Testing the models: NIR imaging and spectroscopy of the benchmark T-dwarf binary Eps Indi B

0.24 MB·English

by  Markus Kasper

Testing the models: NIR imaging and spectroscopy of the
benchmark T-dwarf binary Eps Indi B
M. Kasper
European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
[email protected]
9
0
0
2 A. Burrows

n Princeton University, Department of Astrophysical Sciences, Princeton, NJ 08544-1001, USA
a
and
J

4
1
W. Brandner

]
R Max-Planck-Institut fu¨r Astronomie, K¨onigstuhl 17, 69117 Heidelberg, Germany
S
.
h
p
-
o ABSTRACT
r
t The relative roles of metallicity and surface gravity on the near-infrared spectra of late-T
s
a browndwarfsarenotyetfullyunderstood, andevolutionarymodelsstillneedtobecalibratedin
[ order to provide accurate estimates of brown dwarf physical parameters from measured spectra.

TheT-typebrowndwarfsEpsIndiBaandBbformingthetightlyboundbinaryEpsIndiB,which
1
v orbits the K4V star Eps Indi A, are nowadays the only such benchmark T dwarfs for which all
7 important physical parameters such as metallicity, age and mass are (or soon will be) known.
9 We present spatially resolved VLT/NACO images and low resolution spectra of Eps Indi B in
9
the J, H and K near-infrared bands. The spectral types of Eps Indi Ba and Bb are determined
1
bydirectcomparisonoftheflux-calibratedJHKspectrawithTdwarfstandardtemplatespectra
.
1 and also by NIR spectral indices. Eps Indi Bb is confirmed as a T6 while the spectral type of
0
Eps Indi Ba is T1.5 so somewhat later than the previously reported T1. Constrained values for
9
surface gravity and effective temperature are derived by comparison with model spectra. The
0
v: evolutionarymodelspredictmassesaround∼53MJ forEpsIndiBaand∼34MJ forEpsIndiBb,
slightly higher than previously reported values. The suppressed J-band and enhanced K-band
i
X flux of Eps Indi Ba indicates that a noticeable cloud layer is still present in a T1.5 dwarf while
r no clouds are needed to model the spectrum of Eps Indi Bb.
a
Subject headings: stars: late-type — stars: low-mass, brown dwarfs – stars: binaries: general
1. Introduction
samechemicalcomposition,ageandmetallicityof
themainsequenceprimarydeterminedbywelles-
Multiple stellar systems with one component
tablished methods also apply to the brown dwarf
being a main sequence star and at least one other
companion(s). Furthermore, sufficiently tight bi-
component having a mass below the hydrogen-
nary systems would allow one to directly measure
burninglimit,andhenceresidinginthedomainof
thesystemmassbydeterminingtheorbitandeven
brown dwarfs are the key to calibrate and bench-
to measure the individual component masses via
mark brown dwarf evolution models. Since all
monitoring of radial velocities. Hence, the mea-
components are expected to be coeval and of the
1
sured spectral energy distribution (SED) of such with high precision. While the Eps Indi B binary
acompanioncanbeusedtocalibratetheeffective isseparatedfromtheEpsIndiAby1500AU(sev-
temperature and surface gravity of evolutionary eralarcminutesonskyatthedistanceof3.626pc),
models and provide the required benchmarks. thetwobrowndwarfsareseparatedonlybyabout
One such precious object, Eps Indi B, was re- 0.7(cid:48)(cid:48) corresponding to a projected spatial separa-
solved into a binary T1/T6 dwarf (McCaughrean tion of 2.6 AU (McCaughrean et al. 2004). This
et al. 2004) orbiting the K4.5V primary Eps Indi suggeststhatthesystemmassmightbemeasured
A during the commissioning of the Simultaneous fromorbitalmotionwithinafewyears. Frommea-
Differential Imager (SDI, Close et al. 2005) at surements on Eps Indi A, the system age is rea-
the VLT. Initial mass estimates based on spec- sonably well determined to be in the range 0.8 to
tralpropertiesandanassumedageof1.3Gyrwere 2 Gyr (Lachaume et al. 1999), and a near-to-solar
47±10M and28±7M forEpsIndBaandBb, metallicity([Mg/H]=-0.05,[Al/H]=-0.04,[Fe/H]
Jup Jup
respectively (McCaughrean et al. 2004). Another = -0.06) is reported by Beir˜ao et al. (2005).
T6 dwarf companion, SCR 1845 B, was recently
found by Biller et al. (2006) around the M8.5 pri- 2. Observations and Data Reduction
mary SCR 1845 A using the same instrument and
Eps Indi B was observed with NACO mounted
characterized by NIR imaging and spectroscopy
onUT4oftheVLTon23May2008(imaging)and
by Kasper et al. (2007). Both, Eps Indi B and
9 June 2008 (spectroscopy). The Adaptive Optics
SCR 1845 B, are extremely nearby (closer than 4
used 90% of the light from the binary as a guide
pc), so near that they can efficiently be observed
star for its infrared wavefront sensor while 10%
withmodernAdaptiveOpticsinstrumentsdespite
of the light were directed towards the scientific
their intrinsic faintness and rather small orbital
camera. In this way, we obtained well corrected
distance from the primary. Other benchmark ob-
images that easily resolve the 0.58(cid:48)(cid:48) binary (see
jects include HN Peg (Leggett et al. 2008), Gl
Figure1).
570D (Saumon et al. 2006) and HD 130948 BC
(Dupuy et al. 2008; Potter et al. 2002; Goto et al.
2.1. Imaging
2002).
It is now widely accepted that, besides T Fortheimagingobservations,theS13cameraof
eff
(a function of bolometric luminosity and radius NACOwasusedintheJ,H-andKsnear-infrared
which are both a function of age), surface gravity filters, with a pixel scale of 13.20±0.05mas per
(a function of mass and radius) and metallicity pixel. The target was observed using a standard
are centrally important in determining the spec- dithering procedure integrating for three minutes
tral energy distribution of late-T dwarfs (Saumon on source in each of the bands.
et al. 2007; Liu et al. 2007). The effects of metal- Image data reduction followed standard proce-
licity and surface gravity on the NIR spectra are, dures including sky removal, bad pixel cleaning,
however,notyetfullyunderstood(Burgasseretal. and flat fielding. Differential photometry and as-
2006a; Liu et al. 2007). Calibration of theoretical trometry were done on the reduced images by si-
models against observations has started with the multaneously fitting a model PSF profile to the
work by Burgasser et al. (2006a); Saumon et al. binary. The individual magnitudes for Eps Indi
(2006); Leggett et al. (2007). Both mass and age,
i.e., the main underlying physical parameters for
any modelling of brown dwarf evolution and at-
mospheres, are, however, still highly uncertain for
most known T dwarfs.
Precise mass estimates can only be derived for
browndwarf,whicharemembersofbinaryormul-
tiple systems, and for which orbital parameters
Fig. 1.— NACO broad band JHK images of the
have been determined. Currently, Eps Indi Bb is
0(cid:48).(cid:48)583 binary Eps Indi Bb. North is up, East is
the only late T dwarf for which physical parame-
left.
ters,andinparticularitsmass,willsoonbeknown
2
Ba and Bb were finally calculated from the rela- ibrated. The halo of Eps Indi Ba at the po-
tivemagnitudesandtheintegrated2MASSmagni- sition of Bb was estimated at its opposite po-
tudesoftheunresolvedsystem(J=11.91,H=11.31 sition with respect to Eps Indi Ba. The spec-
and Ks=11.21). The error introduced by the dif- tra were divided by the telluric calibrator and
ference between NACO and 2MASS instrument multiplied by a Kurucz template spectrum of
responses has been calculated from the spectra, Vega (http://kurucz.harvard.edu/stars/VEGA/)
as described in the next section, and has been smoothed to the actual resolution of the data in
taken into account. Table1 displays 2MASS pho- order to remove the hydrogen absorption features
tometry of Eps Indi Ba and Bb. On 23 May of the A0V telluric calibrator. Synthetic magni-
2008, the separation between the two components tudeswerecalculatedasdescribedinKasperetal.
was 0(cid:48).(cid:48)583±0(cid:48).(cid:48)003, and the position angle was (2007) in order to determine and apply the cor-
153◦.9±0◦.5. The errors of the astrometric posi- rection factors needed to convert the flux ratios
tions and the flux are the 95% confidence levels from the NACO to the 2MASS photometric sys-
provided by the MATLAB fitting algorithm, and tem. Flux calibration of the spectra was finally
the error on position angle is introduced by the established using the calculated 2MASS magni-
uncertainty of the NACO FoV orientation. tudes of both components (see table1) and the
2MASSzeromagnitudein-bandflux(Cohenetal.
2.2. Spectroscopy 2003).
It was verified that the errors introduced by
NACOwasusedinlong-slitspectroscopicmode
thedifferenceoftheisophotalwavelengthsforEps
to obtain R ≈ 400 spectra in the J and HK spec-
Indi B and and the spectral calibrator do not ex-
troscopic filters (modes S54-4-SJ and S54-4-SHK,
ceed the photometric errors. We also estimated
2nm/px pixel dispersion, S54 camera with 54.3
noise from the standard deviation of the individ-
mas/pixel). By turning the Nasmyth adaptor ro-
ual spectra that were generated during the data
tator, the slit was aligned with the binary axis.
analysis and averaged to obtain the final result.
Dithering the binary at two positions along the
The SNR for each individual data point is around
slit, exposures of 26 minutes total in each of the
30 for Eps Indi Ba and most parts of Eps Indi Bb
modes were obtained. A telluric calibrator (HIP
spectra. In the K-band and in the depression be-
113982, spectral type A0V) has been observed in
tween the J-band peaks, the SNR of the Eps Indi
a similar manner just after the object at a simi-
Bb spectrum drops to 3-10. The deduced NIR
lar airmass. The effects of differential slit losses
spectrum is consistent with the optical to mid-IR
between Eps Indi B and the spectral calibrator
spectra presented by King et al. (2008).
(Goto et al. 2003) were mitigated by observing
both properly centered on the slit with about the
3. Results
same orientation of the parallactic angle with re-
spect to the dispersion direction. A spectrum of
3.1. Spectral Types
the NACO internal Tungsten lamp was recorded
for wavelength calibration. The most direct way to determine the spectral
Spectroscopicdatareductionfollowedstandard typeofastarisdirectcomparisontospectralstan-
procedures. After sky removal, bad pixel clean- dards. Figures2 and 3 plot the NIR spectrum of
ing and flat fielding, the spectra from Eps Indi EpsIndiBaandBb(blacklines)ontopofTdwarf
Ba and Bb were extracted and wavelength cal- spectra obtained by Knapp et al. (2004); Strauss
et al. (1999); Leggett et al. (2000); Chiu et al.
(2006) and defined as spectral standards by Bur-
gasser et al. (2006b). The NIR spectrum of Eps
Table 1: 2MASS photometry of Eps Indi Ba and
IndiBafitssomewhatinbetweenthestandardT1
Bb from 23 May 2008 (JD 2454609.9).
andT2spectra. Usingthespectralindicesdefined
Star J H Ks
byBurgasseretal.(2006b),three(H O-J,H O-H,
Ba 12.29±0.03 11.52±0.03 11.36±0.03 2 2
CH -K) suggest T1 while CH -H suggests T2 and
Bb 13.23±0.03 13.21±0.03 13.47±0.03 4 4
CH -JevenT3. Hence,weslightlyrefinethespec-
4
traltypeofEpsIndiBatoT1.5. Thespectrumof
3
Eps Indi Bb fits the T6 standard well.
3.2. Fitting Models to the Eps Indi Ba+b
Data
Assuming the Eps Indi B components have the
same metallicity (near solar, Beir˜ao et al. (2005)) 3 Eps Indi Ba
and age (∼ 0.8−2 Gyr, Lachaume et al. (1999))
2.5 T1
adlwloawrfseuvsoltuotieoxnplaonredtahtemaocscpuhrearceytohfeobrays.icEbvroowlun- Constant 2
+
ttoihtoheneaorr,nyeTthehffaenoadrny,dmpgraorsavsvi,ditaeygse(t,gha)en(mdfoarrpapadiinuggisvbeannetdwm,eeoetnna,ltliohcne- Normalized F λ 1.15 T2
ity). For given T and gravity, we have a pre-
eff 0.5 T3
dicted radius, and with the known distance we
can then make absolute comparisons between the 0
meaured spectra and theoretical models. For the 1 1.2 1.4 1.6 1.8 2 2.2 2.4
λ [μm]
evolutionary theory, we use Burrows et al. (1997).
Forthespectralandatmospheretheory,weusethe
Fig. 2.— Spectrum of Eps Indi Ba (black line)
approach described in Burrows et al. (2006) and
plotted on top of T1-3 dwarf spectral standards
Sharp & Burrows (2007). The goal is to deter-
obtainedfromSandyLeggett’sLandTdwarfdata
mineT andgravityforeachobject, andthento
eff archive.
determinewhetheraconsistenttheoreticalpicture
emerges. In particular, we can ascertain whether
the models are consistent with the assumption of
coevality.
These current models have some limitations.
First, since Eps Indi Ba is clearly near the L/T
transition region above a T of ∼1100 K, it
eff
is necessary to incorporate the effects of silicate
clouds (Ackerman & Marley 2001; Tsuji 2002;
Burrows et al. 2006). The modeling of clouds 4 Eps Indi Bb
(e.g.,theirmodalparticlesizes,spatialextent,op-
3.5
tpcilrcooaubldlpemrmoapotdeircet.liedNse,esavcnreidrbtehddeelgeirsnese,(oBwfuepraurotscwehstinheeetss”a)Cli.ass2qe0u0Ei6t”e) + Constant 2.35 T5
F λ
cwliotuhd15isµamsssupmheedrictaolfboerstceormitpergisreadin.soWfewhinicithiatlhlye malized 1.25
also tested 10 and 20µm particles but did not Nor T6
1
achieve better fits. Second, though the absorp-
tionfeatureduetothehotbandsofmethanefrom 0.5 T7
∼1.6 to ∼1.75µm is a defining signature of brown 0
1 1.5 2 2.5
dwarfs, the strengths of these bands are not well- λ [μm]
determined theoretically, and have not been mea-
sured in the laboratory. Since it is as good as any Fig. 3.— Spectrum of Eps Indi Bb (black line)
credible formulation described in the extant lit- plotted on top of T5-7 dwarf spectral standards
erature and though it clearly underestimates the obtainedfromSandyLeggett’sLandTdwarfdata
strength of these bands, we use the prescription archive.
described in Sharp & Burrows (2007). Finally,
thewingsoftheKIresonanceabsorptiondoublet
at ∼0.77 µm determine the continuum of brown
4
dwarf spectra out to as far as ∼1.05 µm (Burrows Eps Indi Bb, taken from Burrows et al. (1997), is
et al. 2000). The shape of these wings, in particu- ∼0.93 R . Generally, the theoretical radii do not
J
lartheeffectivecutoffwavelength,isstillasubject vary by more than ∼10%, with lower radii associ-
of active investigation. ated with higher values of g. Figure5 displays the
Given these caveats, we have identified the measured spectrum together with the best fitting
range of spectral models which approximately re- model.
produce the observations. For Eps Indi Ba, we ThefitstoEpsIndiBaaresomewhatlessgood,
ran grids of models spanning the T range 1150- but constraining. Fitting the Z and J bands was
eff
1300Kwitharesolutionof50Kandthelog(g[cm compromisedbyambiguitiesinthecloudopacities
s−2]) range 4.9-5.3 with a resolution of 0.1. For and KI wings. To approximately achieve the ob-
Eps Indi Bb, we ran grids of models spanning the served flux level in the K band required clouds to
T range850-950Kwitharesolutionof25Kand suppress the fluxes in the Z and J bands. With-
eff
the log(g) range 4.7-5.1 with a resolution of 0.1. out cloud opacity, which naturally is most effica-
The models were than compared to the measured cious in these bands, the Z and J fluxes would be
spectra by visual inspection looking for the small- too high, while the K band flux would simultane-
est residuals between data and model over the ouslybetoolowasshowninfigure4. Cloudslevel
complete spectral region. We further calculated out these bands and without clouds there are no
least-squares residuals between data and model good fits to Eps Indi Ba. An initial investigation
and found that the results agree very well with showed that the best fit modal particle sizes were
the by-eye fits. We did not attempt to use more ∼15 ± 5 µm. This is an interesting conclusion,
sophisticatednumericalmethodssuchasχ2statis- albeit arrived at in lieu of a robust theory of con-
tics(Cushingetal.2008;Burgasseretal.2007)to densates in brown dwarf atmospheres. The three
quantifythegoodness-of-fitbecausecurrentbrown bestfittingmodelsarederivedfor(T /log(g))of
eff
dwarfspectraltheorieshaveyettoomanyunquan- (1250/5.3),(1250/5.2)and(1300/5.3)inorder
tified systematic problems that make a rigorous of preference corresponding to masses and ages of
numerical treatment difficult and to some extent (55M / 1.7Gyr), (48M / 1.2Gyr) and (56M
J J J
still arbitrary. Masses and ages were derived for / 1.5Gyr). We finally estimate mass and age of
thethreebestfittingmodelsfromtheevolutionary Eps Indi Ba of 53+9−7 M and 1.5+0.8−0.5 Gyr.
J
tracksofBurrowsetal.(1997)bylinearinterpola- The radius of Eps Indi Ba, taken from Burrows
tion. We then determined the final mass and age et al. (1997), is ∼0.83 R . Figure4 displays the
J
estimate by calculating a weighted average of the measured spectrum together with the best fitting
threebestfittingmodelsassigningweightsof1,0.5 model and a plot portraying a model, but with-
and0.33inorderofpreference. Errorsonthesefi- out clouds, demonstrating the important role of
nal estimates were derived from the mass and age clouds.
variationsintroducedbyourlog(g)stepsizeof0.1 There is a remarkable agreement in the best
which dominate over the variations introduced by fitting ages of Eps Indi Ba and Bb of 1.5 Gyr, so
the chosen Teff resolution. the models are consistent with the assumption of
For Eps Indi Bb, no cloud models were nec- coevality. However, the age range over which the
essary and we derive well matching fits for (T modelsproducereasonablefitsisoftheorder±0.5
eff
/ log(g)) of (900 / 5.0), (925 / 5.1) and (875 / Gyr, so in total the modelling results agree well
4.9) in order of preference. The best-fit values of with the age estimate of Eps Indi A of 1.3−0.5+0.7
T andg arecorrelated,withthehighergravities Gyr by Lachaume et al. (1999) suggesting overall
eff
paired with the higher T s. Using the evolution- spectral and evolutionary model consistency. Our
eff
ary tracks, the corresponding masses and ages are massestimatesaresomewhathigherthanprevious
(34/,M /1.5Gyr),(38M /1.8Gyr)and(30M / estimates (47 and 28 M for Eps Indi Ba and Bb,
J J J J
1.1Gyr). Using the weighted average described in respectively, McCaughrean et al. (2004)), but still
the previous paragraph, we finally estimate mass consistent with those considering the error bars.
andageofEpsIndiBbof34±5MJ and1.5±0.5Gyr There is enough leeway in these fits that does
withtheerrorbarsderivedfromthevariationsin- not allow us to further constrain the ages and
troduced by the log(g) resolution. The radius of masses beyond this range. Ambiguities in con-
5
densate modeling and the age of Eps Indi A and
degeneracies along the T /g line in the spectral
eff
models are the culprits. Nevertheless, Eps Indi B
is the most promising binary brown dwarf system
with which to make absolute comparisons and for
1
T =1250K, g=2×105cm/s2, clouds which astronomers can test the general theory of
0.9 eff
Teff=1200K, g=1.4×105cm/s2, no clouds browndwarfs. Hence,thesedatashouldbeagoad
0.8 Eps Indi Ba observation
to further measurements and a motivation for im-
−1m]0.7 CH4 proved theory.
−2μW cm 00..56 CH4 4. Conclusions
Flux [1e−17 00..34 H2O H2O CH4 H2O newWeH-haavnedpKre-bseanntdedsptehcetr1astofJE-bpasndIndaisBwaellanads
0.2
CH Bb. By comparison with theoretical models we
0.1 4
were able to derive strong constraints on effec-
00.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 tive temperature, surface gravity and age of both
λ [μm]
brown dwarfs. The results confirm the approxi-
mate coevality of the stellar and the two brown
Fig. 4.— NIR spectrum of Eps Indi Ba. The un-
dwarf components of the Eps Indi system. The
derlyingsolidgraycurveshowsthebestmodelfit.
suppressed J-band and enhanced K-band flux of
The dotted line shows a similar model not consid-
EpsIndBaindicatesthatanoticeablecloudlayer
ering clouds.
is still present in a T1.5 dwarf like Eps Indi Ba
at an age of ≈1.5Gyr, whereas for an T6 dwarf
like Eps Ind Bb clouds become much less impor-
tant at the same age as clouds either sink below
the photosphere (larger scale heights) or thin out
(see, e.g., Cushing et al. (2008)). Once dynam-
ical mass estimates derived from monitoring of
theorbitalmotionofthesystembecomeavailable,
our slightly increased mass predictions (compared
to McCaughrean et al. (2004)) of 53+9−7M and
3.5 J
T =900K, g=1×105cm/s2 34±5M forEpsIndiBaandBb,respectively,will
3 CH4 Eepfsf Indi Bb observation provideJa crucial test for the validity of our mod-
CH
2.5 4 eling. Recent dynamical mass estimates point in-
−1m] H2O deed towards a significantly higher system mass
−2μm 2 H2O for the Eps Ind Ba/Bb system (Cardoso et al.
Flux [1e−18 W c 1.15 CH4 2bb0reo0aw8)nb.ednHwcheanmrfcaeervktoholeubtjEieocpntsafrIoynrdatiensdstyinsattgemmaonsdcpohcnaetrliiinbcurmaestointdog-
0.5
H2O CH4 els.
0
−0.5 This research has made use of data products
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
λ [μm] from the Two Mircon All Sky Survey, a joint
project of the University of Massachusetts and
Fig. 5.— NIR spectrum of Eps Indi Bb. The IPAC/Caltech, funded by NASA and the NSF.
underlying gray curve shows the best model fit. We thank Sandy Leggett for providing electronic
M and T dwarf spectra through her Web site.
AB would like to acknowledge support from the
NASA Astrophysics Theory Program under grant
6
NNX08AU16G. WB acknowledges support by Goto, M., Hayano, Y., Kobayashi, N., et al. 2003,
the Deutsches Zentrum fu¨r Luft- und Raumfahrt in Presented at the Society of Photo-Optical
(DLR), F¨orderkennzeichen 50 OR 0401. Instrumentation Engineers (SPIE) Conference,
Facilities: VLT (NACO). Vol. 4839, Society of Photo-Optical Instrumen-
tation Engineers (SPIE) Conference Series, ed.
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