We present the identification of 634 variable stars in the Milky Way dwarf spheroidal (dSph) satellite Sculptor based on archival ground-based optical observations spanning ∼24 yr and covering ...∼2.5 deg2. We employed the same methodologies as the ‘Homogeneous Photometry’ series published by Stetson. In particular, we have identified and characterized one of the largest (536) RR Lyrae samples so far in a Milky Way dSph satellite. We have also detected four Anomalous Cepheids, 23 SX Phoenicis stars, five eclipsing binaries, three field variable stars, three peculiar variable stars located above the horizontal branch – near to the locus of BL Herculis – that we are unable to classify properly. Additionally, we identify 37 long period variables plus 23 probable variable stars, for which the current data do not allow us to determine the period. We report positions and finding charts for all the variable stars, and basic properties (period, amplitude, mean magnitude) and light curves for 574 of them. We discuss the properties of the RR Lyrae stars in the Bailey diagram, which supports the coexistence of subpopulations with different chemical compositions. We estimate the mean mass of Anomalous Cepheids (∼1.5 M⊙) and SX Phoenicis stars (∼1 M⊙). We discuss in detail the nature of the former. The connections between the properties of the different families of variable stars are discussed in the context of the star formation history of the Sculptor dSph galaxy.
We identified and characterized the largest (536) RR Lyrae (RRL) sample in a Milky Way dSph satellite (Sculptor) based on optical photometry data collected over ∼24 years. The RRLs display a spread ...in V-magnitude (∼0.35 mag) which appears larger than photometric errors and the horizontal branch (HB) luminosity evolution of a mono-metallic population. Using several calibrations of two different reddening free and metal independent period–Wesenheit relations we provide a new distance estimate μ = 19.62 mag (σμ = 0.04 mag) that agrees well with literature estimates. We constrained the metallicity distribution of the old population, using the M
I period–luminosity relation, and we found that it ranges from −2.3 to −1.5 dex. The current estimate is narrower than suggested by low and intermediate spectroscopy of RGBs (Δ Fe/H≤ 1.5). We also investigated the HB morphology as a function of the galactocentric distance. The HB in the innermost regions is dominated by red HB stars and by RRLs, consistent with a more metal-rich population, while in the outermost regions it is dominated by blue HB stars and RRLs typical of a metal-poor population. Our results suggest that fast chemical evolution occurred in Sculptor, and that the radial gradients were in place at an early epoch.
Open clusters (OCs) are crucial for studying the formation and evolution of the Galactic disc. However, the lack of a large number of OCs analysed homogeneously hampers the investigations about ...chemical patterns and the existence of Galactocentric radial and vertical gradients, or an age–metallicity relation. To overcome this, we have designed the Open Cluster Chemical Abundances from Spanish Observatories (OCCASO) survey. We aim to provide homogeneous radial velocities, physical parameters and individual chemical abundances of six or more red clump stars for a sample of 25 old and intermediate-age OCs visible from the Northern hemisphere. To do so, we use high-resolution spectroscopic facilities (R ≥ 62 000) available at Spanish observatories. We present the motivation, design and current status of the survey, together with the first data release of radial velocities for 77 stars in 12 OCs, which represents about 50 per cent of the survey. We include clusters never studied with high-resolution spectroscopy before (NGC 1907, NGC 6991, NGC 7762), and clusters in common with other large spectroscopic surveys like the Gaia-ESO Survey (NGC 6705) and Apache Point Observatory Galactic Evolution Experiment (NGC 2682 and NGC 6819). We perform internal comparisons between instruments to evaluate and correct internal systematics of the results, and compare our radial velocities with previous determinations in the literature, when available. Finally, radial velocities for each cluster are used to perform a preliminary kinematic study in relation with the Galactic disc.
Abstract
We report the discovery of six ultra-faint Milky Way satellites identified through matched-filter searches conducted using Dark Energy Camera (DECam) data processed as part of the second ...data release of the DECam Local Volume Exploration (DELVE) survey. Leveraging deep Gemini/GMOS-N imaging (for four candidates) as well as follow-up DECam imaging (for two candidates), we characterize the morphologies and stellar populations of these systems. We find that these candidates all share faint absolute magnitudes (
M
V
≥ −3.2 mag) and old, metal-poor stellar populations (
τ
> 10 Gyr, Fe/H < −1.4 dex). Three of these systems are more extended (
r
1/2
> 15 pc), while the other three are compact (
r
1/2
< 10 pc). From these properties, we infer that the former three systems (Boötes V, Leo Minor I, and Virgo II) are consistent with ultra-faint dwarf galaxy classifications, whereas the latter three (DELVE 3, DELVE 4, and DELVE 5) are likely ultra-faint star clusters. Using data from the Gaia satellite, we confidently measure the proper motion of Boötes V, Leo Minor I, and DELVE 4, and tentatively detect a proper-motion signal from DELVE 3 and DELVE 5; no signal is detected for Virgo II. We use these measurements to explore possible associations between the newly discovered systems and the Sagittarius dwarf spheroidal, the Magellanic Clouds, and the Vast Polar Structure, finding several plausible associations. Our results offer a preview of the numerous ultra-faint stellar systems that will soon be discovered by the Vera C. Rubin Observatory and highlight the challenges of classifying the faintest stellar systems.
Abstract
Eridanus II (Eri II) is an ultrafaint dwarf (UFD) galaxy (
M
V
= −7.1) located at a distance close to the Milky Way virial radius. Early shallow color–magnitude diagrams (CMDs) indicated ...that it possibly hosted an intermediate-age or even young stellar population, which is unusual for a galaxy of this mass. In this paper, we present new Hubble Space Telescope/Advanced Camera for Surveys CMDs reaching the oldest main-sequence turnoff with excellent photometric precision and derive a precise star formation history (SFH) for this galaxy through CMD fitting. This SFH shows that the bulk of the stellar mass in Eri II formed in an extremely short star formation burst at the earliest possible time. The derived star formation rate profile has a width at half maximum of 500 Myr and reaches a value compatible with null star formation 13 Gyr ago. However, tests with mock stellar populations and with the CMD of the globular cluster M92 indicate that the star formation period could be shorter than 100 Myr. From the quantitative determination of the amount of mass turned into stars in this early star formation burst ( ∼2 × 10
5
M
⊙
) we infer the number of supernova (SN) events and the corresponding energy injected into the interstellar medium. For reasonable estimates of the Eri II virial mass and values of the coupling efficiency of the SN energy, we conclude that Eri II could be quenched by SN feedback alone, thus casting doubts on the need to invoke cosmic reionization as the preferred explanation for the early quenching of old UFD galaxies.
Abstract
We report the discovery of Pegasus IV, an ultra-faint dwarf galaxy found in archival data from the Dark Energy Camera processed by the DECam Local Volume Exploration Survey. Pegasus IV is a ...compact, ultra-faint stellar system (
r
1
/
2
=
41
−
6
+
8
pc;
M
V
= −4.25 ± 0.2 mag) located at a heliocentric distance of
90
−
6
+
4
kpc
. Based on spectra of seven nonvariable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV’s velocity dispersion, measuring
σ
v
=
3.3
−
1.1
+
1.7
km s
−1
(after excluding three velocity outliers); this implies a mass-to-light ratio of
M
1
/
2
/
L
V
,
1
/
2
=
167
−
99
+
224
M
⊙
/
L
⊙
for the system. From the five stars with the highest signal-to-noise spectra, we also measure a systemic metallicity of Fe/H =
−
2.63
−
0.30
+
0.26
dex, making Pegasus IV one of the most metal-poor ultra-faint dwarfs. We tentatively resolve a nonzero metallicity dispersion for the system. These measurements provide strong evidence that Pegasus IV is a dark-matter-dominated dwarf galaxy, rather than a star cluster. We measure Pegasus IV’s proper motion using data from Gaia Early Data Release 3, finding (
μ
α
*
,
μ
δ
) = (0.33 ± 0.07, −0.21 ± 0.08) mas yr
−1
. When combined with our measured systemic velocity, this proper motion suggests that Pegasus IV is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. Lastly, we identify three potential RR Lyrae variable stars within Pegasus IV, including one candidate member located more than 10 half-light radii away from the system’s centroid. The discovery of yet another ultra-faint dwarf galaxy strongly suggests that the census of Milky Way satellites is still incomplete, even within 100 kpc.
Abstract
Open clusters have long been used to study the chemodynamical evolution of the Galactic disc. This requires a homogeneously analysed sample covering a wide range of ages and distances. In ...this paper, we present the Open Clusters Chemical Abundances from Spanish Observatories (OCCASO) second data release. This comprises a sample of high-resolution (R > 65 000) and high signal-to-noise spectra of 115 red clump stars in 18 open clusters. We derive atmospheric parameters (T
eff, log g, ξ), and Fe/H abundances using two analysis techniques: equivalent widths and spectral synthesis. A detailed comparison and a critical review of the results of the two methods are made. Both methods are carefully tested between them, with the Gaia FGK benchmark stars, and with an extensive sample of literature values. We perform a membership study using radial velocities and the resulting abundances. Finally, we compare our results with a chemodynamical model of the Milky Way thin disc concluding that the oldest open clusters are consistent with the models only when dynamical effects are taken into account.
Abstract
We discuss the largest and most homogeneous spectroscopic data set of field RR Lyrae variables (RRLs) available to date. We estimated abundances using both high-resolution and low-resolution ...(
Δ
S method) spectra for fundamental (RRab) and first overtone (RRc) RRLs. The iron abundances for 7941 RRLs were supplemented with similar estimates that are available in the literature, ending up with 9015 RRLs (6150 RRab, 2865 RRc). The metallicity distribution shows a mean value of 〈Fe/H〉 = −1.51 ± 0.01, and
σ
(standard deviation) = 0.41 dex with a long metal-poor tail approaching Fe/H ≃ − 3 and a sharp metal-rich tail approaching solar iron abundance. The RRab variables are more metal-rich (〈Fe/H〉
ab
= −1.48 ± 0.01,
σ
= 0.41 dex) than RRc variables (〈Fe/H〉
c
= −1.58 ± 0.01,
σ
= 0.40 dex). The relative fraction of RRab variables in the Bailey diagram (visual amplitude versus period) located along the short-period (more metal-rich) and the long-period (more metal-poor) sequences are 80% and 20%, while RRc variables display an opposite trend, namely 30% and 70%, respectively. We found that the pulsation period of both RRab and RRc variables steadily decreases when moving from the metal-poor to the metal-rich regime. The visual amplitude shows the same trend, but RRc amplitudes are almost two times more sensitive than RRab amplitudes to metallicity. We also investigated the dependence of the population ratio (N
c
/N
tot
) of field RRLs on the metallicity and we found that the distribution is more complex than in globular clusters. The population ratio steadily increases from ∼0.25 to ∼0.36 in the metal-poor regime, it decreases from ∼0.36 to ∼0.18 for −1.8 ≤ Fe/H ≤ −0.9 and it increases to a value of ∼0.3 approaching solar iron abundance.
Abstract
We performed the largest and most homogeneous spectroscopic survey of field RR Lyraes (RRLs). We secured ≈6300 high-resolution (HR,
R
∼ 35,000) spectra for 143 RRLs (111 fundamental, RRab; ...32 first-overtone, RRc). The atmospheric parameters were estimated by using the traditional approach and the iron abundances were measured by using an LTE line analysis. The resulting iron distribution shows a well-defined metal-rich tail approaching solar iron abundance. This suggests that field RRLs experienced a complex chemical enrichment in the early halo formation. We used these data to develop a new calibration of the Δ
S
method. This diagnostic, based on the equivalent widths of Ca
ii
K and three Balmer (H
δ
,
γ
,
β
) lines, traces the metallicity of RRLs. For the first time, the new empirical calibration: (i) includes spectra collected over the entire pulsation cycle; (ii) includes RRc variables; (iii) relies on spectroscopic calibrators covering more than three dex in iron abundance; and (iv) provides independent calibrations based on one/two/three Balmer lines. The new calibrations were applied to a data set of both SEGUE-SDSS and degraded HR spectra totalling 6451 low-resolution (
R
∼ 2000) spectra for 5001 RRLs (3439 RRab, 1562 RRc). This resulted in an iron distribution with a median
η
= −1.55 ± 0.01 and
σ
= 0.51 dex, in good agreement with literature values. We also found that RRc are 0.10 dex more metal-poor than RRab variables, and have a distribution with a smoother metal-poor tail. This finding supports theoretical prescriptions suggesting a steady decrease in the RRc number when moving from metal-poor to metal-rich stellar environments.
Abstract
We report the discovery of a new ultra-faint stellar system found near the Magellanic Clouds in the DECam Local Volume Exploration Survey. This new system, DELVE J0155−6815 (DELVE 2), is ...located at a heliocentric distance of
D
⊙
= 71 ± 4 kpc, which places it at a 3D physical separation of 12 ± 3 kpc from the center of the Small Magellanic Cloud and
from the center of the Large Magellanic Cloud (LMC). DELVE 2 is identified as a resolved overdensity of old (
τ
> 13.3 Gyr) and metal-poor (
dex) stars with a projected half-light radius of
and an absolute magnitude of
. The size and luminosity of DELVE 2 are consistent with both the population of recently discovered ultra-faint globular clusters and the smallest ultra-faint dwarf galaxies. However, its photometrically derived age and metallicity would place it among the oldest and most metal-poor globular clusters in the Magellanic system. In the absence of spectroscopic measurements of the system’s metallicity dispersion and internal kinematics, we are unable to conclusively classify this system at this time. DELVE 2 is detected in
Gaia
DR2 with a clear proper-motion signal, with multiple blue horizontal-branch stars near the centroid of the system with proper motions consistent with the systemic mean. We measure the system proper motion to be
=
mas yr
−1
. We compare the spatial position and proper motion of DELVE 2 with simulations of the accreted satellite population of the LMC and find that it is very likely to be associated with the LMC.
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