We analysed red giant branch stars in 16 Galactic globular clusters, computing their atmospheric parameters both from the photometry and from excitation and ionisation balances. The spectroscopic ...parameters are lower than the photometric ones and this discrepancy increases with decreasing metallicity, reaching differences of ~350 K in effective temperature and ~1 dex in surface gravity at Fe/H ~ –2.5 dex. We demonstrate that the spectroscopic parameters are inconsistent with the position of the stars in the colour-magnitude diagram, providing overly low temperatures and gravities, and predicting that the stars are up to about 2.5 magnitudes brighter than the observed magnitudes. The parameter discrepancy is likely due to inadequacies in the adopted physics; in particular the assumption of a one-dimensional geometry could be the origin of the observed slope between iron abundances and excitation potential that leads to low temperatures. However, the current modelling of 3D/NLTE radiative transfer for giant stars seems to be unable to totally erase this slope. We conclude that the spectroscopic parameters are incorrect for metallicity lower than –1.5 dex and that photometric temperatures and gravities should be adopted for these red giant stars. We provide a simple relation to correct the spectroscopic temperatures in order to put them onto a photometric scale.
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We present new colour–effective temperature (
T
eff
) transformations based on the photometry of the early third data release (EDR3) of the ESA/
Gaia
mission. These relations are calibrated on a ...sample of about 600 dwarf and giant stars for which
T
eff
has previously been determined with the infrared flux method from dereddened colours. The 1
σ
dispersion of the transformations is of 60–80 K for the pure
Gaia
colours (BP−RP)
0
, (BP−
G
)
0
, and (
G
−RP)
0
, improving to 40–60 K for colours including the 2MASS
K
s
-band, namely (BP−
K
s
)0
, (RP−
K
s
)
0
, and (G−
K
s
)
0
. We validate these relations in the most challenging case of dense stellar fields, where the
Gaia
EDR3 photometry could be less reliable, providing guidance for the safe use of
Gaia
colours in crowded environments. We compare the
T
eff
from the
Gaia
EDR3 colours with those obtained from standard (
V
−
K
s
)
0
colours for stars in three Galactic globular clusters of different metallicity, namely NGC 104, NGC 6752, and NGC 7099. The agreement between the two estimates of
T
eff
is excellent, with mean differences of between –50 and +50 K, depending on the colour, and with 1
σ
dispersions around the mean
T
eff
differences of 25–50 K for most of the colours and below 10 K for (BP−
K
s
)
0
and (
G
−
K
s
)
0
. This demonstrates that these colours are analogous to (
V
−
K
s
)
0
as
T
eff
indicators.
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Aims.
We present the first three-dimensional internal motions for individual stars in the Draco dwarf spheroidal galaxy.
Methods.
By combining first-epoch
Hubble
Space Telescope observations and ...second-epoch
Gaia
Data Release 2 positions, we measured the proper motions of 149 sources in the direction of Draco. We determined the line-of-sight velocities for a sub-sample of 81 red giant branch stars using medium resolution spectra acquired with the DEIMOS spectrograph at the Keck II telescope. Altogether, this resulted in a final sample of 45 Draco members with high-precision and accurate 3D motions, which we present as a table in this paper.
Results.
Based on this high-quality dataset, we determined the velocity dispersions at a projected distance of ∼120 pc from the centre of Draco to be
σ
R
= 11.0
−1.5
+2.1
km s
−1
,
σ
T
= 9.9
−3.1
+2.3
km s
−1
and
σ
LOS
= 9.0
−1.1
+1.1
km s
−1
in the projected radial, tangential, and line-of-sight directions. This results in a velocity anisotropy
β
= 0.25
−1.38
+0.47
at
r
≳ 120 pc. Tighter constraints may be obtained using the spherical Jeans equations and assuming constant anisotropy and Navarro-Frenk-White (NFW) mass profiles, also based on the assumption that the 3D velocity dispersion should be lower than ≈1/3 of the escape velocity of the system. In this case, we constrain the maximum circular velocity
V
max
of Draco to be in the range of 10.2−17.0 km s
−1
. The corresponding mass range is in good agreement with previous estimates based on line-of-sight velocities only.
Conclusions.
Our Jeans modelling supports the case for a cuspy dark matter profile in this galaxy. Firmer conclusions may be drawn by applying more sophisticated models to this dataset and with new datasets from upcoming
Gaia
releases.
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Abstract
We present the first results of the Multi-Instrument Kinematic Survey of Galactic Globular Clusters (GGCs), a project aimed at exploring the internal kinematics of a representative sample of ...GGCs from the radial velocity of individual stars, covering the entire radial extension of each system. This is achieved by exploiting the formidable combination of multi-object and integral field unit spectroscopic facilities of the ESO Very Large Telescope. As a first step, here we discuss the results obtained for 11 clusters from high and medium resolution spectra acquired through a combination of FLAMES and KMOS observations. We provide the first kinematical characterization of NGC 1261 and NGC 6496. In all the surveyed systems, the velocity dispersion profile declines at increasing radii, in agreement with the expectation from the King model that best fits the density/luminosity profile. In the majority of the surveyed systems, we find evidence of rotation within a few half-mass radii from the center. These results are in general overall agreement with the predictions of recent theoretical studies, suggesting that the detected signals could be the relic of significant internal rotation set at the epoch of the cluster’s formation.
The Galactic bulge is dominated by an old, metal-rich stellar population. The possible presence and the amount of a young (a few gigayears old) minor component is one of the major issues debated in ...the literature. Recently, the bulge stellar system Terzan 5 was found to harbor three sub-populations with iron content varying by more than one order of magnitude (from 0.2 up to two times the solar value), with chemical abundance patterns strikingly similar to those observed in bulge field stars. Here we report on the detection of two distinct main-sequence turnoff points in Terzan 5, providing the age of the two main stellar populations: 12 Gyr for the (dominant) sub-solar component and 4.5 Gyr for the component at super-solar metallicity. This discovery classifies Terzan 5 as a site in the Galactic bulge where multiple bursts of star formation occurred, thus suggesting a quite massive progenitor possibly resembling the giant clumps observed in star-forming galaxies at high redshifts. This connection opens a new route of investigation into the formation process and evolution of spheroids and their stellar content.
ABSTRACT
We used a combination of optical and near-UV
Hubble Space Telescope
photometry and FLAMES/ESO-VLT high-resolution spectroscopy to characterize the stellar content of the old and massive ...globular cluster (GC) NGC 121 in the Small Magellanic Cloud (SMC). We report on the detection of multiple stellar populations, the first case in the SMC stellar cluster system. This result enforces the emerging scenario in which the presence of multiple stellar populations is a distinctive-feature of old and massive GCs regardless of the environment, as far as the light-element distribution is concerned. We find that second-generation (SG) stars are more centrally concentrated than first-generation (FG) ones. More interestingly, at odds with what is typically observed in Galactic GCs, we find that NGC 121 is the only cluster so far to be dominated by FG stars that account for more than 65% of the total cluster mass. In the framework where GCs were born with 90%–95% of FG stars, this observational finding would suggest that either NGC 121 experienced a milder stellar mass-loss with respect to Galactic GCs or it formed a smaller fraction of SG stars.
This is the second paper in our series about the search for multiple populations in Magellanic Cloud star clusters using the Hubble Space Telescope. Here we report the detection of multiple stellar ...populations in the colour-magnitude diagrams of the intermediate-age clusters Lindsay 1, NGC 416 and NGC 339. With ages between 6.0 and 7.5 Gyr, these clusters are the youngest ones in which chemical abundance spreads have been detected so far. This confirms that the appearance of multiple populations is not restricted to only ancient globular clusters, but may also be a common feature in clusters as young as 6 Gyr. Our results are in agreement with a recent spectroscopic study of Lindsay 1. We found that the fraction of enriched stars in NGC 416 is ~45 per cent whereas it is ~25 per cent in NGC 339 and ~36 per cent in Lindsay 1. Similar to NGC 121, these fractions are lower than the average value for globular clusters in the Milky Way.
ABSTRACT
In our HST photometric survey, we have been searching for multiple stellar populations (MPs) in Magellanic Clouds (MCs) massive star clusters which span a significant range of ages ...(∼1.5–11 Gyr). In the previous papers of the series, we have shown that the age of the cluster represents one of the key factors in shaping the origin of the chemical anomalies. Here, we present the analysis of four additional clusters in the MCs, namely Lindsay 38, Lindsay 113, NGC 2121, and NGC 2155, for which we recently obtained new UV HST observations. These clusters are more massive than ∼104 M⊙ and have ages between ∼2.5 and ∼6 Gyr, i.e. located in a previously unexplored region of the cluster age/mass diagram. We found chemical anomalies, in the form of N spreads, in three out of four clusters in the sample, namely in NGC 2121, NGC 2155, and Lindsay 113. By combining data from our survey and HST photometry for three additional clusters in the Milky Way (namely 47 Tuc, M15, and NGC 2419), we show that the extent of the MPs in the form of N spread is a strong function of age, with older clusters having larger N spreads with respect to the younger ones. Hence, we confirm that cluster age plays a significant role in the onset of MPs.
Abstract
We have recently shown that the ∼2 Gyr old Large Magellanic Cloud star cluster NGC 1978 hosts multiple populations in terms of star-to-star abundance variations in N/Fe. These can be seen as ...a splitting or spread in the subgiant and red giant branches (SGB and RGB) when certain photometric filter combinations are used. Because of its relative youth, NGC 1978 can be used to place stringent limits on whether multiple bursts of star formation have taken place within the cluster, as predicted by some models for the origin of multiple populations. We carry out two distinct analyses to test whether multiple star formation epochs have occurred within NGC 1978. First, we use ultraviolet colour--magnitude diagrams (CMDs) to select stars from the first and second population along the SGB, and then compare their positions in optical CMDs, where the morphology is dominantly controlled by age as opposed to multiple population effects. We find that the two populations are indistinguishable, with age differences of 1 ± 20 Myr between them. This is in tension with predictions from the asymptotic giant branch scenario for the origin of multiple populations. Second, we estimate the broadness of the main-sequence turn-off (MSTO) of NGC 1978 and we report that it is consistent with the observational errors. We find an upper limit of ∼65 Myr on the age spread in the MSTO of NGC 1978. This finding is in conflict with the age spread scenario as origin of the extended MSTO in intermediate-age clusters, while it fully supports predictions from the stellar rotation model.
Abstract
Recent photometric results have identified a new population among globular cluster stars. This population, referred to as the ‘extended P1', has been suggested to be the manifestation of a ...new abundance pattern where the initial mass fraction of He changes among cluster stars that share the same CNO values. The current paradigm for the formation of the multiple stellar populations in globular clusters assumes that variations in He are the product of chemical ‘enrichment’ by the ashes of the CNO-cycle (which changes He and other elements like C, N and O simultaneously). We obtained MIKE@Magellan spectra of six giant stars in NGC 2808, a cluster with one of the strongest examples of the extended P1 population. We provide the first complete characterization of the light elements abundances for the stars along a significant range of the extended P1 photometric group. The stars from our sample appear to be homogeneous in C, N, O, Na, Mg and Al. The lack of a significant change in these products of the CNO-cycle suggests that unlike the rest of the populations identified to date, the photometric changes responsible for the extended P1 feature are a consequence of an alternative mechanism. Our measurements are consistent with the interpretations where the changes of the He mass fraction among these stars could be a consequence of p–p chain nucleosynthesis (which could increase the He in stars without affecting heavier elements). Having said that, direct measurements of He are necessary to conclude if variations of this element are present among extended P1 stars.
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