1,220 research outputs found
The distribution of old stars around the Milky Way's central black hole I: Star counts
(abridged) In this paper we revisit the problem of inferring the innermost
structure of the Milky Way's nuclear star cluster via star counts, to clarify
whether it displays a core or a cusp around the central black hole. Through
image stacking and improved PSF fitting we push the completeness limit about
one magnitude deeper than in previous, comparable work. Contrary to previous
work, we analyse the stellar density in well-defined magnitude ranges in order
to be able to constrain stellar masses and ages. The RC and brighter giant
stars display a core-like surface density profile within a projected radius
R<0.3 pc of the central black hole, in agreement with previous studies, but
show a cusp-like surface density distribution at larger R. The surface density
of the fainter stars can be described well by a single power-law at R<2 pc. The
cusp-like profile of the faint stars persists even if we take into account the
possible contamination of stars in this brightness range by young pre-main
sequence stars. The data are inconsistent with a core-profile for the faint
stars.Finally, we show that a 3D Nuker law provides a very good description of
the cluster structure. We conclude that the observed stellar density at the
Galactic Centre, as it can be inferred with current instruments, is consistent
with the existence of a stellar cusp around the Milky Way's central black hole,
Sgr A*. This cusp is well developed inside the influence radius of about 3 pc
of Sgr A* and can be described by a single three-dimensional power-law with an
exponent gamma=1.23+-0.05. The apparent lack of RC stars and brighter giants at
projected distances of R < 0.3 pc (R<8") of the massive black hole may indicate
that some mechanism has altered their distribution or intrinsic luminosity.Comment: Accepted for publication A&
The distribution of stars around the Milky Way's central black hole II: Diffuse light from sub-giants and dwarfs
This is the second of three papers that search for the predicted stellar cusp
around the Milky Way's central black hole, Sagittarius A*, with new data and
methods. We aim to infer the distribution of the faintest stellar population
currently accessible through observations around Sagittarius A*. We use
adaptive optics assisted high angular resolution images obtained with the NACO
instrument at the ESO VLT. Through optimised PSF fitting we remove the light
from all detected stars above a given magnitude limit. Subsequently we analyse
the remaining, diffuse light density. The analysed diffuse light arises from
sub-giant and main-sequence stars with KS ~ 19 - 20 with masses of 1 - 2 Msol .
These stars can be old enough to be dynamically relaxed. The observed power-law
profile and its slope are consistent with the existence of a relaxed stellar
cusp around the Milky Way's central black hole. We find that a Nuker law
provides an adequate description of the nuclear cluster's intrinsic shape
(assuming spherical symmetry). The 3D power-law slope near Sgr A* is \gamma =
1.23 +- 0.05. At a distance of 0.01 pc from the black hole, we estimate a
stellar mass density of 2.3 +- 0.3 x 10^7 Msol pc^-3 and a total enclosed
stellar mass of 180 +- 20 Msol. These estimates assume a constant mass-to-light
ratio and do not take stellar remnants into account. The fact that no cusp is
observed for bright (Ks 16) giant stars at projected distances of roughly
0.1-0.3 pc implies that some mechanism has altered their appearance or
distribution.Comment: Accepted for publication A&
Near-Infrared Variability Study of the Central 2.3 arcmin x 2.3 arcmin of the Galactic Centre I. Catalog of Variable Sources
We used four-year baseline HST/WFC3 IR observations of the Galactic Centre in
the F153M band (1.53 micron) to identify variable stars in the central
~2.3'x2.3' field. We classified 3845 long-term (periods from months to years)
and 76 short-term (periods of a few days or less) variables among a total
sample of 33070 stars. For 36 of the latter ones, we also derived their periods
(<3 days). Our catalog not only confirms bright long period variables and
massive eclipsing binaries identified in previous works, but also contains many
newly recognized dim variable stars. For example, we found \delta Scuti and RR
Lyrae stars towards the Galactic Centre for the first time, as well as one BL
Her star (period < 1.3 d). We cross-correlated our catalog with previous
spectroscopic studies and found that 319 variables have well-defined stellar
types, such as Wolf-Rayet, OB main sequence, supergiants and asymptotic giant
branch stars. We used colours and magnitudes to infer the probable variable
types for those stars without accurately measured periods or spectroscopic
information. We conclude that the majority of unclassified variables could
potentially be eclipsing/ellipsoidal binaries and Type II Cepheids. Our source
catalog will be valuable for future studies aimed at constraining the distance,
star formation history and massive binary fraction of the Milky Way nuclear
star cluster.Comment: has been accepted to be published in MNRAS, 64 pages, 26 figures. The
complete lists of table 3, 4, 8 and 9 will be published onlin
An Automatic Tree Search Algorithm for the Tisserand Graph
The Tisserand graph (TG) is a graphical tool commonly employed in the
preliminary design of gravity-assisted trajectories. The TG is a
two-dimensional map showing essential orbital information regarding the
Keplerian orbits resulting from the close passage by one or more massive
bodies, given the magnitude of the hyperbolic excess speed () and
the minimum allowed pericenter height for each passage. Contours of constant
populate the TG. Intersections between contours allow to link
consecutive flybys and build sequences of encounters en route to a selected
destination. When the number of perturbing bodies is large and many
levels are considered, the identification of all the possible
sequences of encounters through the visual inspection of the TG becomes a
laborious task. Besides, if the sequences are used as input for a numerical
code for trajectory design and optimization, an automated examination of the TG
is desirable. This contribution describes an automatic technique to explore the
TG and find all the encounter paths. The technique is based on a tree search
method, and the intersections between contours are found using the regula-falsi
scheme. The method is validated through comparisons with solutions available in
the open literature. Examples are given of application to interplanetary
mission scenarios, including the coupling with a trajectory optimizer
Lactobacillus plantarum CUL66 can impact cholesterol homeostasis in Caco-2 enterocytes
Hypercholesterolemia drives the development of cardiovascular disease, the leading cause of mortality in western society. Supplementation with probiotics that interfere with cholesterol metabolism may provide a contribution to disease prevention. Lactobacillus plantarum CUL66 (NCIMB 30280) has been assessed in vitro for its ability to impact cholesterol absorption. L. plantarum CUL66 tested positive for bile salt hydrolase activity and the ability to assimilate cholesterol from culture media. RT-qPCR analysis showed that the bacterium significantly decreased the expression of Niemann-Pick C1-like 1 and ATP-binding cassette transporter-1 in polarised Caco-2 cells after 6 h exposure. Conversely, the expression of ATP-binding cassette sub-family G member (ABCG)-5 and ABCG-8, and 3-hydroxy-3-methylglutaryl-CoA reductase were significantly increased. Using a radiolabelled assay, we also observed significant reductions in the uptake and basolateral efflux of cholesterol by Caco-2 cells exposed to L. plantarum CUL66. This in vitro study identified L. plantarum CUL66 as a cholesterol lowering bacteria by highlighting its ability to beneficially regulate multiple in vitro events associated with intestinal cholesterol metabolism and provides evidence of efficacy for its inclusion in future in vivo studies
Enhancing methane production from the invasive macroalga Rugulopteryx okamurae through anaerobic co-digestion with olive mill solid waste: process performance and kinetic analysis
The biomass valorisation of the invasive brown alga Rugulopteryx okamurae (Dictyotales, Phaeophyceae) is key to curbing the expansion of this invasive macroalga which is generating tonnes of biomass on southern Spain beaches. As a feasible alternative for the biomass management, anaerobic co-digestion is proposed in this study. Although the anaerobic digestion of macroalgae barely produced 177 mL of CH4 g−1 VS, the co-digestion with a C-rich substrate, such as the olive mill solid waste (OMSW, the main waste derived from the two-phase olive oil manufacturing process), improved the anaerobic digestion process. The mixture improved not only the methane yield, but also its biodegradability. The highest biodegradability was found in the mixture 1 R. okamurae—1 OMSW, which improved the biodegradability of the macroalgae by 12.9% and 38.1% for the OMSW. The highest methane yield was observed for the mixture 1 R. okamurae—3 OMSW, improving the methane production of macroalgae alone by 157% and the OMSW methane production by 8.6%. Two mathematical models were used to fit the experimental data of methane production time with the aim of assessing the processes and obtaining the kinetic constants of the anaerobic co-digestion of different combination of R. okamurae and OMSW and both substrates independently. First-order kinetic and the transference function models allowed for appropriately fitting the experimental results of methane production with digestion time. The specific rate constant, k (first-order model) for the mixture 1 R. okamurae- 1.5 OMSW, was 5.1 and 1.3 times higher than that obtained for the mono-digestion of single OMSW and the macroalga, respectively. In the same way, the transference function model revealed that the maximum methane production rate (Rmax) was also found for the mixture 1 R. okamurae—1.5 OMSW (30.4 mL CH4 g−1 VS day−1), which was 1.6 and 2.2 times higher than the corresponding to the mono-digestions of the single OMSW and sole R. okamurae (18.9 and 13.6 mL CH4 g−1 VS day−1), respectively.Ministerio de Ciencia e Innovación PID2020-114975RB-10
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