1,066 research outputs found
The Tilt of the Fundamental Plane of Elliptical Galaxies: I. Dynamical and Structural Effects
In this paper we explore several structural and dynamical effects on the
projected velocity dispersion as possible causes of the fundamental plane (FP)
tilt of elliptical galaxies. Specifically, we determine the size of the
systematic trend along the FP in the orbital radial anisotropy, in the dark
matter (DM) content and distribution relative to the bright matter, and in the
shape of the light profile that would be needed to produce the tilt, under the
assumption of a constant stellar mass to light ratio. Spherical, non rotating,
two--components models are constructed, where the light profiles resemble the
law. For the investigated models anisotropy cannot play a major role
in causing the tilt, while a systematic increase in the DM content and/or
concentration may formally produce it. Also a suitable variation of the shape
of the light profile can produce the desired effect, and there may be some
observational hints supporting this possibility. However, fine tuning is always
required in order to produce the tilt, while preserving the {\it tightness} of
the galaxies distribution about the FP.Comment: 12 pages MNRAS-TeX (mn.tex v1.5 incl.), 6 figures (.ps included)
uuencoded, gzip'ed tar file, accepted by MNRA
The origin of the spurious iron spread in the globular cluster NGC 3201
NGC 3201 is a globular cluster suspected to have an intrinsic spread in the
iron content. We re-analysed a sample of 21 cluster stars observed with
UVES-FLAMES at the Very Large Telescope and for which Simmerer et al. found a
0.4 dex wide [Fe/H] distribution with a metal-poor tail. We confirmed that when
spectroscopic gravities are adopted, the derived [Fe/H] distribution spans ~0.4
dex. On the other hand, when photometric gravities are used, the metallicity
distribution from Fe I lines remains large, while that derived from Fe II lines
is narrow and compatible with no iron spread. We demonstrate that the
metal-poor component claimed by Simmerer et al. is composed by asymptotic giant
branch stars that could be affected by non local thermodynamical equilibrium
effects driven by iron overionization. This leads to a decrease of the Fe I
abundance, while leaving the Fe II abundance unaltered. A similar finding has
been already found in asymptotic giant branch stars of the globular clusters M5
and 47 Tucanae. We conclude that NGC 3201 is a normal cluster, with no evidence
of intrinsic iron spread.Comment: Accepted for publication by ApJ, 7 pages, 4 figure
No evidence of chemical anomalies in the bimodal turnoff cluster NGC 1806 in the LMC
We have studied the chemical composition of NGC 1806, a massive,
intermediate-age globular cluster that shows a double main sequence turnoff. We
analyzed a sample of high-resolution spectra (secured with FLAMES at the Very
Large Telescope) for 8 giant stars, members of the cluster, finding an average
iron content of [Fe/H]=--0.60 +- 0.01 dex and no evidence of intrinsic
star-to-star variations in the abundances of light elements (Na, O, Mg, Al).
Also, the (m_(F814W); m_(F336W)-m_(F814W)) color-magnitude diagram obtained by
combining optical and near-UV Hubble Space Telescope photometry exhibits a
narrow red giant branch, thus ruling out intrinsic variations of C and N
abundances in the cluster. These findings demonstrate that NGC 1806 does not
harbor chemically distinct sub-populations, at variance with what was found in
old globular clusters. In turn, this indicates that the double main sequence
turnoff phenomenon cannot be explained in the context of the self-enrichment
processes usually invoked to explain the chemical anomalies observed in old
globulars. Other solutions (i.e., stellar rotation, merging between clusters or
collisions with giant molecular clouds) should be envisaged to explain this
class of globulars.Comment: Accepted for publication by ApJ Letters; 15 pages, 4 figures, 1 tabl
Another brick in understanding chemical and kinematical properties of BSSs: NGC 6752
We used high-resolution spectra acquired with the multifiber facility FLAMES
at the Very Large Telescope of the European Southern Observatory to investigate
the chemical and kinematical properties of a sample of 22 Blue Straggler Stars
(BSSs) and 26 red giant branch stars in the nearby globular cluster NGC 6752.
We measured radial and rotational velocities and Fe, O and C abundances.
According to radial velocities, metallicity and proper motions we identified 18
BSSs as likely cluster members. We found that all the BSSs rotate slowly (less
than 40 km/s), similar to the findings in 47 Tucanae, NGC 6397 and M30. The Fe
abundance analysis reveals the presence of 3 BSSs affected by radiative
levitation (showing [Fe/H] significantly higher than that measured in "normal"
cluster stars), confirming that element transport mechanisms occur in the
photosphere of BSSs hotter than 8000 K. Finally, BSS C and O abundances are
consistent with those measured in dwarf stars. No C and O depletion ascribable
to mass transfer processes has been found on the atmospheres of the studied
BSSs (at odds with previous results for 47 Tucanae and M30), suggesting the
collisional origin for BSSs in NGC 6752 or that the CO-depletion is a transient
phenomenon.Comment: ApJ accepte
The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440
We used a combination of deep optical and Halpha images of the Galactic
globular cluster NGC 6440, acquired with the Hubble Space Telescope, to
identify the optical counterpart to the accreting millisecond X-ray pulsar SAX
J1748.9-2021during quiescence. A strong Halpha emission has been detected from
a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15"
from the nominal position of the X-ray source. The position of the star also
agrees with the optical counterpart found by Verbunt et al. (2000) during an
outburst. We propose this star as the most likely optical counterpart to the
binary system. By direct comparison with isochrones, we estimated that COM-SAX
J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun
and a superficial temperature of 5250pm80 K. These parameters combined with the
orbital characteristics of the binary suggest that the system is observed at a
very low inclination angle (~8 deg -14 deg) and that the star is filling or
even overflowing its Roche Lobe. This, together with the equivalent width of
the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The
possibile presence of such a on-going mass transfer during a quiescence state
also suggests that the radio pulsar is not active yet and thus this system,
despite its similarity with the class of redback millisecond pulsars, is not a
transitional millisecond pulsar.Comment: 8 pages, 6 figures. Accepted for publication in Ap
FLAMES and XSHOOTER spectroscopy along the two BSS sequences of M30
We present spectroscopic observations acquired with FLAMES and XSHOOTER at
the Very Large Telescope for a sample of 15 Blue Straggler Stars (BSSs) in the
globular cluster (GC) M30. The targets have been selected to sample the two BSS
sequences discovered, with 7 BSSs along the blue sequence and 8 along the red
one. No difference in the kinematical properties of the two groups of BSSs has
been found. In particular, almost all the observed BSSs have projected
rotational velocity lower than ~30 km/s, with only one (blue) fast rotating BSS
(>90 km/s), identified as a W UMa binary. This rotational velocity distribution
is similar to those obtained in 47 Tucanae and NGC 6397, while M4 remains the
only GC studied so far harboring a large fraction of fast rotating BSSs. All
stars hotter than ~7800 K (regardless of the parent BSS sequence) show iron
abundances larger than those measured from normal cluster stars, with a
clearcut trend with the effective temperature. This behaviour suggests that
particle trasport mechanisms driven by radiative levitation occur in the
photosphere of these stars, as already observed for the BSSs in NGC 6397.
Finally, 4 BSSs belonging to the red sequence (not affected by radiative
levitation) show a strong depletion of [O/Fe], with respect to the abundance
measured in Red Giant Branch and Horizontal Branch stars. This O-depletion is
compatible with the chemical signature expected in BSSs formed by mass transfer
processes in binary systems, in agreement with the mechanism proposed for the
formation of BSSs in the red sequence.Comment: Accepted for publication in Ap
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