Testing the validity of the ray-tracing code GYOTO

In the next few years, the near-infrared interferometer GRAVITY will be able to observe the Galactic center. Astrometric data will be obtained with an anticipated accuracy of 10 $\mu$as. To analyze these future data, we have developed a code called GYOTO to compute orbits and images. We want to assess the validity and accuracy of GYOTO in a variety of contexts, in particular for stellar astrometry in the Galactic center. Furthermore, we want to tackle and complete a study made on the astrometric displacements that are due to lensing effects of a star of the central parsec with GYOTO. We first validate GYOTO in the weak-deflection limit (WDL) by studying primary caustics and primary critical curves obtained for a Kerr black hole. We compare GYOTO results to available analytical approximations and estimate GYOTO errors using an intrinsic estimator. In the strong-deflection limit (SDL), we choose to compare null geodesics computed by GYOTO and the ray-tracing code named Geokerr. Finally, we use GYOTO to estimate the apparent displacements of a star for different angles from Sagittarius A* (Sgr A*). We have demonstrated that GYOTO is accurate to a very high level, orders of magnitude better than the GRAVITY requirements. GYOTO is also valid in weak- and strong-deflection regimes and for very long integrations. At the astrometric precision that GRAVITY is aiming for, lensing effects must always be taken into account when fitting stellar orbits in the central parsec of the Galaxy.Comment: 11 pages, 12 figure

Hot molecular hydrogen in the central parsec of the Galaxy through near-infrared 3D fitting

Aims. We have investigated neutral gas in the central cavity of the circumnuclear disk (CND) at the Galactic Center, where the ionized minispiral lies, to describe the H2 distribution and properties in this ionized environment. Methods. This study was carried out through a spectro-imaging data cube of the central cavity obtained with SPIFFI on the VLT. The observed field of view is 36"x 29" , with a spectral resolution R = 1 300 in the near-infrared. These observations cover several H2 lines. To preserve the spatial resolution and avoid edge effects, we applied a new line-fitting method that consists of a regularized 3D fitting. We also applied a more classical 1D fitting to compare the relative strength of the H2 lines. Results. We present high spatial and spectral resolution maps of the intensity, velocity, and width of five H2 lines and an extinction map derived from H2. Molecular gas is detected everywhere in the field. In particular, in addition to the known CND features, we detected an emission from the northern arm cloud and from the minicavity. The excitation diagrams allow us to estimate the temperature, mass, and density of these features. Conclusions. We interpret the CND emission as coming from a hot, thermalized, thin layer at the surface of the clouds. The observed H2 corresponds only to a small fraction of the total H2 mass. The emission remains fairly strong in the whole central cavity, but it is not thermalized. A strong deviation from thermal equilibrium is detected near the minicavity. We suggest that this emission is caused by constantly forming H2 that is destroyed again before it reaches ortho/para equilibrium

Clumpiness of the interstellar medium in the central parsec of the Galaxy from H2 flux extinction correlation

The central parsec of the Galaxy contains a young star cluster embedded in a complex interstellar medium. The latter mainly consists of a torus of dense clumps and streams of molecular gas (the circumnuclear disk, CND) enclosing streamers of ionized gas (the Minispiral). In this complex environment, knowledge of the local extinction that locally affects each feature is crucial to properly study and disentangle them. We previously studied molecular gas in this region and inferred an extinction map from two H2 lines. Extinction appears to be correlated with the dereddened flux in several contiguous areas in the field of view. Here, we discuss the origin of this local correlation. We model the observed effect with a simple radiative transfer model. H2 emission arises from the surfaces of clumps (i.e., shells) that are exposed to the ambient ultraviolet (UV) radiation field. We consider the shell at the surface of an emitting clump. The shell has a varying optical depth and a screen of dust in front of it. The optical depth varies from one line of sight to another, either because of varying extinction coefficient from the shell of one clump to that of another or because of a varying number of identical clumps on the line of sight. In both scenarios, the model accurately reproduces the dependence of molecular gas emission and extinction. The reason for this correlation is that, in the central parsec, the molecular gas is mixed everywhere with dust that locally affects the observed gas emission. In addition, there is extinction due to foreground (screen) dust. This analysis favors a scenario where the central parsec is filled with clumps of dust and molecular gas. Separating foreground from local extinction allows for a probe for local conditions (H2 is mixed with dust) and can also constrain the three-dimensional (3D) position of objects under study

New results on the Galactic Center Helium stars

The cluster of helium stars around Sgr A* has been re‐observed with the BEAR spectro‐imager on CFHT, in the 2.06 μm helium line, at a spectral resolution of 52 km s^(–1) and on a field of ≃40″. This new analysis confirms and completes a previous study at a spectral resolution of 74 km s^(–1) and on a smaller field of 24″ , corresponding to the central parsec (Paumard et al. 2001). Nineteen stars are confirmed as helium stars. These observations led to a clear differentiation between two groups of hot stars based on their emission linewidth, their magnitude and their positions relative to Sgr A*. The first class of 6 members is characterized by narrow‐line profiles (FWHM ≃200 km s^(–1)) and by their brightness. The other, fainter in K by an average of 2 mag, has a much broader emission component of width ≃1,000 km s^(–1). Several of the emission lines show a P Cygni profile. From these results, we propose that the narrow‐line group is formed of stars in the LBV phase, while the broad‐line group is formed of stars in or near the WR phase. The division into two groups is also shown by their spatial distribution, with the narrow‐line stars in a compact central cluster (IRS 16) and the other group distributed at the periphery of the central cluster of hot stars. HST‐NICMOS data in Paα (1.87 μm) of the same field reveal a similar association. The identification of the Paα counterpart to the He I stars provides an additional element to characterize the two groups. Bright Paα emitters are found generally associated with the narrow‐line class stars while the weak Paαemitters are generally associated with the broad‐line stars. A few particular cases are discussed. This confirms the different status of evolution of the two groups of massive, hot stars in the central cluster. As a by‐product, about 20 additional candidate emission stars are detected in the central, high‐resolution 19″ field from the NICMOS data

The Galactic Center Source IRS 13E: a Star Cluster

High spatial resolution, near‐infrared observations of the Galactic Center source, close to Sgr A*, known historically as IRS13, are presented. These observations include ground‐based adaptive optics images in the H, K' and L bands, HST‐NICMOS observations in filters between 1.1 and 2.2 μm, and spectroimaging data in the He I 2.06 μm line and the Brγ line. Analysis of all these data has made possible the resolution of the main component, IRS 13E, into a cluster of seven individual stars within a projected diameter of ∼0.5″ (0.02 pc), and to build their SED. The main sources, 13E1, 13E2, 13E3 (a binary), and 13E4, are hot stars of different nature. 13E2 and 13E4 are emission line stars. The spectral type of the various members goes from O5I to WR, including dusty WRs like IRS 21 (Tanner et al. 2002). All these sources have a common westward proper motion. Two weaker sources, 13E5 and 13E6, are also detected within the compact cluster, with 13E5 proposed as another dusty WR and 13E6 as a O5V star. An extended halo seen around the cluster, part of the mini‐spiral of dust is particularly enhanced in the L band. It is interpreted as a contribution of the scattered light from the inner cluster and the thermal emission from the dust. IRS 13E is proposed to be the remaining core of a massive, young star cluster which was disrupted in the vicinity of Sgr A*, and hence, the possible source of the young stars in the central parsec, from the helium stars to the S stars

Star formation in the central 0.5 pc of the Milky Way

The supermassive black hole candidate at the Galactic Center is surrounded by a parsec-scale star cluster, which contains a number of early type stars. The presence of such stars has been called a "paradox of youth" as star formation in the immediate vicinity of a supermassive black hole seemed difficult, as well as the transport of stars from far out in a massive-star lifetime. I will recall 30 years of technological developments which led to the current understanding of the nuclear cluster stellar population. The number of early type stars known at present is sufficient to access the 3D structure of this population and its dynamics, which in turn allows discriminating between the various possible origins proposed along the years.Comment: 8 pages, invited review for the conference "The Universe under the Microscope" (AHAR 2008), to be published in Journal of Physics: Conference Series by Institute of Physics Publishin

The GRAVITY instrument software / High-level software

GRAVITY is the four-beam, near- infrared, AO-assisted, fringe tracking, astrometric and imaging instrument for the Very Large Telescope Interferometer (VLTI). It is requiring the development of one of the most complex instrument software systems ever built for an ESO instrument. Apart from its many interfaces and interdependencies, one of the most challenging aspects is the overall performance and stability of this complex system. The three infrared detectors and the fast reflective memory network (RMN) recorder contribute a total data rate of up to 20 MiB/s accumulating to a maximum of 250 GiB of data per night. The detectors, the two instrument Local Control Units (LCUs) as well as the five LCUs running applications under TAC (Tools for Advanced Control) architecture, are interconnected with fast Ethernet, RMN fibers and dedicated fiber connections as well as signals for the time synchronization. Here we give a simplified overview of all subsystems of GRAVITY and their interfaces and discuss two examples of high-level applications during observations: the acquisition procedure and the gathering and merging of data to the final FITS file.Comment: 8 pages, 7 figures, published in Proc. SPIE 9146, Optical and Infrared Interferometry IV, 91462

Multiple star systems in the Orion nebula

This is the author accepted manuscript. The final fersion is available from EDP Sciences via the DOI in this record.This work presents an interferometric study of the massive-binary fraction in the Orion Trapezium cluster with the recently comissioned GRAVITY instrument. We observed a total of 16 stars of mainly OB spectral type. We find three previously unknown companions for θ1 Ori B, θ2 Ori B, and θ2 Ori C. We determined a separation for the previously suspected companion of NU Ori. We confirm four companions for θ1 Ori A, θ1 Ori C, θ1 Ori D, and θ2 Ori A, all with substantially improved astrometry and photometric mass estimates. We refined the orbit of the eccentric high-mass binary θ1 Ori C and we are able to derive a new orbit for θ1 Ori D. We find a system mass of 21.7 M⊙ and a period of 53 days. Together with other previously detected companions seen in spectroscopy or direct imaging, eleven of the 16 high-mass stars are multiple systems. We obtain a total number of 22 companions with separations up to 600 AU. The companion fraction of the early B and O stars in our sample is about two, significantly higher than in earlier studies of mostly OB associations. The separation distribution hints toward a bimodality. Such a bimodality has been previously found in A stars, but rarely in OB binaries, which up to this point have been assumed to be mostly compact with a tail of wider companions. We also do not find a substantial population of equal-mass binaries. The observed distribution of mass ratios declines steeply with mass, and like the direct star counts, indicates that our companions follow a standard power law initial mass function. Again, this is in contrast to earlier findings of flat mass ratio distributions in OB associations. We excluded collision as a dominant formation mechanism but find no clear preference for core accretion or competitive accretion.Marie Skłodowska-Curie Grant AgreementFCT-PortugalERC Starting Gran

The GRAVITY+ Project: Towards All-sky, Faint-Science, High-Contrast Near-Infrared Interferometry at the VLTI

The GRAVITY instrument has been revolutionary for near-infrared interferometry by pushing sensitivity and precision to previously unknown limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) in GRAVITY+, these limits will be pushed even further, with vastly improved sky coverage, as well as faint-science and high-contrast capabilities. This upgrade includes the implementation of wide-field off-axis fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser guide stars in an upgraded facility. GRAVITY+ will open up the sky to the measurement of black hole masses across cosmic time in hundreds of active galactic nuclei, use the faint stars in the Galactic centre to probe General Relativity, and enable the characterisation of dozens of young exoplanets to study their formation, bearing the promise of another scientific revolution to come at the VLTI.Comment: Published in the ESO Messenge