112 research outputs found
First evidence of multiple populations along the AGB from Str\"omgren photometry
Spectroscopic studies have demonstrated that nearly all Galactic globular
clusters (GCs) harbour multiple stellar populations with different chemical
compositions. Moreover, colour-magnitude diagrams based exclusively on
Str\"omgrem photometry have allowed us to identify and characterise multiple
populations along the RGB of a large number of clusters. In this paper we show
for the first time that Str\"omgren photometry is also very effcient at
identifying multiple populations along the AGB, and demonstrate that the AGB of
M3, M92, NGC362, NGC1851, and NGC6752 are not consistent with a single stellar
population. We also provide a catalogue of RGB and AGB stars photometrically
identified in these clusters for further spectroscopic follow-up studies.We
combined photometry and elemental abundances from the literature for RGB and
AGB stars in NGC6752 where the presence of multiple populations along the AGB
has been widely debated. We find that, while the MS, SGB, and RGB host three
stellar populations with different helium and light element abundances, only
two populations of AGB stars are present in the cluster. These results are
consistent with standard evolutionary theory.Comment: 9 pages, 3 figures, 1 table in the main article, 3 tables in the
appendix of which 2 tables containing coordinates and photometry of
photometrically identified RGB and AGB star
An automated search for transiting exocomets
This paper discusses an algorithm for detecting single transits in photometric time-series data. Specifically, we aim to identify asymmetric transits with ingress that is more rapid than egress, as expected for cometary bodies with a significant tail. The algorithm is automated, so can be applied to large samples and only a relatively small number of events need to be manually vetted. We applied this algorithm to all long cadence light curves from the Kepler mission, finding 16 candidate transits with significant asymmetry, 11 of which were found to be artefacts or symmetric transits after manual inspection. Of the 5 remaining events, four are the 0.1% depth events previously identified for KIC 3542116 and 11084727. We identify HD 182952 (KIC 8027456) as a third system showing a potential comet transit. All three stars showing these events have H-R diagram locations consistent with 100Myr-old open cluster stars, as might be expected given that cometary source regions deplete with age, and giving credence to the comet hypothesis. If these events are part of the same population of events as seen for KIC 8462852, the small increase in detections at 0.1% depth compared to 10% depth suggests that future work should consider whether the distribution is naturally flat, or if comets with symmetric transits in this depth range remain undiscovered. Future searches relying on asymmetry should be more successful if they focus on larger samples and young stars, rather than digging further into the noise
Assessing telluric correction methods for Na detections with high-resolution exoplanet transmission spectroscopy.
Using high-resolution ground-based transmission spectroscopy to probe
exoplanetary atmospheres is difficult due to the inherent telluric
contamination from absorption in Earth's atmosphere. A variety of methods have
previously been used to remove telluric features in the optical regime and
calculate the planetary transmission spectrum. In this paper we present and
compare two such methods, specifically focusing on Na detections using
high-resolution optical transmission spectra: (a) calculating the telluric
absorption empirically based on the airmass, and (b) using a model of the
Earth's transmission spectrum. We test these methods on the transmission
spectrum of the hot Jupiter HD 189733 b using archival data obtained with the
HARPS spectrograph during three transits. Using models for Centre-to-Limb
Variation and the Rossiter-McLaughlin effect, spurious signals which are
imprinted within the transmission spectrum are reduced. We find that correcting
tellurics with an atmospheric model of the Earth is more robust and produces
consistent results when applied to data from different nights with changing
atmospheric conditions. We confirm the detection of sodium in the atmosphere of
HD 189733 b, with doublet line contrasts of -0.64 0.07 % (D2) and -0.53
0.07 % (D1). The average line contrast corresponds to an effective
photosphere in the Na line located around 1.13 . We also confirm an
overall blueshift of the line centroids corresponding to net atmospheric
eastward winds with a speed of 1.8 1.2 km/s. Our study highlights the
importance of accurate telluric removal for consistent and reliable
characterisation of exoplanetary atmospheres using high-resolution transmission
spectroscopy
The fast transient sky with Gaia
The ESA Gaia satellite scans the whole sky with a temporal sampling ranging
from seconds and hours to months. Each time a source passes within the Gaia
field of view, it moves over 10 CCDs in 45 s and a lightcurve with 4.5 s
sampling (the crossing time per CCD) is registered. Given that the 4.5 s
sampling represents a virtually unexplored parameter space in optical time
domain astronomy, this data set potentially provides a unique opportunity to
open up the fast transient sky. We present a method to start mining the wealth
of information in the per CCD Gaia data. We perform extensive data filtering to
eliminate known on-board and data processing artefacts, and present a
statistical method to identify sources that show transient brightness
variations on ~2 hours timescales. We illustrate that by using the Gaia
photometric CCD measurements, we can detect transient brightness variations
down to an amplitude of 0.3 mag on timescales ranging from 15 seconds to
several hours. We search an area of ~23.5 square degrees on the sky, and find
four strong candidate fast transients. Two candidates are tentatively
classified as flares on M-dwarf stars, while one is probably a flare on a giant
star and one potentially a flare on a solar type star. These classifications
are based on archival data and the timescales involved. We argue that the
method presented here can be added to the existing Gaia Science Alerts
infrastructure for the near real-time public dissemination of fast transient
events.Comment: 10 pages, 5 figures and 5 tables; MNRAS in pres
NGTS-4b: A sub-Neptune transiting in the desert
We report the discovery of NGTS-4b, a sub-Neptune-sized planet transiting a 13th magnitude K-dwarf in a 1.34 d orbit. NGTS-4b has a mass M = 20.6 ± 3.0 M⊕ and radius R = 3.18 ± 0.26 R⊕, which places it well within the so-called ‘Neptunian Desert’. The mean density of the planet (3.45 ± 0.95 g cm−3) is consistent with a composition of 100  per cent H2O or a rocky core with a volatile envelope. NGTS-4b is likely to suffer significant mass loss due to relatively strong EUV/X-ray irradiation. Its survival in the Neptunian desert may be due to an unusually high-core mass, or it may have avoided the most intense X-ray irradiation by migrating after the initial activity of its host star had subsided. With a transit depth of 0.13 ± 0.02 per cent, NGTS-4b represents the shallowest transiting system ever discovered from the ground, and is the smallest planet discovered in a wide-field ground-based photometric survey
The Monitor project: the search for transits in the open cluster NGC 2362
We present the results of a systematic search for transiting planets in a ~5
Myr open cluster, NGC 2362. We observed ~1200 candidate cluster members, of
which ~475 are believed to be genuine cluster members, for a total of ~100
hours. We identify 15 light curves with reductions in flux that pass all our
detection criteria, and 6 of the candidates have occultation depths compatible
with a planetary companion. The variability in these six light curves would
require very large planets to reproduce the observed transit depth. If we
assume that none of our candidates are in fact planets then we can place upper
limits on the fraction of stars with hot Jupiters (HJs) in NGC 2362. We obtain
99% confidence upper limits of 0.22 and 0.70 on the fraction of stars with HJs
(f_p) for 1-3 and 3-10 day orbits, respectively, assuming all HJs have a
planetary radius of 1.5R_Jup. These upper limits represent observational
constraints on the number of stars with HJs at an age <~10 Myr, when the vast
majority of stars are thought to have lost their protoplanetary discs. Finally,
we extend our results to the entire Monitor Project, a survey searching young,
open clusters for planetary transits, and find that the survey as currently
designed should be capable of placing upper limits on f_p near the observed
values of f_p in the solar neighbourhood.Comment: 17 pages, 11 figures, accepted to MNRA
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