J/MNRAS/356/1583 RV of Sigma Ori low-mass candidate members (Burningham+, 2005)
Contamination and exclusion in the σ Orionis young group.
Burningham B., Naylor T., Littlefair S.P., Jeffries R.D.
<Mon. Not. R. Astron. Soc., 356, 1583-1591 (2005)>
=2005MNRAS.356.1583B 2005MNRAS.356.1583B
ADC_Keywords: Clusters, open ; Stars, pre-main sequence ; Stars, dwarfs ;
Radial velocities ; Equivalent widths
Keywords: techniques: radial velocities - techniques: spectroscopic -
stars: low-mass, brown dwarfs - stars: pre-main-sequence -
open clusters and associations: individual: σ Orionis
Abstract:
We present radial velocities for 38 low-mass candidate members of the
σ Orionis young group. We have measured their radial velocities
by cross-correlation of high-resolution (R≫6000) AF2/Wide Field Fibre
Optical Spectrograph (WYFFOS) spectra of the gravity-sensitive NaI
doublet at 8183,8195Å, on the nights of 2003 January 3 and 4. The
total sample contained 117 objects, of which 54 have sufficient
signal-to-noise ratio to detect NaI at an equivalent width of 3Å;
however, we only detect NaI in 38 of these. This implies that very
low-mass members of this young group display weaker NaI absorption
than similarly aged objects in the Upper Scorpius OB association. We
develop a technique to assess membership using radial velocities with
a range of uncertainties that does not bias the selection when large
uncertainties are present. The resulting membership probabilities are
used to assess the issue of exclusion in photometric selections, and
we find that very few members are likely to be excluded by such
techniques. We also assess the level of contamination in the expected
pre-main-sequence region of colour-magnitude space brighter than I=17.
We find that contamination by non-members in the expected
pre-main-sequence region of the colour-magnitude diagram is small. We
conclude that although radial velocity alone is insufficient to
confirm membership, high signal-to-noise ratio observations of the NaI
doublet provide the opportunity to use the strength of NaI absorption
in concert with radial velocities to asses membership down to the
lowest masses, where lithium absorption no longer distinguishes youth.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 98 54 Equivalent widths and radial velocities measured
using the NaI doublet.
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See also:
J/MNRAS/356/89 : sigma Ori low-mass stars (Kenyon+, 2005)
J/AJ/128/2316 : BVRIJHKs photometry in sigma Ori cluster (Sherry+, 2004)
J/A+A/419/249 : RJHKs phot. of sigma Ori low-mass stars (Scholz+, 2004)
J/AN/325/705 : RIJHKs of low-mass stars in sigma Ori (Bejar+, 2004)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 4 F4.2 --- Field Field number (1)
6- 9 I4 --- Star Object number (1)
11- 12 I2 h RAh Right ascension (J2000)
14- 15 I2 min RAm Right ascension (J2000)
17- 22 F6.3 s RAs Right ascension (J2000)
24 A1 --- DE- Declination sign (J2000)
25- 26 I2 deg DEd Declination (J2000)
28- 29 I2 arcmin DEm Declination (J2000)
31- 35 F5.2 arcsec DEs Declination (J2000)
37- 42 F6.3 mag Icmag Cousins I magnitude
44- 48 F5.3 mag e_Icmag rms uncertainty on Imag
50- 51 A2 --- q_Icmag Quality flag in Icmag (2)
53- 57 F5.3 0.1nm Rc-Ic Cousins R-I colour index
59- 63 F5.3 0.1nm e_Rc-Ic rms uncertainty on Rc-Ic
65- 66 A2 --- q_Rc-Ic Quality flag in Rc-Ic (2)
68- 72 F5.2 km/s EW Equivalent width of NaI doublet (8183/8195Å)
integrated over the range 8180-8202Å (3)
74- 77 F4.2 km/s e_EW rms uncertainty on EW
79- 84 F6.2 km/s RVbar ?=- Barycentric radial velocity (4)
85 A1 --- f_RVbar [X] X=cross-correlation failed
87- 91 F5.2 % e_RVbar ?=- rms uncertainty on RVbar
93- 98 F6.2 % Pvel ?=- Membership probability from velocity
measurements
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Note (1): The field number and object number are those used in the
photometric catalog of Kenyon et al. (2005, Cat. J/MNRAS/356/89,
file "catalog.dat"). Objects identified in Simbad as
[KJN2005] F.FF NNNN
Note (2): Quality flags are as described in Burningham et al.
(2003, cat. J/MNRAS/346/1143) as follows:
OO = OK for Vmag and Icmag
OV = OK for Vmag, Variable for Icmag
VV = Variable for Vmag and Icmag
Note (3): The continuum was measured from bands at 8151-8173 and 8233-8255Å.
Note (4): Radial velocities were measured by cross-correlating against GJ3517,
and corrected to give barycentric radial velocities.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 13-Mar-2006