Improving our knowledge on nearby stars and brown dwarfs Ralf-Dieter Scholz Astrophysikalisches Institut Potsdam Thüringer Landessternwarte Tautenburg, December 13, 2005

A neighbourhood of dwarfs Our ignorance of neighbours Distances and proper motions Using old catalogues/archives Spectroscopic distances Visitors from the halo Finding the coolest neighbours

Proxima Centauri – the next neighbour (?) Red dwarf, spectral type: M5, distance: 1.3 pc Three different epoch images (SuperCOSMOS Sky Surveys = SSS): blue (Bj) extremely red (I) red (R) Image size: 2 arcmin by 2 arcmin Proper motion: 3.9 arcsec/yr (among TOP 20 of fastest stars in the sky)

The Solar neighbourhood (

Stars with accurate (trigonometric) distances HRD-CMD for about stars measured by the Hipparcos satellite + about 1000 stars with ground-based parallaxes most red (and white ?) dwarfs not yet included !

The missing stellar neighbours 5pc sample: 44 systems (complete) Henry et al. (1997) 10pc sample: 229 known, 130 missing (36%) Henry et al. (1997) 25pc sample: ~2000 known systems ~3500 missing systems (63%) Henry et al. (2002) Assumptions: 1) constant space density 2) complete counts within 5pc ? ?

The missing stellar neighbours 5pc sample: 44 systems (complete) Henry et al. (1997) but: 3 new systems discovered since then!! 10pc sample: 229 known, 130 missing (36%) Henry et al. (1997) 25pc sample: ~2000 known systems ~3500 missing systems (63%) Henry et al. (2002) Assumptions: 1) constant space density 2) complete counts within 5pc ? ?

Why care about neighbours? ● nearest representatives of a given class of stars can be studied down to the smallest detail ● many search methods for extra-solar planets work only with nearby stars ● new classes of low-luminosity objects (late M, L, T, Y? dwarfs/subdwarfs + cool white dwarfs) are still being discovered – best chances to find them nearby ● complete picture of stellar/substellar luminosity functions, binary/multiple systems statistics, velocity distributions, etc. of different populations in a galaxy – only possible in Solar neighbourhood

Proper motion as rough distance measure ● Proper motion µ = apparent motion on the sky (large values range from ~0.1 to ~10 arcsec/yr) ● Real velocity [in km/s] can only be estimated if the distance from the Sun d [in pc] is known: v tan = 4.76 · µ · d ● Typical relative velocity of local Galactic disk stars ~ 40 km/s ● Disk star with µ = 1 arcsec/yr lies typically at d ~ 10 pc ● Halo stars do not take part in Galactic rotation (~220 km/s at the location of the Sun)  same µ indicates 5 times larger distance

Proper motion samples are halo biased Galactic space velocities UVW for proper motion stars from Lepine, Shara & Rich (2003): Dotted and dashed ellipses - 2σ velocity dispersions of local disk and halo stars, respectively (Chiba & Beers 2000) Solid circles - limit for stars gravitationally bound to the Galaxy (model of Dauphole & Colins 1999) Normal red dwarfs red subdwarfs

Search tools for nearby red dwarfs Discovery of an M6 dwarf (LHS 2090) at spectroscopic distance of only 6 pc Scholz, Meusinger & Jahreiß (2001) Old catalogues (e.g. + new deeper and/or near-infrared Luyten Half Second sky surveys (e.g. Two Micron = LHS) und Archives All Sky Survey = 2MASS) Proper motion + extremely red colour

Continuing search among known pm stars 322 red candidates from NLTT (Scholz, Meusinger & Jahreiß 2005)

How did we select them? ● observable from Calar Alto (delta > -30 deg) ● proper motion stars from the New Luyten Two Tenths (NLTT) catalogue (µ > 0.18 arcsec/yr) ● faint + red (optical or optical-to-NIR) objects (90%) red giants are excluded ● + Luyten Half Second (LHS) stars (pm > 0.5 arcsec/yr) lacking spectroscopy or bibliography in SIMBAD (10%) Problem: poor NLTT positions and photometry Solution: NLTT cross-Id. with 2MASS (2 nd incremental data release), SSS+USNO A2.0, CMC, Tycho-2

Preliminary photometric distance estimates a) SSS or USNO A2.0 R magnitudes combined with 2MASS Ks: absolute Ks vs. R-Ks relation obtained from objects with known trigonometric parallaxes (Figure) targets selected with d < 30 pc ______________________________________________________________ b) V magnitudes from CMC, Tycho-2 or other sources available: according to absolute V vs. V-Ks relation of Reid & Cruz (2002) targets selected with d < 25 pc (Scholz, Meusinger & Jahreiß 2005)

Low-resolution spectra: Alto (Scholz, Meusinger & Jahreiß 2005)

Some statistics for our sample (Scholz, Meusinger & Jahreiß 2005) H = Hipparcos, Y = Yale parallax catalogue 1..4 = distances from Reid & collaborators

Comparison with other spectroscopy R03 - Reid et al. (2003b) C02 - Cruz & Reid (2002) G00 - Gizis (2000) H96 - Hawley, Gizis & Reid (1996) K95 - Kirkpatrick, Henry & Simons (1995) FBS - Gigoyan, Hambaryan & Azzopardi (1998) HIC - Turon et al. (1993) PPM - Röser & Bastian (1988) B98 - Buscombe (1998) L84 - Lee (1984) T00 - Torres et al. (2000) T94 - Tokovinin (1994) L98 - Li & Hu (1998)

Absolute magnitude/spectral type calibration based on data from the CNS4 (Jahreiß, in prep.)

Two white dwarfs (Scholz, Meusinger & Jahreiß 2005) LHS 1200: T eff ~13000K, d=110±20pc, (U,V,W)=(-116,-184,-219)km/s, halo object LHS 2288: T eff ~4000K, d=44±7pc, v tan =140km/s, thick disk object

Subdwarfs (candidates) with large velocities - our estimates (K and M dwarfs) + - objects with much smaller trigonometric parallaxes (obviously sub- dwarfs) x - white dwarfs ∘ - assumed sub- dwarfs (values reduced by 50%) 5% of our targets are at d>50pc, 11 objects with 250<v tan <1200 km/s are assumed to be in fact subdwarfs, if so, their distances and v tan would be reduced by ~50% (we can not distinguish normal K/M dwarfs and subdwarfs based on our spectra) (Scholz, Meusinger & Jahreiß 2005)

Results of our search among NLTT stars ● 85% of our K and M dwarfs are within 30pc (72% within 25pc)  high success rate! (purely photometric study with meanwhile completed 2MASS may be worth doing) ● 50 objects have spectroscopic distances d<15pc, 8 are within 10pc (three of the latter had no previous spectral types) ● Our spectroscopic distances are generally in good agreement with previous estimates based on higher resolution spectra ● Compared to expected number of missing stars in 25pc sample our study investigated/uncovered only a small fraction of them  further work is needed ● The relatively hot (~DA3.5) white dwarf LHS1200 as well as 11 red (sub)dwarfs (which were selected mainly based on their proper motion) have very large space velocities typical of the Galactic halo ● There are no other brown dwarfs in Luyten’s NLTT catalogue except the M9.5 dwarf LP  deeper high pm searches are needed!

How brown dwarfs differ from stars? ( Mass in Solar units: Failed to accumulate critical mass of stars (~0.08 M solar ) during formation  no hydrogen burning in their cores Low luminosity + red colour (low surface temperature), methane, lithium,... absorption in their spectra, mass not measurable in most cases!

Spectra of late spectral types (red and brown dwarfs) Kirkpatrick et al. (1999) New spectral classes L and T defined only very recently Objects with ~ <1000 K surface temperature, i.e. even cooler than the coolest M dwarfs Few late-M, 50% of early-L and all late-L and T dwarfs in the Solar neighbourhood are brown dwarfs

Discovery of a bright L dwarf (Scholz & Meusinger 2002) Detected in SSS high pm survey; spectroscopic classification with Alto; More high pm L dwarfs classified with ESO 3.6m and NTT (Lodieu et al. 2002, 2005) SSSPM J : About 1 mag brighter than Kelu 1 Estimated distance: 12 pc

A visitor from the Galactic halo: SSSPM J1444−2019 Extremely large proper motion: 3.5 arcsec/yr sdM9, but L-type spectral features (RbI, FeH, CrH, no VO!) (Scholz, Lodieu & McCaughrean 2004)

Subdwarf classification by spectral indices Lower temperature Lower metallicity ( Reid et al. 1995, Gizis 1997 ) Lepine et al. (2003), Scholz et al. (2004)

SSSPM J : a sub-stellar subdwarf? VLT/FORS2 spectra: Lithium not detected (Scholz, Lodieu & McCaughrean 2004)

Metallicity of the coolest subdwarfs Scholz, Lodieu & McCaughrean (2004)

Statistics on all known L and T dwarfs 476 objects as of Nov. 28, 2005 in spider.ipac.caltech.edu/staff/davy/ARCHIVE/

The (sub-)stellar luminosity function Courtesy of Peter Allen M L T Y? Cruz (2005)

Discovery of the nearest brown dwarf Scholz et al. (2003) as result of SSS high proper motion search

Discovery of the nearest brown dwarf Scholz et al. (2003) pm + parallax + orbital motion (simulation)... resolved as a T1+T6 binary McCaughrean et al. (2004) as result of SSS high proper motion search using adaptive optics at VLT+NACO 0.7 arcsec

Search for cooler, Y-type brown dwarfs New deep near-infrared survey Pinfield et al. (2005)

Search for coolest Y dwarfs near the Sun High proper motion survey using SDSS pm of late-L dwarf (Finkbeiner et al. 2003) SDSS positional accuracy (Pier et al. 2004) SDSS multi-epoch (5 years) imaging data (Ivezic 2003)  can be used for pm search!

Search for coolest Y dwarfs near the Sun High proper motion survey using SDSS pm of late-L dwarf (Finkbeiner et al. 2003) SDSS positional accuracy (Pier et al. 2004) SDSS multi-epoch (5 years) imaging data (Ivezic 2003)  can be used for pm search! Corresponding SDSS project just started at AIP (Scholz et al.)

+Ba, Bb Are we surrounded by as many brown dwarfs as stars? >90% unknown!