This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.The first L subdwarf was a discovered only ten years ago. Less than ten L subdwarfs been published in the literature to date. Metal-poor ultracool atmospheres has not been well understood. Halo mass function cross substellar limit has not been measured. We used UKIDSS and SDSS to search for L subdwarfs. We have confirmed some new L subdwarfs and are following up more candidates with ground based large telescopes. We discussed spectral features of L subdwarfs and halo brown dwarfs

A.C. Day-Jones

B. Burningham

Baraffe

Burgasser

Burrows

Cushing

D.J. Pinfield

F. Marocco

H.R.A. Jones

J. Gomes

Lodieu

M.C. Galvez-Ortiz

Roberto Saglia

Witte

Z.H. Zhang

EPJ Web of Conferences

English

The first L subdwarf was a discovered only ten years ago. Less than ten L subdwarfs been published in the literature to date. Metal-poor ultracool atmospheres has not been well understood. Halo mass function cross substellar limit has not been measured. We used UKIDSS and SDSS to search for L subdwarfs. We have confirmed some new L subdwarfs and are following up more candidates with ground based large telescopes. We discussed spectral features of L subdwarfs and halo brown dwarfs

EDP Sciences OAI-PMH repository (1.2.0)

Purple dwarfs: New L subdwarfs from UKIDSS and SDSS

Zhang, Z. H.

Pinfield, D.J.

Burningham, B.

Jones, H.R.A.

Day-Jones, A. C.

Marocco, F.

Gomes, J.

Galvez-Ortiz, M. C.

University of Hertfordshire Research Archive

EPJ Web of Conferences 47, 06007 (2013)DOI: 10.1051/epjconf/20134706007C© Owned by the authors, published by EDP Sciences, 2013Purple dwarfs: New L subdwarfs from UKIDSS and SDSSZ.H. Zhang1,a, D.J. Pinfield1, B. Burningham1, H.R.A. Jones1,A.C. Day-Jones2, F. Marocco1, J. Gomes1 and M.C. Galvez-Ortiz31Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK2Department of Astronomy, Universidad de Chile, Casilla postal 36D, Santiago, Chile3Centro de Astrobiologia - INTA-CSICAbstract. The first L subdwarf was a discovered only ten years ago. Less than ten L subdwarfs beenpublished in the literature to date. Metal-poor ultracool atmospheres has not been well understood. Halomass function cross substellar limit has not been measured. We used UKIDSS and SDSS to search for Lsubdwarfs. We have confirmed some new L subdwarfs and are following up more candidates with groundbased large telescopes. We discussed spectral features of L subdwarfs and halo brown dwarfs.1. INTRODUCTIONL subdwarfs are a mixture of lowest mass stars and brown dwarfs with subsolar abundance and Teff≤ 2500 K. L subdwarfs are kinematically associated with Galactic halo and thick disk and form thelow mass end of the stellar Population II. They have higher proper motion and tangential velocity thanL dwarfs. L subdwarfs exhibit characteristic spectral signatures of strong metal hydrides (CaH, MgH,AlH and FeH), weak or absent metal oxides (TiO, VO and CO), and enhanced collision-induced H2absorption (CIA H2). Near infrared flux of L subdwarfs is suppressed due to the CIA H2. L subdwarfsare red in optical, and bluer than same effective temperature L dwarfs in near infrared, thus could alsobe called as purple dwarfs.L subdwarfs are very rare compare with L dwarfs. There are over one thousand L dwarfsbeen discovered while there only seven L subdwarfs been published [1–6]. Only 2MASSJ05325346+8246465 (J0532) has estimated mass (based on models of [7, 8]) just blow the sustainedhydrogen burning limit, and is therefore a possible substellar subdwarf or a halo brown dwarf. Thelow metallicity ultracool atmosphere models (e.g. [9]) have not been tested properly, the L subdwarfclassification scheme has not been established due to the lack of known L subdwarfs. Currently Lsubdwarfs are classified by compare their optical spectra with that of L dwarfs. Model spectra show bothlow metallicity and high gravity suppress the near infrared flux of L subdwarfs, and shift the flux peakto shorter wavelength [9]. Near infrared spectra must be considered in order to characterize L subdwarfsproperly.It is critical to discover a sample of L subdwarfs with different temperature and metallicity.L subdwarfs could not only help us to understand metal-poor ultracool atmospheres but also theformation and structure of the Galactic halo through study of the luminosity and mass function andkinematics. We started a project to identify a sample of L subdwarfs with UKIDSS and SDSS. We haveae-mail: zenghuazhang@gmail.comThis is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permitsunrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20134706007EPJ Web of ConferencesFigure 1. Metallicity sequences of optical and near infrared spectra of L7 dwarfs and subdwarfs. The absence2.3 m CO band which presents in spectra of M7 – T1 dwarfs might be a indicator of L subdwarfs. Spectra arenormalized at 1.08 m.confirmed three new L subdwarfs and followed up 10 candidates with the Magellan Telescope and GranTelescopio Canarias. We are also following up another 41 candidates with the Gran Telescopio Canarias.2. METAL CLASSES OF L SUBDWARFSThe overall profile of optical spectra of L subdwarfs is less affected by the chemical abundancebecause the dust cloud delays the suppressing of spectra below one micron [9]. Thus optical spectracould represent the effective temperature better than near infrared spectra. The infrared spectral ofL subdwarfs/dwarfs are sensitive to both chemical abundance and effective temperature. Figure 1top shows optical spectra of L7 dwarfs/subdwarfs. Similar overall profile indicates similar effective06007-p.2Hot Planets and Cool StarsFigure 2. Left: spectra of M7 – L2 dwarfs. Right: optical and near infrared spectra of four L type extreme subdwarfs.Deep 1.4 m H2O absorption in ULAS 1519 and 2MASS J0532 which are different from 2MASS J1626 indicatesthat they might be halo brown dwarfs or substellar subdwarfs. All spectra are normalized at 1.6 m.temperature, while stronger metal hydrides (e.g. FeH) indicates lower metallicity. Figure 1 bottom showsthe near infrared spectra of L7 dwarfs/subdwarfs. A smooth suppressing of J , H and K band spectramoving across objects classified as, red L7, normal L7, blue L7, d/sdL7, sdL7 and esdL7. This spectralsequence also represents a sequence of brown dwarfs with similar effective temperature but differentmasses (from the lowest to the highest mass brown dwarfs). This is an intuitive proof of the evolutionof brown dwarfs.The most notable features of an L7 subdwarf spectrum that differ from an L7 dwarf spectrum are themuch stronger FeH lines and absence of the CO band around 2.3 m. The 2.3 m CO band appears inspectra of M7 to T1 dwarfs, and disappear after T2 due to the strengthening of CH4 absorption (Fig. 2of [10]). The absence of 2.3 m CO band in the spectra of known L subdwarfs suggests that it is anindicator of a subsolar abundance population.3. HALO BROWN DWARFSThe most massive brown dwarfs have spectral types of late M and are usually found in ∼ 100 Myr oldyoung open clusters. Brown dwarfs in the Galactic disk are usually a few Gyr old start with spectraltype of L0. After about 10 Gyr of cooling, brown dwarfs would have temperature lower than ∼ 1600 K(e.g. [8]). Thus spectral types of old brown dwarfs in the halo should start with late L types.The esdL7 type J0532 is the first L subdwarf discovered and is the only one with theoreticalpredicated mass (∼ 0.08M) just below the hydrogen burning minimum mass. Figure 2 (left) showsspectra of M7 to L2 dwarfs which cover the star-brown dwarf transit region. H2O absorption bands arestrengthening from M7 to L2. Figure 2 (right) shows spectra of four L subdwarfs. The depth of theH2O absorption band around 1.4 m in the spectrum of J1626 is similar to that seen in late M dwarfs,a few Gyr old lowest mass stars. While the H2O absorption depth (around 1.4 m) of the spectrum ofJ1519 (Zhang et al. in preparation) is similar to that of early L dwarfs, a few Gyr old high mass browndwarfs. The 1.15 m H2O band in J1519 is also much stronger than that of J1626. With only two subtypedifference, spectra of J1626 and J1519 are showing significantly different features. Could the strengthenof 1.4 m H2O absorption band in J1519 be an indicator of a substellar subdwarf? If this is true J0532,J0616 [3], J1519 and J0216 (Zhang et al. in preparation) would be the first four brown dwarfs discoveredin the Galactic halo. Further analysis based on the latest model grids and new astrometric observationsare needed to confirmed this hypothesis.06007-p.3EPJ Web of ConferencesReferences[1] Burgasser A. J., et al., ApJ, 592, 1186 (2003)[2] Burgasser A. J., ApJ, L73, 614 (2004)[3] Cushing M. C., Looper D., Burgasser A. J., Kirkpatrick J. D., Faherty J., Cruz K. L., Sweet A.,Sanderson R. E., ApJ, 696, 986 (2009)[4] Sivarani T., Lépine S., Kembhavi A. K., Gupchup J., ApJ, 694, L140 (2009)[5] Lodieu N., Zapatero Osorio M. R., Martín E. L., Solano E., Aberasturi M., ApJ, 708, L107 (2010)[6] Lodieu N., Espinoza Contreras M., Zapatero Osorio M. R., Solano E., Aberasturi M., MartínE. L., A&A, 542, A105 (2012)[7] Baraffe I., Chabrier G., Allard F., Hauschildt P. H., A&A, 327, 1054 (1997)[8] Burrows A., Hubbard W. B., Lunine J. I., Liebert J., RvMP, 73, 719 (2001)[9] Witte S., Helling C., Hauschildt P. H., A&A, 506, 1367 (2009)[10] Burgasser A. J., ApJ, 659, 655 (2007)06007-p.4

Purple dwarfs : New L subdwarfs from UKIDSS and SDSS

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