii) ratio for weakly active M dwarf exoplanet host stars is 10 ± 3.