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1. HST/ACS observations of NGC 346
ID:
ivo://CDS.VizieR/J/ApJS/166/549
Title:
HST/ACS observations of NGC 346
Short Name:
J/ApJS/166/549
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a photometric study of the star-forming region NGC 346 and its surrounding field in the Small Magellanic Cloud, using data taken with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). The data set contains both short and long exposures for increased dynamic range, and photometry was performed using the ACS module of the stellar photometry package DOLPHOT. We detected almost 100,000 stars over a magnitude range of V~11 to V~28 mag, including all stellar types from the most massive young stars to faint lower main-sequence and pre-main-sequence stars. We find that this region, which is characterized by a plethora of stellar systems and interesting objects, is an outstanding example of mixed stellar populations. We take into account different features of the color-magnitude diagram of all the detected stars to distinguish the two dominant stellar systems: the stellar association NGC 346 and the old spherical star cluster BS 90. These observations provide a complete stellar sample of a field about 5'x5' around the most active star-forming region in this galaxy. Considering the importance of these data for various investigations in the area, we provide the full stellar catalog from our photometry. This paper is the first part of an ongoing study to investigate in detail the two dominant stellar systems in the area and their surrounding field.
2. HST color-magnitude diagrams of the LMC
ID:
ivo://CDS.VizieR/J/AJ/117/2244
Title:
HST color-magnitude diagrams of the LMC
Short Name:
J/AJ/117/2244
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present results on the analysis of background field stars found in Hubble Space Telescope Wide Field Planetary Camera 2 observations of six of the old globular clusters of the Large Magellanic Cloud. Treated as contaminants by the globular cluster analysts, we produce (V-I, V) color-magnitude diagrams (CMDs) of the field stars and use them to explore the LMC's star formation history. The photometry approaches V {~} 26, well below the turnoff of an ancient ({~} 14 Gyr) LMC population of stars. The field star CMDs are generally characterized by an upper main sequence broadened by stellar evolution, an old red giant branch, a prominent red clump, and an unevolved lower main sequence. The CMDs also contain a few visual differences, the most obvious of which is the smeared appearance of the NGC 1916 field caused by heavy differential reddening. More subtly, the base of the subgiant branch near the old turnoff appears extended in V, and the red giant branch appears broad in V-I in four of the fields but not in the NGC 1754 field.
3. HST observations of old clusters in the LMC
ID:
ivo://CDS.VizieR/J/ApJ/527/199
Title:
HST observations of old clusters in the LMC
Short Name:
J/ApJ/527/199
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present V, V-I color-magnitude diagrams (CMDs) for three old star clusters in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257, and Hodge 11. Our data extend {~}3 mag below the main-sequence turnoff, allowing us to determine accurate relative ages and the blue straggler frequencies. Based on a differential comparison of the CMDs, any age difference between the three LMC clusters is less than 1.5 Gyr. Comparing their CMDs to those of M92 and M3, the LMC clusters, unless their published metallicities are significantly in error, are the same age as the old Galactic globulars. The similar ages to Galactic globulars are shown to be consistent with hierarchical clustering models of galaxy formation. The blue straggler frequencies are also similar to those of Galactic globular clusters. We derive a true distance modulus to the LMC of (m - M)_0_ = 18.46 {+/-} 0.09 [assuming (m - M)_0_ = 14.61 for M92] using these three LMC clusters.
4. HST Observations of SMC N88A
ID:
ivo://CDS.VizieR/J/A+A/347/841
Title:
HST Observations of SMC N88A
Short Name:
J/A+A/347/841
Date:
21 Oct 2021
Publisher:
CDS
Description:
High-resolution Hubble Space Telescope images have allowed us for the first time to resolve the compact SMC ionized ``blob'' N88A (diameter ~3.5arcsec or 1pc). This very young HII, region, which is hatching from its natal molecular cloud, is heavily affected by absorbing dust associated with the cloud. The interstellar reddening towards N88A is on average A_v_~1.5mag and strikingly rises to more than 3.5mag in a narrow dust band crossing the core of the HII region. Such a high extinction is unprecedented for an HII region in the metal-poor SMC. We present the photometry of some 70 stars lying towards the OB association at the center of which lies N88A. The exciting star(s) of N88A is not detected, due to the heavy extinction. The chronology of star formation is discussed for the whole region.
5. HST VI Photometry of Six LMC Old Globular Clusters
ID:
ivo://CDS.VizieR/J/MNRAS/300/665
Title:
HST VI Photometry of Six LMC Old Globular Clusters
Short Name:
J/MNRAS/300/665
Date:
21 Oct 2021
Publisher:
CDS
Description:
The following tables contain the results of photometry performed on Hubble Space Telescope WFPC2 images of the Large Magellanic Cloud globular clusters NGC 1754, 1835, 1898, 1916, 2005, and 2019. The magnitudes reported here were measured from Planetary Camera F555W and F814W images using DoPHOT (Schechter, Mateo, & Saha 1993) and afterwards transformed to Johnson V/Kron-Cousins I using equation 9 of Holtzman et al. (1995PASP..107.1065H). We carried out photometry on both long (1500 sec combined in F555W, 1800 sec in F814W) and short (40 sec combined in F555W, 60 sec in F814W) exposures. Where the short exposure photometry produced smaller errors, we report those magnitudes in place of those measured from the long exposures. For each star, we give an integer identifier, its x and y pixel position as measured in the F555W PC image, its V and I magnitude, the photometric errors reported by DoPHOT, both the V and I DoPHOT object types (multiplied by 10 if the reported magnitude was measured in the short exposure frame), and a flag if the star was removed during our procedure for statistical field star subtraction. Summary of data reduction and assessment of photometric accuracy: Cosmic ray rejection, correction for the y-dependent CTE effect (Holtzman et al. 1995a), geometric distortion correction, and bad pixel flagging were applied to the images before performing photometry. For the photometry, we used version 2.5 of DoPHOT, modified by Eric Deutsch to handle floating-point images. We found that there were insufficient numbers of bright, isolated stars in the PC frames for producing aperture corrections. Aperture corrections as a function of position in the frame were instead derived using WFPC2 point spread functions kindly provided by Peter Stetson. As these artificially generated aperture corrections agree well with ones derived from isolated stars in the WF chips, we trust that they are reliable to better than 0.05 mag. In agreement with the report of Whitmore & Heyer (1997), we found an offset in mean magnitudes between the short- and long-exposure photometry. We corrected for this effect by adjusting the short-exposure magnitudes to match, on average, those of the long exposures. Finally, we merged the short- and long- exposure lists of photometry as described above and transformed the magnitudes from the WFPC2 system to Johnson V/Kron-Cousins I, applying the Holtzman et al. (1995PASP..107.1065H) zero points. Statistical field star subtraction was performed using color-magnitude diagrams of the field stars produced from the combined WF frames. Completeness and random and systematic errors in the photometry were extensively modelled through artificial star tests. Crowding causes the completeness to be a strong function of position in the frame, with detection being most difficult near the cluster centers. In addition, we found that crowding introduces systematic errors in the photometry, generally <0.05 mag, that depend on the V-I and V of the star. Fortunately, these errors are well-understood. However, unknown errors in the zero points may persist at the ~0.05 mag level.
6. LMC far-ultraviolet imaging
ID:
ivo://CDS.VizieR/J/AJ/117/206
Title:
LMC far-ultraviolet imaging
Short Name:
J/AJ/117/206
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present an analysis of the deepest pure UV observations with the highest angular resolution ever performed, a set of 12 exposures with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 and F160BW filter obtained in parallel observing mode, which covers ~12arcmin^2^ in the Large Magellanic Cloud (LMC), north of the bar, and in the "general field" regime of the LMC. The 341 independent measurements of 198 objects represent an accumulated exposure of >=2x10^4^s and reveal stars as faint as m_UV_~=22mag. The observations show that about two-thirds of the UV emission from the LMC is emitted by our HST-detected UV stars in the field, that is, not in clusters or associations. We identified optical counterparts in the Royal Observatory Edinburgh/Naval Research Laboratory photometric catalog for about one-third of the objects.
7. Old Stellar Populations of the SMC
ID:
ivo://CDS.VizieR/J/ApJ/562/303
Title:
Old Stellar Populations of the SMC
Short Name:
J/ApJ/562/303
Date:
21 Oct 2021
Publisher:
CDS
Description:
Photometry is given for six data sets of SMC photometry, all in the V and I bands. Field names in the file summary below are the names used in the STScI archive for the WFPC2 observations. WFPC2 observations of NGC 121 and the SMC fields reach limiting magnitudes of V=26 and I=25.5; ground-based observations cover a square region of 14.5 arcmin on a side and reach V=25.5 and I=24.5.
8. VI photometry of NGC 2157
ID:
ivo://CDS.VizieR/J/AJ/115/592
Title:
VI photometry of NGC 2157
Short Name:
J/AJ/115/592
Date:
21 Oct 2021
Publisher:
CDS
Description:
We have carried out Wide Field Planetary Camera 2 V- and I-band imaging of the young LMC cluster NGC 2157. Construction of a color-magnitude diagram and isochrone fitting yield an age of {tau}=10^8^yr, a reddening E(B-V)=0.1, and a distance modulus of 18.4mag. Our data cover the mass range 0.75M_{sun}_<=m<=5.1M_{sun}_. We find that the cluster mass function changes significantly from the inner regions to the outer regions, becoming steeper (larger number of low-mass stars relative to high-mass stars) at larger radii. The age of NGC 2157 is comparable to its two-body relaxation timescale only in the cluster core. The observed steepening of the mass function at larger radii is therefore most likely an initial condition of the cluster stars. Such initial conditions are predicted in models of cluster star formation in which dissipative processes act more strongly upon more massive stars.
9. WFPC 2 imaging of young LMC clusters
ID:
ivo://CDS.VizieR/J/AJ/119/1748
Title:
WFPC 2 imaging of young LMC clusters
Short Name:
J/AJ/119/1748
Date:
21 Oct 2021
Publisher:
CDS
Description:
We have carried out Wide Field Planetary Camera 2 (WFPC2) F160BW, F555W, and F656N imaging of four young populous clusters: NGC 330, in the Small Magellanic Cloud, and NGC 1818, 2004, and 2100, in the Large Magellanic Cloud. We report photometric results for these four clusters, including identification using photometric colors of the cluster Be star population. We present theoretical WFPC2 and broadband colors and bolometric corrections for LMC and SMC metallicities. The use of the far-UV F160BW filter enables accurate determination of the effective temperatures for stars in the vicinity of the main-sequence turnoff and on the unevolved main sequence.