383 research outputs found
Photometry and spectroscopy of faint candidate spectrophotometric standard DA white dwarfs
We present precise photometry and spectroscopy for 23 candidate
spectrophotometric standard white dwarfs. The selected stars are distributed in
the Northern hemisphere and around the celestial equators and are all fainter
than r ~ 16.5 mag. This network of stars, when established as standards,
together with the three Hubble Space Telescope primary CALSPEC white dwarfs,
will provide a set of spectrophotometric standards to directly calibrate data
products to better than 1%. These new faint standard white dwarfs will have
enough signal-to-noise ratio in future deep photometric surveys and facilities
to be measured accurately while still avoiding saturation in such surveys. They
will also fall within the dynamic range of large telescopes and their
instruments for the foreseeable future. This paper discusses the provenance of
the observational data for our candidate standard stars. The comparison with
models, reconciliation with reddening, and the consequent derivation of the
full spectral energy density distributions for each of them is reserved for a
subsequent paper.Comment: 31 pages, 17 figures, 10 tables, ApJ in press (accepted on December
23rd, 2018
Broad Balmer Wings in BA Hyper/Supergiants Distorted by Diffuse Interstellar Bands: Five Examples in the 30 Doradus Region from the VLT-FLAMES Tarantula Survey
Extremely broad emission wings at Hβ and Hα have been found in VLT-FLAMES Tarantula Survey data for five very luminous BA supergiants in or near 30 Doradus in the Large Magellanic Cloud. The profiles of both lines are extremely asymmetrical, which we have found to be caused by very broad diffuse interstellar bands (DIBs) in the longward wing of Hβ and the shortward wing of Hα. These DIBs are well known to interstellar but not to many stellar specialists, so that the asymmetries may be mistaken for intrinsic features. The broad emission wings are generally ascribed to electron scattering, although we note difficulties for that interpretation in some objects. Such profiles are known in some Galactic hyper/supergiants and are also seen in both active and quiescent Luminous Blue Variables (LBVs). No prior or current LBV activity is known in these 30 Dor stars, although a generic relationship to LBVs is not excluded; subject to further observational and theoretical investigation, it is possible that these very luminous supergiants are approaching the LBV stage for the first time. Their locations in the HRD and presumed evolutionary tracks are consistent with that possibility. The available evidence for spectroscopic variations of these objects is reviewed, while recent photometric monitoring does not reveal variability. A search for circumstellar nebulae has been conducted, with an indeterminate result for one of them
The Resolved Stellar Populations in NGC 1705
We present HST photometry of the resolved stellar population in the dwarf
irregular galaxy NGC 1705. The galaxy has been observed with both WFPC2 and
NICMOS, and successful images have been obtained in the F555W, F814W, F110W and
F160W bands. The optical fields cover most of the galaxy, while the infrared
field (NIC2) maps only its central regions. The optical photometry provides
\~20000 objects down to m_F555W ~ 29 in the PC field of view and ~ 20000 in the
three WFCs. In the infrared we have been able to resolve ~ 2400 stars down to
m_{F110W}, m_{F160W} ~ 26. A subsample of 1834 stars have been unambiguously
measured in all the four bands. The corresponding color-magnitude diagrams
(CMDs) confirm the existence of an age gradient, showing that NGC 1705 hosts
both young (a few Myr old) and very old (up to 15 Gyr old) stars, with the
former strongly concentrated toward the galactic center and the latter present
everywhere, but much more easily visible in the external regions. The tip of
the red giant branch (TRGB) is clearly visible both in the optical and in the
infrared CMDs and allows us to derive the galaxy distance. Taking into account
the uncertainties related to both the photometry and the TRGB magnitude --
distance relation, we find that the distance modulus of NGC 1705 is
(m-M)_0=28.54 +- 0.26, corresponding to a distance D=5.1 +- 0.6 Mpc.Comment: aastex, 37 pages, 13 (reduced resolution) .ps figures. Accepted by
AJ. Full resolution figures are available upon request to the first autho
LEGUS Discovery of a Light Echo Around Supernova 2012aw
We have discovered a luminous light echo around the normal Type II-Plateau
Supernova (SN) 2012aw in Messier 95 (M95; NGC 3351), detected in images
obtained approximately two years after explosion with the Wide Field Channel 3
on-board the Hubble Space Telescope (HST) by the Legacy ExtraGalactic
Ultraviolet Survey (LEGUS). The multi-band observations span from the
near-ultraviolet through the optical (F275W, F336W, F438W, F555W, and F814W).
The apparent brightness of the echo at the time was ~21--22 mag in all of these
bands. The echo appears circular, although less obviously as a ring, with an
inhomogeneous surface brightness, in particular, a prominent enhanced
brightness to the southeast. The SN itself was still detectable, particularly
in the redder bands. We are able to model the light echo as the time-integrated
SN light scattered off of diffuse interstellar dust in the SN environment. We
have assumed that this dust is analogous to that in the Milky Way with R_V=3.1.
The SN light curves that we consider also include models of the unobserved
early burst of light from the SN shock breakout. Our analysis of the echo
suggests that the distance from the SN to the scattering dust elements along
the echo is ~45 pc. The implied visual extinction for the echo-producing dust
is consistent with estimates made previously from the SN itself. Finally, our
estimate of the SN brightness in F814W is fainter than that measured for the
red supergiant star at the precise SN location in pre-SN images, possibly
indicating that the star has vanished and confirming it as the likely SN
progenitor.Comment: 10 pages, 9 figures, to appear in the Astrophysical Journa
On the stellar content of the starburst galaxy IC10
We investigate the stellar content of the starburst dwarf galaxy IC10 using
accurate and deep optical data collected with the Advanced Camera for Surveys
and with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope.
The comparison between theory and observations indicates a clear change in age
distribution when moving from the center toward the external regions. Moreover,
empirical calibrators and evolutionary predictions suggest the presence of a
spread in heavy element abundance of the order of one-half dex. The comparison
between old and intermediate-age core He-burning models with a well defined
overdensity in the color-magnitude diagram indicates the presence of both
intermediate-age, red clump stars and of old, red horizontal branch stars.Comment: 7 pages, 4 figures, 1 table. Accepted for publication in ApJ
Uniform detection of the pre-main sequence population in the 5 embedded clusters related to the H\,II region NGC\,2174 (Sh2-252)
We investigate 5 embedded clusters (ECs) and the extended stellar group
itself of the prominent H\,II region NGC\,2174 (Sh2-252), which presents scarce
and heterogeneous information, coming from the optical and infrared.
Considering the discrepant values of distance and age, the clusters and the
H\,II region appear to be physically unrelated. The analysis is based on
field-star decontaminated 2MASS photometry, which allows sampling the pre-main
sequence (PMS). We find that Sh2-252A, C, E, NGC\,2175s, and Teu\,136 are small
ECs (radius within \,pc) characterised by a similar age
(\,Myr), reddening (\aV\sim1), distance from the Sun
(\ds\sim1.4\,kpc), and low mass (60-200\,\ms). This age is consistent with
the H\,II region, the presence of O and B stars still in the MS, and the
dominance (\ga95% in number) of PMS stars in colour-magnitude diagrams
(CMDs). NGC\,2175 is not a star cluster, but an extended stellar group that
encompasses the ECs Sh2-252\,A and C. It contains of the member stars
(essentially PMS) in the area, with the remaining belonging to the 2 ECs. CMDs
of the overall star-forming region and the ECs provide \ds=1.4\pm0.4\,kpc for
the NGC\,2174 complex, consistent with the value estimated for the
physically-related association Gem\,OB1. Our uniform approach shows that
NGC\,2174 and its related ECs (except, perhaps, for Teu\,136) are part of a
single star-forming complex. CMD similarities among the ECs and the overall
region suggest a coeval (to within \,Myr) star-forming event extending
for several Myr. At least 4 ECs originated in the event, together with the
off-cluster star formation that probably gave rise to the scattered stars of
NGC\,2175.Comment: Accepted by MNRA
Star Formation Histories of the LEGUS dwarf galaxies. II. Spatially resolved star formation history of the Magellanic irregular NGC 4449
We present a detailed study of the Magellanic irregular galaxy NGC 4449 based
on both archival and new photometric data from the Legacy Extragalactic UV
Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys
and Wide Field Camera 3. Thanks to its proximity ( Mpc) we
reach stars 3 magnitudes fainter than the tip of the red giant branch in the
F814W filter. The recovered star formation history spans the whole Hubble time,
but due to the age-metallicity degeneracy of the red giant branch stars, it is
robust only over the lookback time reached by our photometry, i.e.
Gyr. The most recent peak of star formation is around 10 Myr ago. The average
surface density star formation rate over the whole galaxy lifetime is
M yr kpc. From our study it emerges that NGC 4449 has
experienced a fairly continuous star formation regime in the last 1 Gyr with
peaks and dips whose star formation rates differ only by a factor of a few. The
very complex and disturbed morphology of NGC 4449 makes it an interesting
galaxy for studies of the relationship between interactions and starbursts, and
our detailed and spatially resolved analysis of its star formation history does
indeed provide some hints on the connection between these two phenomena in this
peculiar dwarf galaxy.Comment: 16 pages, 25 figures. Accepted for publication in the Astrophysical
Journa
BS196: an old star cluster far from the SMC main body
We present B and V photometry of the outlying SMC star cluster BS196 with the
4.1-m SOAR telescope. The photometry is deep (to V~25) showing ~3 mag below the
cluster turnoff point (TO) at Mv=2.5 (1.03 Msun). The cluster is located at the
SMC distance. The CMD and isochrone fittings provide a cluster age of 5.0+-0.5
Gyr, indicating that this is one of the 12 oldest clusters so far detected in
the SMC. The estimated metallicity is [Fe/H]=-1.68+-0.10. The structural
analysis gives by means of King profile fittings a core radius Rc=8.7+-1.1
arcsec (2.66+-0.14 pc) and a tidal radius Rt=69.4+-1.7 arcsec (21.2+-1.2 pc).
BS196 is rather loose with a concentration parameter c=0.90. With
Mv=-1.89+-0.39, BS196 belongs to the class of intrinsically fainter SMC
clusters, as compared to the well-known populous ones, which starts to be
explored.Comment: 8 pages, 10 figures; accepted by MNRA
HST astrometry in the 30 Doradus region: II. Runaway stars from new proper motions in the Large Magellanic Cloud
We present a catalog of relative proper motions for 368,787 stars in the 30
Doradus region of the Large Magellanic Cloud (LMC), based on a dedicated
two-epoch survey with the Hubble Space Telescope (HST) and supplemented with
proper motions from our pilot archival study. We demonstrate that a relatively
short epoch difference of 3 years is sufficient to reach a 0.1 mas
yr level of precision or better. A number of stars have relative proper
motions exceeding a 3-sigma error threshold, representing a mixture of Milky
Way denizens and 17 potential LMC runaway stars. Based upon 183 VFTS OB-stars
with the best proper motions, we conclude that none of them move faster than
0.3 mas yr in each coordinate -- equivalent to 70 km
s. Among the remaining 351 VFTS stars with less accurate proper motions,
only one candidate OB runaway can be identified. We rule out any OB star in our
sample moving at a tangential velocity exceeding 120 km s. The
most significant result of this study is finding 10 stars over wide range of
masses, which appear to be ejected from the massive star cluster R136 in the
tangential plane to angular distances from out to
, equivalent to 8-98 pc. The tangential velocities of these
runaways appear to be correlated with apparent magnitude, indicating a possible
dependence on the stellar mass.Comment: 45 pages (in referee format), 12 figures, 3 tables. Submitted to AJ.
Comments are welcom
The identification of the optical companion to the binary millisecond pulsar J0610-2100 in the Galactic field
We have used deep V and R images acquired at the ESO Very Large Telescope to
identify the optical companion to the binary pulsar PSR J0610-2100, one of the
black-widow millisecond pulsars recently detected by the Fermi Gamma-ray
Telescope in the Galactic plane. We found a faint star (V~26.7) nearly
coincident (\delta r ~0".28) with the pulsar nominal position. This star is
visible only in half of the available images, while it disappears in the
deepest ones (those acquired under the best seeing conditions), thus indicating
that it is variable. Although our observations do not sample the entire orbital
period (P=0.28 d) of the pulsar, we found that the optical modulation of the
variable star nicely correlates with the pulsar orbital period and describes a
well defined peak (R~25.6) at \Phi=0.75, suggesting a modulation due to the
pulsar heating. We tentatively conclude that the companion to PSR J0610-2100 is
a heavily ablated very low mass star (~ 0.02Msun) that completely filled its
Roche Lobe.Comment: 17 pages, 5 figures - Accepted for pubblication in Ap
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