27 research outputs found
Searching for merger debris in the Galactic halo: Chemodynamical evidence based on local blue HB stars
We report on the discovery of a group of local A-type blue horizontal-branch
(HBA) stars moving in a prograde, comet-like orbit with very similar kinematics
and abundances. This serendipitously discovered group contains 5 or 6 local HBA
stars venturing very close to the Galactic centre; their [Fe/H] is around -1.7,
and they seem to present minimum scatter in at least Mg, Si, Ti, Fe, Al, and Cr
abundances. This ``Cometary Orbit Group'' (COG) was found while we were testing
a new method to detect the debris associated with the merger of smaller,
specific protogalactic entities into our galaxy. The method is primarily
intended to identify field HBA stars with similar kinematics and detailed,
multi-species abundance patterns as seen among members of a surviving remnant
(e.g., omega Centauri). Quite possibly, the COG is the remnant, on a highly
decayed orbit, of a merging event that took place in the relatively remote past
(i.e., at least one revolution ago).Comment: 4 pages and 2 EPS figures, accepted for publication in Astronomy and
Astrophysics Letter
The Horizontal Branch of NGC 1851: Constraints from its RR Lyrae Variables
We use the pulsational properties of the RR Lyrae variables in the globular
cluster NGC 1851 to obtain detailed constraints of the various sub-stellar
populations present along its horizontal branch. On the basis of detailed
synthetic horizontal branch modeling, we find that minor helium variations
(Y~0.248-0.280) are able to reproduce the observed periods and amplitudes of
the RR Lyrae variables, as well as the frequency of fundamental and
first-overtone RR Lyrae stars. Comparison of number ratios amongst the blue and
red horizontal branch components and the two observed subgiant branches also
suggest that the RR Lyrae variables originated from the progeny of the bright
subgiant branch. The RR Lyrae variables with a slightly enhanced helium
(Y~0.270-0.280) have longer periods at a given amplitude, as is seen with
Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with
Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI)
variables. This correlation does suggest that the pulsational properties of RR
Lyrae stars can be very useful for tracing the various sub-populations and can
provide suitable constraints on the multiple population phenomenon. It appears
of great interest to explore whether this conclusion can be generalized to
other globular clusters hosting multiple populations.Comment: accepted to A
Variable stars in the Fornax dSph Galaxy. I. The Globular Cluster Fornax 4
Variable stars have been identified for the first time in Fornax 4, the
globular cluster located near the center of the Fornax dwarf spheroidal galaxy.
By applying the image subtraction technique to B,V time series photometry
obtained with the MagIC camera of the 6.5-m Magellan/Clay telescope and with
the wide field imager of the 4-m Blanco/CTIO telescope, we detected 27 RR Lyrae
stars (22 fundamental mode, 3 first overtone, and 2 double-mode pulsators) in a
2.4'x2.4' area centered on Fornax 4. The average and minimum periods of the
ab-type RR Lyrae stars, = 0.594 d and P(ab,min)=0.5191 d, respectively, as
well as the revised position of the cluster in the horizontal branch
type--metallicity plane, all consistently point to an Oosterhoff-intermediate
status for the cluster, unlike what is seen for the vast majority of Galactic
globular clusters, but in agreement with previous indications for the other
globular clusters in Fornax.
The average apparent magnitude of the RR Lyrae stars located within 30 arcsec
from the cluster center is =21.43 +/- 0.03 mag (sigma=0.10 mag, average
on 12 stars), leading to a true distance modulus of (m-M)o=20.64 +/- 0.09 mag
or (m-M)o=20.53 +/- 0.09 mag, depending on whether a low ([Fe/H]=-2.0) or a
moderately high ([Fe/H]=-1.5) metallicity is adopted.Comment: Apj, in pres