S Ori 70 still a strong cluster planet candidate

a r X i v :a s t r o -p h /0410678v 1 27 O c t 2004S Ori 70:still a strong cluster planet candidate

E.L.Mart′?n (1)

(1)Instituto de Astrof′?sica de Canarias

Avda.V′?a L′a ctea,E-38200La Laguna,Spain

In this paper I show that the coolest σOrionis cluster planet S Ori 70is still a strong candidate

member despite recent claims by Burgasser et al.that it could be a brown dwarf interloper.

The main point of my argument is that the colors of S Ori 70are signi?cantly di?erent to

those of ?eld dwarfs.This object has in fact the reddest H ?K color of all known T dwarfs,

a clear indication of low gravity according to all published models.In a J ?H versus H ?K

diagram,S Ori 70lies in the region where models of ultracool dwarfs predict that low gravity

objects should be located.I conclude that S Ori 70is still a strong candidate member of the

σOrionis open cluster.I brie?y discuss additional observational tests that can be carried out

with existing facilities to verify the σOrionis membership of this cluster planet candidate.

Keywords :stars:very low mass stars and brown dwarfs.1Introduction The term brown dwarf (BD)refers to objects with masses below the limit for stable hydrogen fusion in stellar interiors.For solar composition,this limit was calculated to be 0.08M ⊙by Kumar (1963)and Hayashi &Nakano (1963).Modern calculations have changed the value of this boundary only slightly.For example,Bara?e et al.(1998)give 0.072M ⊙for solar metallicity.There is no consensus in the community about the minimum mass of brown dwarfs (Boss et al.2003).Some argue for the deuterium burning limit at 13Jupiter masses (for solar composition).Some prefer a limit at around 10Jupiter masses,where there appears to be a

sharp rise in the number of extrasolar planets detected by high-precision radial velocity surveys around main-sequence stars.Finally,it has also been proposed to use a limit at 5Jupiter masses where the mass-radius relationship of substellar objects changes its sign.In this paper we continue to use the deuterium limit as the mass boundary between brown dwarfs and planets because we do not have any strong reason to change the nomenclature adopted in our previous papers.

Another term used in this paper is that of ultracool dwarf,which refers to small objects with very cool e?ective temperatures.Since 1997,two new ultracool spectral classes have been adopted to extend the classical OBAFGKM system into cooler temperatures.The L dwarfs are characterized by weak or absent TiO bands,and very broad NaI and KI lines in the optical spectrum (Mart′?n et al.1997,1999;Kirkpatrick et al.1999,2000).The T dwarfs are charac-terized by methane bands in the near-infrared spectra (Oppenheimer et al.1995;Burgasser et al.2002;Geballe et al.2002).Current estimates of the temperatures of ultracool dwarfs range

from about2400K to1400K for L dwarfs,and from1400K to700K for T dwarfs(Basri et al. 2000;Vrba et al.2004).

Most of the known ultracool dwarfs have been identi?ed in the general?eld by the wide area surveys2MASS and SDSS.These objects consist of a mixed population of very low-mass stars,brown dwarfs and free-?oating planets formed in di?erent star-formation events during the lifetime of the Milky Way.Their individual ages,chemical compositions and masses are not known.Our best chance to study a population of ultracool dwarfs of known age,chemical composition,and uniform distance is to?nd them in open clusters where the stellar populations are well characterized.Two of the?rst brown dwarfs identi?ed were located in the Pleiades open cluster(PPl15and Teide1,Stau?er et al.1994;Basri et al.1996;Rebolo et al.1995).Now, we know several dozens of bona?de brown dwarfs in the Pleiades and in other open clusters.

TheσOrionis open cluster has been a region where our group has concentrated many e?orts to reveal the substellar population(see Zapatero Osorio et al.2003for a recent review).It o?ers several advantages:(1)It is young(3-8Myr)and,thus,the substellar objects are relatively bright and hot(B′e jar et al.2001),but not so young that the theoretical models cannot be reliably used to obtain masses(Bara?e et al.2001).(2)It has very little extinction(Lee1968),probably because the parental cloud has been blown away by the O-type star at the center of the cluster.

(3)It is relatively nearby(distance350pc).(4)It is moderately rich and dense.B′e jar et al. (2004,in preparation)estimate a peak central density of0.2members per square arcminute.

So far the coolest and faintest candidate member that we have found in theσOrionis cluster is the T dwarf candidate S Ori70(Zapatero Osorio et al.2002).It was found in a pencil-beam deep mini-survey of only55square arcminutes with a sensitivity of21magnitude in the J and H-bands carried out with the 4.2-meter William Herschel Telescope in the Observatorio del Roque de los Muchachos.Follow-up near-infrared photometry and low-resolution spectroscopy were obtained with NIRC at the10-meter Keck I telescope.The photometry is summarized in Table1,together with the data for?eld T dwarfs of similar spectral type obtained from the literature.A mid-resolution spectrum in the K-band obtained with NIRSPEC on Keck II was presented in Mart′?n&Zapatero Osorio(2003).We have claimed that both the NIRC and the NIRSPEC spectra are best?tted with synthetic spectra with low gravity(log g=3.5),which is consistent with membership in theσOrionis open cluster.We have estimated a mass of3 Jupiter masses for an age of3Myr,making S Ori70the least massive object observed directly outside the solar system.

2A critique of Burgasser et al.’s paper

Burgasser et al.(2004)have carried out an independent analysis of our Keck NIRC and NIR-SPEC data of S Ori70.We sent them our raw and reduced spectra.Burgasser et al.reprocessed our spectra using their own software and found the same results as us for S Ori70but not for the comparison object2MASS J055919-1404.Apparently,our NIRSPEC data reduction for the 2MASS comparison object was incorrect,and it turned out to be a?eld star rather than a T dwarf.

Burgasser et https://www.sodocs.net/doc/9c16747340.html,pared the spectra of S Ori70with spectra of?eld T brown dwarfs(bdTs) obtained by them,and claimed to?nd a good match between our object and old brown dwarfs. In their Figure2,they showed a comparison of their reduction of our NIRSPEC spectrum of S Ori70and their NIRSPEC spectrum of the?eld bdT72MASS1553+1532.In their Figure4, they showed a comparison of our NIRC spectrum of S Ori70with their NIRC spectrum of the ?eld bdT6.52MASS1047+2124.From these comparisons they claimed that S Ori70is a?eld brown dwarf that coincidentally lies in the line of sight of the cluster.If the interpretation of Burgasser et al.is correct,S Ori70should be at a distance of only75to100pc,instead of350 pc.

Table1:Comparison of photometric data of?eld T4-T8dwarfs with S Ori70

SpT J?H Ref.

T40.06±0.04K04 SDSS0000+255414.73±0.05-0.08±0.04

T4.5-0.03±0.07L02 SDSS0926+584715.47±0.03-0.08±0.04

T4.5-0.07±0.04L02 SDSS0742+205515.60±0.03-0.11±0.04

T5.5-0.18±0.04K04 SDSS0741+235115.87±0.030.00±0.07

T5.5-0.19±0.04K04 SOri7020.28±0.100.64±0.25

T6-0.02±0.07L99 SDSS1231+084715.14±0.03-0.06±0.04

T6-0.24±0.04K04 2MASS0937+293114.29±0.03-0.72±0.04

T6-0.29±0.04L02 SDSS1110+011616.12±0.050.17±0.07

T6.5-0.37±0.04K04 SDSS1758+463315.86±0.030.08±0.04

T8-0.42±0.04L02 Gl570D15.33±0.050.01±0.19

T8-0.48±0.04K04

in the range log g=4.0-5.5,consistent with their presumed old ages.S Ori70,on the other hand, is located outside the locus of the?eld bdTs,even taking into account the photometric error bars.Its red H?K color indicates lower gravity than the?eld objects according to the models. In fact,Mart′?n&Zapatero Osorio(2003)derived log g=3.5for S Ori70,which implies a mass of3Jupiters.The red H?K color cannot be due to insterstellar reddening because extinction is very low in this line of sight,and because reddening would also a?ect the J?H color.

3Where do we go from here?

The di?erences in colors and spectral energy distribution between SOri70and?eld bdTs strongly suggest that S Ori70has lower gravity,consistent with young age and low mass.We conclude that,despite Burgasser’s claims,S Ori70is still a strong cluster planet candidate.Nevertheless, more data is necessary to improve our understanding of this object and to con?rm its cluster membership inσOrionis.In this section,I discuss future observations that will provide crucial information about S Ori70.

?The proper motion:Zapatero Osorio et al.(2002)compared images with a3year baseline of the?eld around S Ori70.They placed an upper limit of0.1arcsec per year on the proper motion of S Ori70.If S Ori70is a bdT at75-100pc,the proper motion should be about0.05arcsec per year.The last images of S Ori70were obtained at Keck

I on December2001.We plan to obtain new images in winter2004in order to improve

the limit on the proper motion of S Ori70by a factor of two.Unfortunately,proposals to obtain accurate astrometry of S Ori70with HST in Cycles11,12and13have not been successful.

?The KI near-infrared lines:The detection of the KI doublet at1.25microns reported by Mart′?n&Zapatero Osorio(2003)needs to be con?rmed with higher signal to noise ratio.A proposal to reobserve S Ori70with Keck/NIRSPEC was rejected by the NASA TAC given“the extensive study of S Ori70by Burgasser and his colleagues”.In this paper we have shown that the Burgasser et al.study did not provide the?nal word on the nature of this object,and that S Ori70clearly deserves further investigation.

?The spectral energy distribution(SED):The SED coverage of S Ori70is still in-complete and the error bars in the photometry should be reduced.A proposal to obtain the JH spectrum with Keck/NIRC,and to improve the HK spectrum was rejected by the NASA TAC.A proposal to obtain mid-infrared photometry with SIRTF/IRAC was also turned down.We plan to insist on applying for telescope time to observe S Ori70 so that we can obtain an excellent SED of this benchmark cluster planet,which can be

a reference for future studies of planetary-mass objects in star-forming regions and young

open clusters.In parallel with more detailed observations of S Ori70,there should also be theoretical e?orts to improve the synthetic spectra of T dwarfs.The best?t of the J-band NIRSPEC spectrum found by Mart′?n&Zapatero Osorio(2003)has a T e?=1,100K, signi?cantly hotter than the best?t of the HK NIRC spectrum for T e?=800K obtained by Zapatero Osorio et al.(2002).Burgasser et al.(2004)correctly pointed out this incon-sistency in the comparison of observations with theory and also showed that this?tting technique provides gravities that are too low for bdT7-bdT8?eld objects.However,the gravities obtained from the models are reasonable for bdT6?eld objects,which is closer to the spectral type of S Ori70.

Last but not least,I would like to note that our e?orts to?nd more cluster planets similar to S Ori70have met with poor weather in several observatories(Calar Alto,Canaries,Hawaii and

Figure1:Comparison of S Ori70with?eld bdT4-T8objects.Theoretical models from Marley et al.(2002)are also shown with dotted lines for two di?erent gravities.Note that S Ori70is located in a region outside the locus of?eld T dwarfs,consistent with having lower surface gravity.All the known?eld T dwarfs are bracketed by the

models.

Paranal).We do not have a reliable database to search for these objects yet.Nevertheless,we have found a few candidates that await spectroscopic observations.I showed one of them at the Moriond meeting,a possible planetary mass companion to an M dwarf member in theσOrionis open cluster.Follow-up observations of these objects with several telescopes are planned.Stay tuned!

Acknowledgments

Partial funding for attending this conference was provided by a grant from the RTN network “The Formation and Evolution of Young Stellar Clusters”led by Mark McCaughrean.Adam Burgasser,Jose Antonio Caballero,David Barrado y Navascu′e s and Maria Rosa Zapatero Osorio provided useful comments that helped to improve the manuscript.Mark Marley sent a computer readable version of the models used in Figure1.

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