Improving our knowledge on nearby stars and brown dwarfs

Ralf-Dieter ScholzAstrophysikalisches 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):

1976 1982 1993

blue (Bj) extremely red (I) red (R)

Image size: 2 arcmin by 2 arcminProper motion: 3.9 arcsec/yr (among TOP 20 of fastest stars in

the sky)

The Solar neighbourhood (www.anzwers.org/free/universe/)

Stars with accurate

(trigonometric) distances

HRD-CMD forabout 22000 stars measured by the

Hipparcos satellite+

about 1000 stars with ground-based

parallaxes

most red (and white ?) dwarfs not

yet included !

www.anzwers.org/free/universe/

The missing stellar neighbours5pc 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 density2) complete counts within 5pc

?

?

The missing stellar neighbours5pc 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 density2) 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: vtan = 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 M6dwarf (LHS 2090) at spectroscopic distance of only 6 pc

Scholz, Meusinger & Jahreiß (2001)

Old catalogues (e.g. + new deeper and/or near-infraredLuyten 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 photometrySolution: NLTT cross-Id. with 2MASS (2nd 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 withd < 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: 2.2m@Calar Alto(Scholz, Meusinger & Jahreiß

2005)

Some statistics for our sample(Scholz, Meusinger & Jahreiß 2005)

H = Hipparcos, Y = Yale parallax catalogue1..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: Teff~13000K, d=110±20pc, (U,V,W)=(-116,-184,-219)km/s, halo objectLHS 2288: Teff~4000K, d=44±7pc, vtan=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<vtan<1200 km/s are assumed to be in fact subdwarfs, if so, their distances and vtan 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 944-20 deeper high pm searches are needed!

How brown dwarfs differ from stars?

(www.astron.berkeley.edu/~stars/bdwarfs/)

Mass in Solar units: 1 0.6...0.08 0.08...0.013 0.001

Failed to accumulate critical mass of stars (~0.08 Msolar) 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 ~2000...<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 2.2m@Calar Alto;More high pm L dwarfs classified with ESO 3.6m and NTT (Lodieu et al. 2002, 2005)

SSSPM J0829-1309:About 1 mag brighter than Kelu 1Estimated 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!)

1976

1985

1994 (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 J1444-2019: 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

TY?

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!