Issue |
A&A
Volume 515, June 2010
|
|
---|---|---|
Article Number | A100 | |
Number of page(s) | 23 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361/200913138 | |
Published online | 15 June 2010 |
Online Material
Appendix A: Cluster membership of M 34 Using extant kinematic measurements
There exist two extant kinematic studies of the M 34 cluster which
can assist us further in establishing cluster membership for our
gyrochronology sample. The first, by Jones & Prosser (1996),
relies upon 2-d space motions in the form of proper motions, although
their high-fidelity membership probabilities are magnitude limited to
about
.
While JP96 proper motion results do extend to fainter magnitudes, their
reliability reduces. That is to say, a fainter M 34 star with
a
proper motion probability may indeed still be a bona fide member
of the cluster, insomuch as Poisson-limited proper motion errors could
lead to erroneously lower proper motion probabilities.
The second, by Jones et al. (1997),
employing 1-d heliocentric radial velocities, can be used to establish
cluster membership, or in the very least, establish cluster
non-membership (or binarity). Unfortunately neither membership
probabilities nor velocity errors were reported by Jones et al.,
however we can exploit their measurements in our membership assessment
(see below). Interestingly, in the same manuscript, Jones
et al. report Li I 6708 Å equivalent widths [EWs]
for their target stars. Even though the detection of lithium in solar
type stars is not a requisite for cluster membership, its absence is
almost ubiquitous among the generally older, Galactic field stars. The
very presence of a substantial lithium line in solar-type stellar
spectra is in itself indicative of youth ( Myr; e.g., Soderblom et al. 1993b; James & Jeffries 1997; James et al. 2006).
For each of the M 34 stars in our sample, we have collated these additional membership criteria published by JP96 and Jones et al. (1997) in Table A.1. Its Lowell identifier is listed in Col. 1 in concert with its JP96 identifier in Col. 2. Each star's photometric and radial velocity membership flag, reproduced from Tables 2 and 3, are cited in Cols. 3 and 4 respectively, whereas a flag confirming the detection of lithium 6708 Å in its spectrum (from Jones et al. 1997) is listed in Col. 5. Corresponding Jones et al. radial velocities are detailed in Col. 6, as well as their associated membership probabilities in Col. 7 (see below and Fig. A.1). V-band magnitudes of each star are listed in Col. 8 and finally, cluster membership probabilities based on the proper motions reported in JP96 are reproduced in Col. 9.
In the first instance, we note that all stars in common between
our gyrochronology sample and the Jones et al. sample have a
significant lithium detection, indicative of stellar youth. These stars
are therefore unlikely to be field star interlopers, which when
combined with the photometric membership criterion, can safely be
considered as bona fide cluster members. In the second, the
radial velocity data from Jones et al. study do seem to cluster
around -10 km s-1,
however, specific membership assignments are difficult to assess due to
the associated scatter in the individual measurements. In order to
understand whether the scatter in these radial velocities is due to a
true dispersion in the data or is an admixture of single and binary
member measurements, together with cluster non-members, we have
constructed a histogram for all radial velocities published in the
Jones et al. survey. This histogram, plotted in Fig. A.1,
clearly shows a peak in the distribution representing the cluster's
systemic heliocentric radial velocity. In order to investigate the
dispersion about the central peak, we fit an unweighted Gaussian
function to the velocity histogram, noting that the fit does not
include background contamination due to binarity or cluster
non-members, and assumes that the sample is complete. The Gaussian
function is centred on -12.49 km s-1 with a 1
width of 2.32 km s-1, which we can exploit to determine a membership probability (reported in Col. 7 of Table A.1) for each individual star based on its radial velocity and these Gaussian fit parameters.
A.1 Results
Analysis of the additional membership data reported in Table A.1 yields few surprises. Of all the Lowell periodic variables identified as photometric and/or RV members in Table 2,
only two of them, F3_0172 and F3_0215, have a zero probability of being
proper motion members according to the JP96 survey. Interestingly
however, their V-magnitudes lie right at the point at which the
JP96 proper motions begin to become less reliable due to Poisson-error
limits on their photographic plate measurements, which brings their
validity into question. Furthermore, we have noted in Sect. 5.2
that the gyrochronology sample may indeed suffer from contamination due
to one or two cluster non-members masquerading as bona fide
members, but we argue that their contribution to the period variance,
and hence gyro errors, is small. In fact, F3_0172 and F3_0215
contribute a combined period variance of 0.79 days2, which is only a
effect (see Table 5).
In spite of their low contribution to the period variance, we
remain in the uncomfortable position of choosing whether to include
these photometric and kinematic members of the cluster in our
gyrochronology sample, or to exclude them on the basis of
their proper motion membership probabilities. In order to further
assess their cluster membership status, we have recently obtained high
resolution optical spectra of these two stars using the Canada France
Hawai`i Telescope [CFHT], the analysis of which we discuss below (see
Sect. A.2).
Finally, all bar one of the Jones et al. (1997)
stars are radial velocity members of the cluster, which correlates well
with our own radial velocity data. The one Jones et al. star,
F4_0136, that is formally a cluster non-member based on a Gaussian
probability fit to their radial velocity data plotted in Fig. A.1,
actually shows up as a single cluster star member in the long term
synoptic velocity survey of Meibom et al. (in prep. -
see also Sect. 2.1).
Curiously, over the 9-epochs of observation covering 2.5-years, that
Meibom et al. have for this star, its varies about their average radial velocity of -8.0 km s-1 by only 0.68 km s-1.
Interestingly, a Gaussian fit to all radial velocities for the
70 single and binary M 34 stars for which Meibom et al.
have obtained results yields a cluster systemic velocity of -7.59
1.02 km s-1.
Assuming the Jones et al. velocity is not in error, this star may
be a binary member of the cluster, albeit with either a long-period
orbit or a considerably eccentric one.
Table A.1: Cluster membership assessments for Jones & Prosser (1996) stars in our period sample.
A.2 Cluster membership status for stars F3_0172 and F3_0215:
Two stars exhibiting variability in our differential photometric survey of the M 34 cluster, for which we derive periods, namely F3_0172 and F3_0215, present us with somewhat of a conundrum. While these stars have photometric and radial velocity properties consistent with cluster membership, with relatively short photometric periods indicative of youth (compared to the Galactic field), and result in gyrochronology ages appropriate for a 200 Myr group of stars, they possess proper motion vectors incongruent with the remainder of the cluster. In an attempt to establish or refute genuine cluster membership for these two objects, we have recently observed them at high resolution using the fibre-fed, bench-mounted ESPaDOns échelle spectrograph, located in a Coudé-like instrument chamber in the CFHT Observatory. The primary goal of these observations is to detect lithium at 6708 Å, and measure its equivalent width in these stars, thereby confirming their relative youth and increasing their probability of being bona fide cluster members.
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Figure A.1: Heliocentric radial velocities, in histogram form using 3 km s-1 bins, are plotted for M 34 stars using velocity data detailed in Jones et al. (1997). The red solid line depicts an unweighted Gaussian fit to the data, with a Gaussian centre of -12.49 km s-1 and a sigma of 2.32 km s-1. |
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During the evening of 27 January 2010, high resolution (
)
spectra were acquired for stars F3_0172 and F3_0215 using the fibre-fed
ESPaDOns spectrograph, in service with the 3.6-m Canada France Hawai`i
Telescope located on top of Mauna Kea, Hawai`i, USA. The ESPaDOns
spectrograph consists of a 79 lines mm-1 échelle grating imaged onto a 2048
4608 EEV CCD detector, having 13.5
m square pixels, with photon input delivered by separate 100
m
(1.6 arcsec) diameter sky and target fibres. This set-up yields a
FWHM of cross correlated ThAr arc lines of 0.271 Å at 6700 Å,
and a complete wavelength range of
Å.
Using this set-up, F3_0172 and F3_0215 were observed for exposures
totalling 2000 and 2140 s respectively, resulting in spectra with S/N
20 at 6700 Å.
For each of the targets, we exploit our CFHT ESPaDOns spectra in order
to measure their heliocentric radial velocity and their Li I 6708 Å EW.
The removal of CCD instrumental effects, as well as the
extraction of wavelength-calibrated spectra, have been achieved by two
independent data reduction methodologies. In the first, we
performed the bias-subtraction, flat-fielding, spectral order tracing,
optimal extraction and wavelength calibration using standard IRAF
procedures. In the second, we used the direct output from Libre-ESpRIT
(Donati et al. 1997), the dedicated pipeline software for the ESPaDOns spectrograph. A cross-match of radial velocities and EWs for each
target from the two reduction methods reassuringly yields consistent results to within km s-1 and
3 mÅ respectively.
Table A.2: CFHT spectroscopic data products for M 34 candidate members F3_0172 and F3_0215.
Heliocentric radial velocities were derived, relative to the
well-exposed standard star HD 32963, in the spectral region
of the Mg triplet lines (
Å). Cross correlation of HD 32963 with other radial velocity standard stars observed during the CFHT programme shows
that the zero-point in placing our velocities onto the standard system is
0.2 km s-1. The relatively low S/N of our target spectra results in radial velocity precision errors of
0.5 km s-1.
In order to measure equivalent widths for the Li I 6708 Å line in our targets, each spectrum between
Å, was normalized using continuum fitting after spectra extraction. Each EW
was calculated using both the direct integration and the Gaussian
fitting methods, whose values were within a few percent of each other.
In the lithium region, Li I EWs include contributions from the small Fe I+CN line at 6707.44 Å, leading to measured
Li I EWs which are representative of a slightly (10-20 mÅ) over-estimated photospheric Li presence. Soderblom et al. (1993b) report that this Fe line blend has an EW = [20
(B-V)0
-3] mÅ, determined through an empirical relationship for main
sequence, solar-type stars. For each target star, we removed the
Fe I line contribution before transforming Li I EWs into abundances, N(Li) - on a scale where log N(H) = 12, using the effective temperature-colour (B-V) relation from Soderblom et al. (1993c), and the curves of growth presented in Soderblom et al. (1993b).
Data products, radial velocity and lithium measurements, from the CFHT
spectroscopic observations of F3_0172 and F3_0215 are presented in
Table A.2.
CFHT radial velocities of F3_0172 and F3_0215 are consistent with
cluster membership of M 34 irrespective of whether we compare
their individual values to the Jones et al. (1997)
sample or to the Meibom et al. (in prep.) one. For the Jones
et al. sample, their kinematic membership probabilities are
non-zero although they are quite low at
and
(for F3_0172 and F3_0215 respectively). Their membership
probabilities are far more convincing when compared to the Meibom
et al. sample (with respective values of
and >99%).
![]() |
Figure A.2:
Logarithmic lithium abundances N(Li) =
12 + log(Li/H) versus effective temperature are plotted for stars
in the M 34 cluster (data taken from Jones et al. 1997).
Red filled squares represent abundances for F3_0172 and F3_0215
(JP 49 and JP 41), determined using data reported in
Table A.2, with error parallelograms based on temperature errors of 100 K and |
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The lithium content of both F3_0172 and F3_0215 is substantial indicating that these stars are relatively young compared to the general Galactic field, whose solar-type stellar content is typically old and has had sufficient time to have proton-burned considerable fractions of its natal lithium. In order for these stars to be judged as likely members of the M 34 cluster, not only must they contain lithium in their atmospheres, but it must quantitatively fit into the mass-dependent lithium abundance distribution for the cluster. In Fig. A.2, we plot the lithium abundances for F3_0172 and F3_0215 that we have determined from our CFHT spectra in concert with the extant mass-dependent lithium distribution for M 34 stars (data taken from Jones et al. 1997, who employed identical (B-V)o-Temperature, Fe I line correction and lithium curves of growth as we have used). It is clear that both F3_0172 and F3_0215 have lithium abundances consistent with the mass-dependent lithium distribution of the M 34 cluster, and must be considered lithium abundance members.
JP96 proper motion vectors for F3_0172 and F3_0215 indicate cluster non-membership. However their V-magnitudes render JP96 2-d kinematic membership probabilities questionable, because at or about this magnitude, JP96 proper motion accuracy and precisions begin to have strong dependencies on the Poissonian errors of their centroiding measurements. In consideration that these two stars are both photometric and kinematic members of the cluster, have photometric periods consistent with the remainder of the cluster's distribution, and have measured lithium abundances which lie right along the trend of the mass-dependent lithium distribution for the cluster, they are probable M 34 cluster members and we retain them in our gyrochronology sample.
Appendix B: A period vs. equatorial velocity analysis
A comparison of photometric period and projected equatorial rotation rate for M 34 stars is plotted in Fig. B.1.
For the most part, photometric periods are determined from the Lowell
campaign described in this manuscript, except for a few cases where
Irwin et al. periods are employed where Lowell ones do not exist.
Spectroscopic velocities for stars in common to the Lowell campaign and
Irwin et al. survey are obtained from Jones et al. (1997). Three loci are also shown in the figure, representing equality between period and
data for G0, K0 and M0 dwarfs, where
is assumed for each.
![]() |
Figure B.1:
Projected equatorial velocities ( |
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There are three notable characteristics to the plot. First, the clustering of data-points for stars
with periods
days and
10-14 km s-1
lie to the right of the G0 dwarf locus. Given that the M 34
stars having photometric periods are late-F to early M-dwarfs, these
data-points are most likely indicative of those stars whose
values are
.
Second, there are four stars with periods <3 days and 10 <
< 25 km s-1, whose period,
data place them considerably below even the M0 dwarf locus in the
diagram. Assuming that these stars are bona fide cluster members,
whose period and
values are correctly determined, these stars must be high inclination systems (
;
i.e., becoming more pole-on), whose true equatorial velocities are considerably higher.
Finally, there is one M 34 star (F3)
whose period determinations, from our Lowell data and by the Irwin
et al. study, are seriously discrepant (see also Fig. 2).
The periodicity for this star was detected in three filter/exposure
time observations during our Lowell campaign. It is interesting to
note that the Irwin et al. period is almost half that of our
Lowell one, and their lower value could be due to phase aliasing, power
leakage in the power spectrum as a consequence of their shorter
observing window or multiple spot groups on the surface on the star
during its observation. In any case, either both period
determinations are incorrect, or either value from the Lowell
campaign or the Irwin et al. study is in error.
If we assume that one of the periods for this star is correct, we can
make some predictive statements as to its equatorial velocity. With an
intrinsic B-V colour
of 0.95, its spectral type on the main sequence would be K2 or K3,
placing it close to the central locus of the three plotted in Fig. B.1 (assuming the star is inclined
to the line-of-sight). This scenario is consistent with its Irwin
et al. period of 6.655 days. Conversely, if the Lowell period
of 11.0 days is correct for this object, and it is a single member
of the cluster lying on the main sequence, its inclination angle must
be higher than
(
),
whose appearance is more face-on to the line-of-sight. Hopefully, more
extensive photometric monitoring of this star will reveal its true
nature.
Appendix C: Lowell light-curves for photometrically variable stars in the field of M 34
![]() |
Figure C.1: Period-phased photometric lightcurves for M 34 variables, derived from short-V observations. |
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![]() |
Figure C.1: continued. |
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Figure C.2: Period-phased photometric lightcurves for M 34 variables, derived from short-I observations. |
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Figure C.2: continued. |
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Figure C.3: Period-phased photometric lightcurves for M 34 variables, derived from long-V observations. |
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![]() |
Figure C.3: continued. |
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Figure C.3: continued. |
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Figure C.4: Period-phased photometric lightcurves for M 34 variables, derived from long-I observations. |
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![]() |
Figure C.4: continued. |
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Figure C.4: continued. |
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