astro-ph/9607002 1 Jul 1996 University of California and the Californian Institute of Technology Department of Based Subject con rms the substellar abundances low fundamental parameters genuine dwarfs their masses are estimated at line little if any Teide and Instituto de Astrof sicade Canarias Instituto de Astrof sicade Canarias We on luminosities and at Calar observations are present e mail addresses headings brown we unable Astronomy University of California Berkeley Berkeley CA depletion is have showing Brown dwarf stars nm Con rmed also op en gmarcy etoile b erkeley edu mosorio iac es to obtained m A burn Li Keck a realm low mass obtained nature clear detection Dwarfs clusters and high content inferred from the rrl iac es ege iac es basri soleil b erkeley edu Li at sp ectra R Reb olo E L G Basri G W Marcy M R Zapatero Li Given the the of abundance evidence place ABSTRACT by brown our W M signi cant in of the asso ciations the Jupiter masses two ob jects the and the Teide of Keck dwarfs probable E E for the two observed lines Lithium log Pleiades preservation Observatory Osorio the Mart n Pleiades and N presence La La individual Pleiades age Li Regardless of stars Calar Laguna Laguna nm of Test brown Cluster which the evolution of Li Since Pleiades brown of comfortably this fragile To resonance Pleiades the consistent Tenerife Tenerife is dwarfs app ear their age op erated sub ordinate cluster Teide stars Spain Spain in ApJ Letters line element with in jointly stars their the In USA by the
Introduction
In stellar interiors Li nuclei are destroyed via proton collisions at relatively low
temp eratures K This element has long b een used as a tracer see e g Reb olo
Michaud Charb onneau of internal structure in stars of di erent types The
strong convection of very low mass stars M M causes an extremely e cient mixing
of Li and indeed signi cant depletion has b een observed in these stars even in the very
young ones Zapatero Osorio et al a Garc a L opez Reb olo Mart n Mart n
Reb olo Magazz u Ob jects with masses M M M Jupiter mass
J
units are well b elow the minimum mass for hydrogen burning and cannot reach the Li
burning temp erature so unlike very low mass stars they must preserve a signi cant amount
of their initial Li content during their lifetime This can b e detected sp ectroscopically and
therefore can provide a diagnostic of substellar nature for brown dwarf candidates Reb olo
Mart n Magazz u Magazz u Mart n Reb olo
At the age of the Pleiades cluster even the most massive brown dwarfs M
J
should have preserved a large amount of their initial Li but searches Mart n et al
Marcy Basri Graham of the faintest prop er motion members failed to detect it
An obvious conclusion was that brown dwarfs had not b een discovered yet in the cluster
This prompted the search for new fainter and co oler candidates that led to the discovery of
Teide and Calar Reb olo Zapatero Osorio Mart n Zapatero Osorio Reb olo
Mart n b These are two very late type dwarfs near the center of the cluster with radial
velocities photometric and sp ectroscopic prop erties and a prop er motion measurement in
the case of Teide consistent with Pleiades membership Reb olo et al Zapatero
Osorio et al b Mart n Reb olo Zapatero Osorio Their luminosities and
e ective temp eratures qualify them according to all the available evolutionary mo dels as
brown dwarfs However a de nitive con rmation of their substellar nature can only b e
obtained via the Li test Such con rmation is a key step to consolidate our understanding
of brown dwarf interiors Very recently Li has b een discovered Basri Marcy Graham
in PPl a photometric member Stau er Hamilton Probst of the Pleiades
ab out one magnitude brighter than Teide and Calar which strongly encouraged us to
extend the search for Li in these new ob jects PPl app ears to de ne the Li reapp earance
b oundary in the Pleiades and sits on the frontier b etween stars and brown dwarfs
Observations and results
We rep ort sp ectroscopic observations of Teide and Calar that provide clear evidence
for the presence of Li in b oth The observations were carried out at the m Keck telescop e
Mauna Kea Hawaii on November and and February using the Low
Resolution Imaging Sp ectrograph LRIS The g mm grating provided a disp ersion
of A pixel on the TEK pix detector in the region of the nm Li i
resonance doublet and A pixel in the region of the sub ordinate nm Li i line
The arcsec slit width nally gave an e ective resolution of A The total integration
time for Teide in the sp ectral regions of the nm and nm Li i lines was
and hr resp ectively individual exp osures ranged from to min Only one min
sp ectrum could b e obtained for Calar on February at the wavelength of the
resonance line due to bad weather
The individual images have b een indep endently pro cessed using standard pro cedures
within the IRAF package which included debias at eld optimal extraction and
wavelength calibration using arc lamps The sp ectra in the region of the Li resonance line
were ux calibrated using the standard star G B B Massey et al The nal
sp ectra of Teide and Calar centered at nm are shown in Fig H is seen in
emission equivalent widths of A and A resp ectively and lines of K i at
nm as well as TiO and VO bandheads are clearly detected The new
more accurate radial velocities of Teide and Calar are and km s fully
consistent with Pleiades membership and previous measurements Reb olo et al
Mart net al They were determined by cross correlating with the sp ectrum of vB
v km s Goldb erg obtained with the same instrumental con guration The
r
nm VO absorption index Kirkpatrick Henry Simons previously employed
for sp ectral classi cation Reb olo et al was also measured in the new sp ectra
resulting in values of and in Teide and Calar resp ectively in
very go o d agreement with measurements for other very late type dwarfs
The Li i line at nm is clearly present in the sp ectra of Teide and Calar with
remarkable strength as compared with the nearby TiO bands as can b e seen in Fig a
An accurate measurement of the equivalent width of the Li i nm line cannot b e
p erformed at our sp ectral resolution b ecause of the strong blending with the nearby TiO
lines In order to determine approximate equivalent widths we subtracted the sp ectrum
of other very late type dwarfs vB LHS observed with the same con guration
for which much higher resolution sp ectra do not show any evidence of Li absorption and
then measured the equivalent width of the feature at the p osition of the resonance doublet
We found these pseudo equivalent widths to b e A and A in Teide and
Calar resp ectively They are useful as a reference for future searches although they should
IRAF is distributed by National Optical Astronomy Observatories which is op erated by the Asso ciation of Universities for Research in Astronomy Inc under contract with the National Science Foundation
b e used with caution to derive an abundance from the available theoretical curves of growth
Pavlenko et al since these curves were computed considering pure Li absorption
The same pro cedure was applied to the nal sp ectrum of Teide in the region of the Li i at
nm After subtraction of the sp ectrum of a typical M dwarf obtained averaging
the sp ectra of LP and vB an absorption is apparent see Fig b at this
precise wavelength with a pseudo equivalent width of mA which we interpret
as additional evidence for the presence of Li in the atmosphere It would b e desirable in
principle to con rm this feature with higher sp ectral resolution and S N ratio
Lithium abundances and brown dwarf status
To estimate the Li abundance in our ob jects we have used sp ectral synthesis
computations based on Allard Hauschildt AH mo del atmospheres for very
co ol dwarfs The e ective temp eratures of Teide and Calar were estimated from a
new sp ectral type classi cation using pseudo continuum indices Mart n et al and
the T calibration given in Kirkpatrick et al which was revised Kirkpatrick
e
according to the new AH mo dels For M dwarfs this calibration yields an e ective
temp erature of K The M dwarf PPl would have a T ab out K hotter
e
The Li abundance was determined by comparison with the available sp ectral synthesis
Pavlenko et al and with new computations kindly provided by Ya Pavlenko using
various molecular line lists Kurucz A set of sp ectra with several Li abundances were
generated for the AH mo dels with e ective temp eratures of and K While these
mo dels describ e reasonably well the TiO bands in the vicinity of the Li resonance line co oler
AH mo dels pro duce to o broad and deep TiO absorption bands which do not t the observed
sp ectrum and app ear to indicate that the e ective temp erature of our two ob jects is indeed
higher than K Unfortunately the limitations in the available molecular line lists and
the uncertainties in the e ective temp erature prevent a very precise determination of the Li
abundance We should note that there are other e ective temp erature calibrations in the
literature Brett Tsuji Ohnaka Aoki which give lower values by K
for very late type dwarfs Several computations have b een conducted to determine the
sensitivity of the Li resonance doublet to uncertainties in the e ective temp erature It can
b e safely concluded that Teide and Calar have a rather high Li abundance in the range
log N Li where the usual scale is log N H At present we cannot claim
any di erence in the Li content b etween Calar and Teide Our preliminary analysis
of the Li i nm line in Teide gives Li abundances a factor of higher than the
resonance line strongly suggesting that Li has b een signi cantly preserved in our ob jects
It is known that the Li resonance line is sensitive to chromospheric e ects Pavlenko et al
Houdebine Doyle and might underestimate the true atmospheric abundance
in late type dwarfs We intend to pursue this question in a subsequent investigation
The use of Li abundance and luminosity provides a very e cient way to constrain
masses at the very b ottom of the Pleiades sequence Basri et al We can determine
the luminosity of Teide and Calar using optical and infrared observations Zapatero
Osorio et al b c which give I and K resp ectively Using
the calibration of b olometric magnitude versus I K color Tinney Mould Reid
the resulting values are log L L and for Teide and Calar
The value for Teide is in fact a factor lower than the luminosity estimated from the
sp ectral type and I magnitude in Reb olo et al The new sp ectral classi cation
as M Mart n et al gives a luminosity of for Teide and for Calar
Monet et al which are consistent with those obtained from the I K color For
PPl we adopted I K Stau er et al Zapatero Osorio et al c
and a sp ectral type M Mart net al Its luminosity was obtained as
the average of those resulting from the previous two calibrations This value is consistent
with that obtained by Basri et al and implies that PPl has a mass of ab out
M and a most likely age of Myr Using various sets of computations
J
the Li abundance and luminosity constrain the masses of Teide and Calar to b e b elow
M no matter what their age Nelson Rappap ort Chiang Chabrier Bara e
J
Plez D Antona Hence they are con dently b elow the hydrogen burning
mass limit Given the plausible age interval for the cluster Myr the mass range
is M If the age of PPl were indeed representative of the very low mass ob jects
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in the cluster we infer a likely mass of M for Teide and Calar In Table we
J
summarize the most relevant parameters of our ob jects
The faint end of Li depletion in the Pleiades
In Fig we compare the Li abundances of Teide and Calar with those of Pleiades
members in a wide mass interval F and early G type stars e ective temp eratures
ab ove K show that the Pleiades stars were formed with an initial abundance
ab out log N Li Bo esgaard Budge Ramsay Pre main sequence
convective mixing causes the depletion of Li in late G K and early mid M dwarfs e ective
temp eratures K Garc a L opez et al Mart n et al Marcy et al
So derblom et al The rst indication of the exp ected reapp earance of Li
at the b orderline b etween stars and brown dwarfs is provided by PPl at an e ective
temp erature slightly b elow K This ob ject has retained ab out of its initial Li
Basri et al At co oler e ective temp eratures corresp onding to lower masses Teide
and Calar clearly con rm the exp ectations of a Li abundance higher than that of PPl
The origin of the remarkable spread in Li abundances in Pleiades stars with e ective
temp eratures in the range K has b een sub ject of discussion for several years At
least two interpretations have b een considered so far one of which prop oses the existence of
a large age disp ersion within the cluster members Duncan Jones while the other
suggests that rotation in uences the Li depletion of individual stars see Mart n Claret
and references therein It is well known that the Li depletion around the substellar
limit T K is extremely sensitive to age Stringfellow Magazz u et al
e
Nelson et al Chabrier et al The p ossible role of rotation in the Li depletion
of these ob jects has to b e investigated b oth from the theoretical and observational sides to
take advantage of the reapp earance of this element to shed light on the problem
Final remarks
We have inferred a mass of ab out M for Teide and Calar from their Li
J
abundances and luminosities They are the rst genuine brown dwarfs in the Pleiades and
hence it is interesting to use them to discuss the cluster mass function b eyond the substellar
limit Our ob jects were discovered in an R I survey of the central region of the Pleiades
cluster Zapatero Osorio et al b Teide is lo cated away from the cluster center
while Calar is at southeast from the core The total area covered by the survey was
arcmin only ab out of the cluster There is no reason to think that any of the
studied regions is anomalous regarding their substellar contents If Pleiades brown dwarfs
have a spatial distribution similar to early and mid M dwarfs the exp ected number of
ob jects like Teide and Calar in the whole cluster is in the range as previously
discussed in Reb olo et al The cluster mass function app ears to increase b eyond the
substellar limit but it seems that no reasonable extrap olation of the mass function would
allow brown dwarfs to dominate the mass of the cluster
In this work we have shown that the two b est brown dwarf candidates in the
photometric survey of Zapatero Osorio et al b have succesfully passed the Li test In
the future ob jects with similar or fainter photometric magnitudes and with radial velocity
and prop er motion that guarantee membership in the Pleiades should b e considered as
bona de brown dwarfs It would not b e necessary to apply the Li test to all of them
Nevertheless since the evolutionary mo dels predict a high sensitivity of the Li abundance to
age for masses in the range M we consider it interesting to continue the observations
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of Li b eyond the substellar limit to prob e a p ossible age spread in the cluster
Acknowledgments We thank Ya Pavlenko F D Antona I Bara e and G Chabrier
for providing theoretical computations and for useful discussions Financial supp ort for
E L M and R R participation in the Keck observations was provided by the Spanish
DGICYT pro ject no PB C Supp ort for G W M was provided by NASA Grant
NAGW
Table Pleiades least massive ob jects
PPl Calar Teide
a b b
I
c
I K
d
SpT M M M
e
v km s
r
d
VO
e d
EW A
H
e
EW A
Li
670:8
EW mA
Li
812:6
e
log N Li
log L L
T K
e
Mass M
J
a
Stau er et al
b
Zapatero Osorio et al b
c
Zapatero Osorio et al c
d
Mart net al
e
Basri et al
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A
This preprint was prepared with the AAS L T X macros v E
Fig The optical sp ectrum of Teide ab ove and Calar b elow showing very similar
features Positions of several atomic lines and molecular bands are indicated An o set of
has b een added to the sp ectrum of Teide for clarity
Fig a The sp ectral region of the Li i nm line in Teide and Calar
Overplotted to Teide is the sp ectrum of vB dotted line Also plotted for comparison
is the sp ectrum Basri et al of PPl degraded to the same resolution than the
previous ones b The sp ectral region of the Li i nm line in Teide solid line and
overplotted dotted line the average sp ectrum of the two M dwarfs LP and vB
The substraction of these two sp ectra is also plotted The two strong absorption features
are Na i lines at and nm
Fig Li abundance versus e ective temp erature for Pleiades members in a wide mass
range Abundances are given in the usual scale log N H Op en circles and arrows
which indicate upp er limits are taken from the literature Bo esgaard et al So derblom
et al Garc a L opez et al Mart n et al Marcy et al The Li
abundance of PPl has b een taken from Basri et al
Hα TiO TiOVO KI
LiI
a)
b)