The S-star cluster in the Galactic center allows us to study the physics close to a supermassive black hole, including distinctive dynamical tests of general relativity. Our best estimates for the ...mass of and the distance to Sgr A* using the three stars with the shortest period (S2, S38, and S55/S0-102) and Newtonian models are MBH = (4.15 0.13 0.57) × 106 M and R0 = 8.19 0.11 0.34 kpc. Additionally, we aim at a new and practical method to investigate the relativistic orbits of stars in the gravitational field near Sgr A*. We use a first-order post-Newtonian approximation to calculate the stellar orbits with a broad range of periapse distance rp. We present a method that employs the changes in orbital elements derived from elliptical fits to different sections of the orbit. These changes are correlated with the relativistic parameter defined as rs/rp (with rs being the Schwarzschild radius) and can be used to derive from observational data. For S2 we find a value of = 0.00088 0.00080, which is consistent, within the uncertainty, with the expected value of = 0.00065 derived from MBH and the orbit of S2. We argue that the derived quantity is unlikely to be dominated by perturbing influences such as noise on the derived stellar positions, field rotation, and drifts in black hole mass.
Context.
The Milky Way nuclear star cluster (MWNSC) is a crucial laboratory for studying the galactic nuclei of other galaxies, but its properties have not been determined unambiguously until now.
...Aims.
We aim to study the size and spatial structure of the MWNSC.
Methods.
This study uses data and methods that address potential shortcomings of previous studies on the topic. We use 0.2″ angular resolution
K
s
data to create a stellar density map in the central 86.4 pc × 21 pc at the Galactic center. We include data from selected adaptive-optics-assisted images obtained for the inner parsecs. In addition, we use
Spitzer
/IRAC mid-infrared (MIR) images. We model the Galactic bulge and the nuclear stellar disk in order to subtract them from the MWNSC. Finally, we fit a Sérsic model to the MWNSC and investigate its symmetry.
Results.
Our results are consistent with previous work. The MWNSC is flattened with an axis ratio of
q
= 0.71 ± 0.10, an effective radius of
R
e
= (5.1 ± 1.0) pc, and a Sérsic index of
n
= 2.2 ± 0.7. Its major axis may be tilted out of the Galactic plane by up to −10°. The distribution of the giants brighter than the Red Clump (RC) is found to be significantly flatter than the distribution of the faint stars. We investigate the 3D structure of the central stellar cusp using our results on the MWNSC structure on large scales to constrain the deprojection of the measured stellar surface number density, obtaining a value of the 3D inner power law of
γ
= 1.38 ± 0.06
sys
± 0.01
stat
.
Conclusions.
The MWNSC shares its main properties with other extragalactic NSCs found in spiral galaxies. The differences in the structure between bright giants and RC stars might be related to the existence of not completely mixed populations of different ages. This may hint at recent growth of the MWNSC through star formation or cluster accretion.
We present a comprehensive data description for K sub(s)-band measurements of Sgr A*. We characterize the statistical properties of the variability of Sgr A* in the near-infrared, which we find to be ...consistent with a single-state process forming a power-law distribution of the flux density. We discover a linear rms-flux relation for the flux density range up to 12 mJy on a timescale of 24 minutes. This and the power-law flux density distribution implies a phenomenological, formally nonlinear statistical variability model with which we can simulate the observed variability and extrapolate its behavior to higher flux levels and longer timescales. We present reasons why data with our cadence cannot be used to decide on the question whether the power spectral density of the underlying random process shows more structure at timescales between 25 minutes and 100 minutes compared to what is expected from a red-noise random process.
Context. Due to the extreme extinction towards the Galactic centre (AV ∼ 30 mag), its stellar population is mainly studied in the near-infrared (NIR) regime. Therefore, a proper analysis of the NIR ...extinction curve is necessary to fully characterise the stellar structure and population of the inner part of the galaxy. Aims. We studied the dependence of the extinction index (αλ) in the NIR on the line of sight, wavelength, and extinction. Methods. We used the GALACTICNUCLEUS imaging survey, a high angular resolution catalogue (0.2″) for the inner part of the Galaxy in JHKs, and studied the spatial variation in the extinction index. We also applied two independent methods based on red clump stars to compute the extinction index between different bands and its variation with wavelength. Results. We did not detect any significant line-of-sight or extinction variation in α within the studied region in the nuclear stellar disc. The extinction index between JH and HKs differs by 0.19 ± 0.05. We obtained mean values for the extinction indices αJH = 2.43 ± 0.03 and αHKs = 2.23 ± 0.03. The dependence of the extinction index on the wavelength could explain the differences obtained for αλ in the literature since it was assumed constant for the NIR regime.
We present the results of a large-scale proper motion study of the central ∼36′ × 16′ of the Milky Way, based on our high angular resolution GALACTICNUCLEUS survey (epoch 2015) combined with the HST ...Paschen-
α
survey (epoch 2008). Our catalogue contains roughly 80 000 stars, an unprecedented kinematic dataset for this region. We describe the data analysis and the preparation of the proper motion catalogue. We verify the catalogue by comparing our results with measurements from previous work and data. We provide a preliminary analysis of the kinematics of the studied region. Foreground stars in the Galactic disc can be easily identified via their low reddening. Consistent with previous work and with our expectations, we find that stars in the nuclear stellar disc have a smaller velocity dispersion than inner bulge stars, in particular in the direction perpendicular to the Galactic plane. The rotation of the nuclear stellar disc can be clearly seen in the proper motions parallel to the Galactic plane. Stars on the near side of the nuclear stellar disc are less reddened than stars on its far side. Proper motions enable us to detect co-moving groups of stars that may be associated with young clusters dissolving in the galactic centre that are difficult to detect by other means. We demonstrate a technique based on a density clustering algorithm that can be used to find such groups of stars.
•Variable geometry ejector is assessed first time for small scale desalination unit.•CFD is used for detailed flow modelling and design optimisation.•Variable geometry ejector improves performance ...under variable operating conditions.•Performance of ejector was found highly sensitive to spindle geometry.•Benefit of applying variable geometry is up 400% in performance.
Nowadays, desalination is the primary alternative to increase water supply for domestic, agriculture and industrial use. Energy demanding desalination could be a sustainable solution in places where renewable energy resources are abundant. Multi-effect thermal vapour compression desalination is widely applied; however, its integration with solar energy requires more research effort. One of the key components of these systems is the steam ejector. The present study focuses on the geometry modification of an ejector, designed for a small-scale multi-effect thermal vapour compression unit, by means of computational fluid dynamics simulations. Its performance is evaluated considering a range of operating temperatures of the primary flow (i.e. 120 °C–180 °C) that would be suitable for running the system using low-grade solar thermal energy. Results show that for relatively high primary inlet temperatures, the primary jet undergoes a considerable expansion after leaving the nozzle. Consequently, the effective area for the secondary fluid becomes reduced, leading to a poor ejector performance and the entrainment ratio decreases from 1.2 to 0.3. In order to improve the ejector performance, variable geometry features are tested by allowing two geometry components to vary: the nozzle exit position and the area ratio (rA). The results show that nozzle exit position influences the ejector performance only about 16% within the considered range. The influence of the area ratio on the entrainment ratio is however very significant. The results indicate that the improvement of the entrainment ratio for high primary inlet temperatures could be as high as 400% when compared to a fixed geometry design.
Context. The compact radio and near-infrared (NIR) source Sagittarius A* (Sgr A*) associated with the supermassive black hole in the Galactic center was observed at 7 mm in the context of a NIR ...triggered global Very Long Baseline Array (VLBA) campaign. Aims. Sgr A* shows variable flux densities ranging from radio through X-rays. These variations sometimes appear in spontaneous outbursts that are referred to as flares. Multi-frequency observations of Sgr A* provide access to easily observable parameters that can test the currently accepted models that try to explain these intensity outbursts. Methods. On May 16–18, 2012 Sgr A* has been observed with the VLBA at 7 mm (43 GHz) for 6 h each day during a global multi-wavelength campaign. These observations were triggered by a NIR flare observed at the Very Large Telescope (VLT). Accurate flux densities and source morphologies were acquired. Results. The total 7 mm flux of Sgr A* shows only minor variations during its quiescent states on a daily basis of 0.06 Jy. An observed NIR flare on May 17 was followed ~4.5 h later by an increase in flux density of 0.22 Jy at 43 GHz. This agrees well with the expected time delay of events that are casually connected by adiabatic expansion. Shortly before the peak of the radio flare, Sgr A* developed a secondary radio off-core feature at 1.5 mas toward the southeast. Even though the closure phases are too noisy to place actual constraints on this feature, a component at this scale together with a time delay of 4.5 ± 0.5 h between the NIR and radio flare provide evidence for an adiabatically expanding jet feature.
Proper motion studies of stars in the centre of the Milky Way have typically been limited to the Arches and Quintuplet clusters, and to the central parsec. Here we present the first results of a ...large-scale proper motion study of stars within several tens of parsecs of Sagittarius A* based on our 0.2″ angular resolution GALACTICNUCLEUS survey (epoch 2015) combined with NICMOS/HST data from the Paschen-α survey (epoch 2008). This comprises the first extensive proper motion study of the central ∼36′×16′ of the Galaxy, which is not covered adequately by any of the existing astronomical surveys, such as Gaia, because of the extreme interstellar extinction (AV ≳ 30 mag). Proper motions can help us to disentangle the different stellar populations along the line-of-sight and interpret their properties in combination with multi-wavelength photometry from GALACTICNUCLEUS and other sources. It also allows us to infer the dynamics and interrelationships between different stellar components (Galactic bulge, nuclear stellar disk, nuclear stellar cluster) of the Galactic centre (GC). In particular, we use proper motions to detect co-moving groups of stars which are able to trace low-mass or partially-dissolved young clusters in the GC that can hardly be discovered by any other means. Our pilot study for this work is based on a field in the nuclear bulge associated with HII regions that show the presence of young stars. We have detected the first group of co-moving stars coincident with an H II region. Using colour–magnitude diagrams, we have inferred that the co-moving stars are consistent with the post-main sequence stars with ages of few Myr. Simulations show that this group of stars is a real group that can indicate the existence of a dissolving or low-to-intermediate-mass young cluster. A census of these undiscovered clusters will ultimately help us to constrain star formation at the GC in the past few ten Myr.
Context. There are a number of faint compact infrared excess sources in the central stellar cluster of the Milky Way. Their nature and origin is unclear. In addition to several isolated objects of ...this kind there is a small but dense cluster of comoving sources (IRS13N) located ~3′′ west of SgrA* just 0.5′′ north of the bright IRS13E cluster of Wolf-Rayet and O-type stars. Based on the analysis of their color and brightness, there are two main possibilities: (1) they may be dust-embedded stars older than a few Myr; or (2) very young, dusty stars with ages younger than 1 Myr. Aims. We present a first Ks-band identification and proper motions of the IRS13N members, the high-velocity dusty S-cluster object (DSO, also referred to as G2), and other infrared excess sources in the central field. Goal is to constrain the nature of these source. Methods. The L′- (3.8 μm) Ks- (2.2 μm) and H-band (1.65 μm) observations were carried out using the NACO adaptive optics system at the ESO VLT. Proper motions were obtained by linear fitting of the stellar positions extracted by StarFinder as a function of time, weighted by positional uncertainties, and by Gaussian fitting from high-pass filtered and deconvolved images. We also present results of near-infrared (NIR) H- and Ks-band ESO-SINFONI integral field spectroscopy of the Galactic center cluster ISR13N. Results. We show that within the uncertainties, the positions and proper motions of the IRS13N sources in Ks- and L′-band are identical. The HK−sL′ colors then indicate that the bright L′-band IRS13N sources are indeed dust-enshrouded stars rather than core-less dust clouds. The proper motions also show that the IRS13N sources are not strongly gravitationally bound to each other. Combined with their NIR colors, this implies that they have been formed recently. For the DSO we obtain proper motions and a Ks-L′-color. Conclusions. Most of the compact L′-band excess emission sources have a compact H- or Ks-band counterpart and therefore are likely stars with dust shells or disks. Our new results and orbital analysis from our previous work favor the hypothesis that the infrared excess IRS13N members and other dusty sources close to SgrA* are young dusty stars and that star formation at the Galactic center (GC) is a continuously ongoing process. For the DSO the color information indicates that it may be a dust cloud or a dust-embedded star.
We present high-angular-resolution radio observations of the Arches cluster in the Galactic centre, one of the most massive young clusters in the Milky Way. The data were acquired in two epochs and ...at 6 and 10 GHz with the
Karl G. Jansky
Very Large Array. The rms noise reached is three to four times better than during previous observations and we have almost doubled the number of known radio stars in the cluster. Nine of them have spectral indices consistent with thermal emission from ionised stellar winds, one is a confirmed colliding wind binary, and two sources are ambiguous cases. Regarding variability, the radio emission appears to be stable on timescales of a few to ten years. Finally, we show that the number of radio stars can be used as a tool for constraining the age and/or mass of a cluster and also its mass function.
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