arXiv subjects

T. Shimizu

Publications and source records attributed to T. Shimizu.

At least 73 records · Page 4Linked to original sources

arXiv2022

GRAVITY+ Wide: Towards hundreds of z $\sim$ 2 AGN

As part of the GRAVITY$^{+}$ project, the near-infrared beam combiner GRAVITY and the VLTI are currently undergoing a series of significant upgrades to further improve the performance and sky coverage. The instrumental changes will be transformational, and for instance uniquely position GRAVITY to observe the broad line region of hundreds of Active Galactic Nuclei (AGN) at a redshift of two and higher. The increased sky coverage is achieved by enlarging the maximum angular separation between the celestial science object (SC) and the off-axis fringe tracking (FT) star from currently 2 arcseconds (arcsec) up to unprecedented 30 arcsec, limited by the atmospheric conditions. This was successfully demonstrated at the VLTI for the first time.

astro-ph.IM↗

arXiv2022

GRAVITY faint: reducing noise sources in GRAVITY$^+$ with a fast metrology attenuation system

With the upgrade from GRAVITY to GRAVITY$^+$ the instrument will evolve into an all-sky interferometer that can observe faint targets, such as high redshift AGN. Observing the faintest targets requires reducing the noise sources in GRAVITY as much as possible. The dominant noise source, especially in the blue part of the spectrum, is the backscattering of the metrology laser light onto the detector. To reduce this noise we introduce two new metrology modes. With a combination of small hardware changes and software adaptations, we can dim the metrology laser during the observation without losing the phase referencing. For single beam targets, we can even turn off the metrology laser for the maximum SNR on the detector. These changes lead to an SNR improvement of over a factor of two averaged over the whole spectrum and up to a factor of eight in the part of the spectrum currently dominated by laser noise.

astro-ph.IM↗

arXiv2022

An Analytical Latency Model and Evaluation of the Capacity of 5G NR to Support V2X Services using V2N2V Communications

5G has been designed to support applications such as connected and automated driving. To this aim, 5G includes a highly flexible New Radio (NR) interface that can be configured to utilize different subcarrier spacings (SCS), slot durations, scheduling, and retransmissions mechanisms. This flexibility can be exploited to support advanced V2X services with strict latency and reliability requirements using V2N2V (Vehicle-to-Network-to-Vehicles) communications instead of direct or sidelink V2V (Vehicle-to-Vehicle). To analyze this possibility, this paper presents a novel analytical model that estimates the latency of 5G at the radio network level. The model accounts for the use of different numerologies (SCS, slot durations and Cyclic Prefixes), modulation and coding schemes, full-slots or mini-slots, semi-static and dynamic scheduling, different retransmission mechanisms, and broadcast/multicast or unicast transmissions. The model has been used to first analyze the impact of different 5G NR radio configurations on the latency. We then identify which radio configurations and scenarios can 5G NR satisfy the latency and reliability requirements of V2X services using V2N2V communications. This paper considers cooperative lane changes as a case study. The results show that 5G can support advanced V2X services at the radio network level using V2N2V communications under certain conditions that depend on the radio configuration, bandwidth, service requirements and cell traffic load.

cs.NI↗

arXiv2022

First Light for GRAVITY Wide: Large Separation Fringe Tracking for the Very Large Telescope Interferometer

GRAVITY+ is the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) with wide-separation fringe tracking, new adaptive optics, and laser guide stars on all four 8~m Unit Telescopes (UTs), for ever fainter, all-sky, high contrast, milliarcsecond interferometry. Here we present the design and first results of the first phase of GRAVITY+, called GRAVITY Wide. GRAVITY Wide combines the dual-beam capabilities of the VLTI and the GRAVITY instrument to increase the maximum separation between the science target and the reference star from 2 arcseconds with the 8 m UTs up to several 10 arcseconds, limited only by the Earth's turbulent atmosphere. This increases the sky-coverage of GRAVITY by two orders of magnitude, opening up milliarcsecond resolution observations of faint objects, and in particular the extragalactic sky. The first observations in 2019 - 2022 include first infrared interferometry of two redshift $z\sim2$ quasars, interferometric imaging on the binary system HD 105913A, and repeated observations of multiple star systems in the Orion Trapezium Cluster. We find the coherence loss between the science object and fringe-tracking reference star well described by the turbulence of the Earth's atmosphere. We confirm that the larger apertures of the UTs result in higher visibilities for a given separation due to larger overlap of the projected pupils on sky and give predictions for visibility loss as a function of separation to be used for future planning.

astro-ph.IM↗

arXiv2022

Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg II h & k Lines

The CLASP2 (Chromospheric LAyer SpectroPolarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg II h & k spectral region around 2800 Angstroms along a 200 arcsecond slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg II h & k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semi-empirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry.

astro-ph.SR↗

arXiv2022

Kiloparsec view of a typical star-forming galaxy when the Universe was $\sim$1 Gyr old II. Regular rotating disk and evidence for baryon dominance on galactic scales

We present a kinematic analysis of the main-sequence galaxy HZ4 at $z=5.5$. Our study is based on deep, spatially resolved observations of the [CII] 158 $μ$m transition obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA). From the combined analysis of the disk morphology, the two-dimensional velocity structure, and forward-modeling of the one-dimensional velocity and velocity dispersion profiles, we conclude that HZ4 has a regular rotating disk in place. The intrinsic velocity dispersion in HZ4 is high ($σ_{0}=65.8^{+2.9}_{-3.3}$ km s$^{-1}$), and the ratio between the rotational velocity and the intrinsic velocity dispersion is $V_{\rm rot}/σ_{0}=2.2$. These values are consistent with the expectations from the trends of increasing $σ_{0}$ and decreasing $V_{\rm rot}/σ_{0}$ as a function of redshift observed in main-sequence galaxies up to $z\approx4$. Galaxy evolution models suggest that the high level of turbulence observed in HZ4 can only be achieved if, in addition to stellar feedback, there is radial transport of gas within the disk. Finally, we find that HZ4 is baryon dominated on galactic scales ($\lesssim2\times R_{\rm e}$), with a dark matter fraction at one effective radius of $f_{\rm DM}(R_{\rm e})=0.41^{+0.25}_{-0.22}$. This value is comparable to the dark matter fractions found in lower redshift galaxies that could be the descendants of HZ4: massive ($M_{\star}\approx10^{11}~M_{\odot}$), star-forming galaxies at $z\sim2$, and passive, early type galaxies at $z\approx0$.

astro-ph.GA↗

arXiv2021

Deep Images of the Galactic Center with GRAVITY

Stellar orbits at the Galactic Center provide a very clean probe of the gravitational potential of the supermassive black hole. They can be studied with unique precision, beyond the confusion limit of a single telescope, with the near-infrared interferometer GRAVITY. Imaging is essential to search the field for faint, unknown stars on short orbits which potentially could constrain the black hole spin. Furthermore, it provides the starting point for astrometric fitting to derive highly accurate stellar positions. Here, we present $\mathrm{G^R}$, a new imaging tool specifically designed for Galactic Center observations with GRAVITY. The algorithm is based on a Bayesian interpretation of the imaging problem, formulated in the framework of information field theory and building upon existing works in radio-interferometric imaging. Its application to GRAVITY observations from 2021 yields the deepest images to date of the Galactic Center on scales of a few milliarcseconds. The images reveal the complicated source structure within the central $100\,\mathrm{mas}$ around Sgr A*, where we detected the stars S29 and S55 and confirm S62 on its trajectory, slowly approaching Sgr A*. Furthermore, we were able to detect S38, S42, S60, and S63 in a series of exposures for which we offset the fiber from Sgr A*. We provide an update on the orbits of all aforementioned stars. In addition to these known sources, the images also reveal a faint star moving to the west at a high angular velocity. We cannot find any coincidence with any known source and, thus, we refer to the new star as S300. From the flux ratio with S29, we estimate its K-band magnitude as $m_\mathrm{K}\left(\mathrm{S300}\right)\simeq 19.0 - 19.3$. Images obtained with CLEAN confirm the detection.

astro-ph.GA↗

arXiv2021

The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits

The stars orbiting the compact radio source Sgr A* in the Galactic Centre are precision probes of the gravitational field around the closest massive black hole. In addition to adaptive optics assisted astrometry (with NACO / VLT) and spectroscopy (with SINFONI / VLT, NIRC2 / Keck and GNIRS / Gemini) over three decades, since 2016/2017 we have obtained 30-100 mu-as astrometry with the four-telescope interferometric beam combiner GRAVITY / VLTI reaching a sensitivity of mK = 20 when combining data from one night. We present the simultaneous detection of several stars within the diffraction limit of a single telescope, illustrating the power of interferometry. The new data for the stars S2, S29, S38 and S55 yield significant accelerations between March and July 2021, as these stars pass the pericenters of their orbits between 2018 and 2023. This allows for a high-precision determination of the gravitational potential around Sgr A*. Our data are in excellent agreement with general relativity orbits around a single central point mass, M = 4.30 x 10^6 M_sun with a precision of about +-0.25%. We improve the significance of our detection of the Schwarzschild precession in the S2 orbit to 7 sigma. Assuming plausible density profiles, an extended mass component inside S2's apocentre (= 0.23" or 2.4 x 10^4 R_S) must be 3000 M_sun (1 sigma), or 0.1% of M. Adding the enclosed mass determinations from 13 stars orbiting Sgr A* at larger radii, the innermost radius at which the excess mass beyond Sgr A* tentatively is seen is r = 2.5" >= 10x the apocentre of S2. This is in full harmony with the stellar mass distribution (including stellar-mass black holes) obtained from the spatially resolved luminosity function.

astro-ph.GA↗

arXiv2021

The GRAVITY Young Stellar Object survey. VII. The inner dusty disks of T Tauri stars

These protoplanetary disks in T Tauri stars play a central role in star and planet formation. We spatially resolve at sub-au scales the innermost regions of a sample of T Tauri's disks to better understand their morphology and composition. We extended our homogeneous data set of 27 Herbig stars and collected near-IR K-band observations of 17 T Tauri stars, spanning effective temperatures and luminosities in the ranges of ~4000-6000 K and ~0.4-10 Lsun. We focus on the continuum emission and develop semi-physical geometrical models to fit the interferometric data and search for trends between the properties of the disk and the central star. The best-fit models of the disk's inner rim correspond to wide rings. We extend the Radius-luminosity relation toward the smallest luminosities (0.4-10 Lsun) and find the R~L^(1/2) trend is no longer valid, since the K-band sizes measured with GRAVITY are larger than the predicted sizes from sublimation radius computation. No clear correlation between the K-band half-flux radius and the mass accretion rate is seen. Having magnetic truncation radii in agreement with the K-band GRAVITY sizes would require magnetic fields as strong as a few kG, which should have been detected, suggesting that accretion is not the main process governing the location of the half-flux radius of the inner dusty disk. Our measurements agree with models that take into account the scattered light. The N-to-K band size ratio may be a proxy for disentangling disks with silicate features in emission from disks with weak and/or in absorption silicate features. When comparing inclinations and PA of the inner disks to those of the outer disks (ALMA) in nine objects of our sample, we detect misalignments for four objects.

astro-ph.SR↗

arXiv2021

The GRAVITY Young Stellar Object survey VIII. Gas and dust faint inner rings in the hybrid disk of HD141569

The formation and evolution of planetary systems impact the primordial accretion disk. HD141569 is the only known pre-main sequence star characterized by a hybrid disk. Observations probed the outer-disk structure showing a complex system of rings and interferometric observations attempted to characterize its inner 5 au region, but derived limited constraints. The goal of this work was to explore with new high-resolution interferometric observations the properties of the dust and gas in the internal regions of HD141569. We observed HD141569 on mas scales with GRAVITY/VLTI in the near-infrared at low and high spectral resolution. We interpreted the visibilities and spectral energy distribution with geometrical models and radiative transfer techniques to constrain the dust emission. We analyzed the high spectral resolution quantities to investigate the properties of the Br-Gamma line emitting region. Thanks to the combination of three different epochs, GRAVITY resolves the inner dusty disk in the K band. Data modeling shows that an IR excess of about 6% is spatially resolved and that the origin of this emission is confined in a ring of material located at a radius of 1 au from the star with a width smaller than 0.3 au. The MCMax modeling suggests that this emission could originate from a small amount of QHPs, while large silicate grain models cannot reproduce at the same time the observational constraints on the properties of near-IR and mid-IR fluxes. The differential phases in the Br-Gamma line clearly show an S-shape that can be best reproduced witha gas disk in Keplerian rotation, confined within 0.09 au. This is also hinted at by the double-peaked Br-Gamma emission line shape. The modeling of the continuum and gas emission shows that the inclination and position angle of these two components are consistent with a system showing relatively coplanar rings on all scales.

astro-ph.SR↗

arXiv2021

The mass of Beta Pictoris c from Beta Pictoris b orbital motion

We aim to demonstrate that the presence and mass of an exoplanet can now be effectively derived from the astrometry of another exoplanet. We combined previous astrometry of $β$ Pictoris b with a new set of observations from the GRAVITY interferometer. The orbital motion of $β$ Pictoris b is fit using Markov chain Monte Carlo simulations in Jacobi coordinates. The inner planet, $β$ Pictoris c, was also reobserved at a separation of 96\,mas, confirming the previous orbital estimations. From the astrometry of planet b only, we can (i) detect the presence of $β$ Pictoris c and (ii) constrain its mass to $10.04^{+4.53}_{-3.10}\,M_{\rm Jup}$. If one adds the astrometry of $β$ Pictoris c, the mass is narrowed down to $9.15^{+1.08}_{-1.06}\,M_{\rm Jup}$. The inclusion of radial velocity measurements does not affect the orbital parameters significantly, but it does slightly decrease the mass estimate to $8.89^{+0.75}_{-0.75}\,M_{\rm Jup}$. With a semimajor axis of $2.68\pm0.02$\,au, a period of $1221\pm15$ days, and an eccentricity of $0.32\pm0.02$, the orbital parameters of $β$ Pictoris c are now constrained as precisely as those of $β$ Pictoris b. The orbital configuration is compatible with a high-order mean-motion resonance (7:1). The impact of the resonance on the planets' dynamics would then be negligible with respect to the secular perturbations, which might have played an important role in the eccentricity excitation of the outer planet.

astro-ph.EP↗

arXiv2021

A geometric distance to the supermassive black Hole of NGC 3783

The angular size of the broad line region (BLR) of the nearby active galactic nucleus (AGN) NGC 3783 has been spatially resolved by recent observations with VLTI/GRAVITY. A reverberation mapping (RM) campaign has also recently obtained high quality light curves and measured the linear size of the BLR in a way that is complementary to the GRAVITY measurement. The size and kinematics of the BLR can be better constrained by a joint analysis that combines both GRAVITY and RM data. This, in turn, allows us to obtain the mass of the supermassive black hole in NGC3783 with an accuracy that is about a factor of two better than that inferred from GRAVITY data alone. We derive $M_\mathrm{BH}=2.54_{-0.72}^{+0.90}\times 10^7\,M_\odot$. Finally, and perhaps most notably, we are able to measure a geometric distance to NGC 3783 of $39.9^{+14.5}_{-11.9}$ Mpc. We are able to test the robustness of the BLR-based geometric distance with measurements based on the Tully-Fisher relation and other indirect methods. We find the geometric distance is consistent with other methods within their scatter. We explore the potential of BLR-based geometric distances to directly constrain the Hubble constant, $H_0$, and identify differential phase uncertainties as the current dominant limitation to the $H_0$ measurement precision for individual sources.

astro-ph.GA↗

arXiv2021

The GRAVITY Young Stellar Object Survey. VI. Mapping the variable inner disk of HD 163296 at sub-au scales

Protoplanetary disks drive some of the formation process (e.g., accretion, gas dissipation, formation of structures, etc.) of stars and planets. Understanding such physical processes is one of the main astrophysical questions. HD 163296 is an interesting young stellar object for which infrared and sub-millimeter observations have shown a prominent circumstellar disk with gaps plausibly created by forming planets. This study aims at characterizing the morphology of the inner disk in HD 163296 with multi-epoch near-infrared interferometric observations performed with GRAVITY at the Very Large Telescope Interferometer (VLTI). Our goal is to depict the K-band (lambda_0 ~ 2.2 um) structure of the inner rim with milliarcsecond (sub-au) angular resolution. Our data is complemented with archival PIONIER (H-band; lambda_0 ~ 1.65 um) data of the source. We performed a Gradient Descent parametric model fitting to recover the sub-au morphology of our source. Our analysis shows the existence of an asymmetry in the disk surrounding the central star of HD 163296. We confirm variability of the disk structure in the inner ~2 mas (0.2 au). While variability of the inner disk structure in this source has been suggested by previous interferometric studies, this is the first time that it is confirmed in the H- and K-bands by using a complete analysis of the closure phases and squared visibilities over several epochs. Because of the separation from the star, position changes, and persistence of this asymmetric structure on timescales of several years, we argue that it is a dusty feature (e.g., a vortex or dust clouds), probably, made by a mixing of sillicate and carbon dust and/or refractory grains, inhomogeneously distributed above the mid-plane of the disk.

astro-ph.SR↗

arXiv2021

Constraining particle acceleration in Sgr A* with simultaneous GRAVITY, Spitzer, NuSTAR and Chandra observations

We report the time-resolved spectral analysis of a bright near-infrared and moderate X-ray flare of Sgr A*. We obtained light curves in the $M$-, $K$-, and $H$-bands in the mid- and near-infrared and in the $2-8~\mathrm{keV}$ and $2-70~\mathrm{keV}$ bands in the X-ray. The observed spectral slope in the near-infrared band is $νL_ν\propto ν^{0.5\pm0.2}$; the spectral slope observed in the X-ray band is $νL_ν\propto ν^{-0.7\pm0.5}$. We tested synchrotron and synchrotron self-Compton (SSC) scenarios. The observed near-infrared brightness and X-ray faintness, together with the observed spectral slopes, pose challenges for all models explored. We rule out a scenario in which the near-infrared emission is synchrotron emission and the X-ray emission is SSC. A one-zone model in which both the near-infrared and X-ray luminosity are produced by SSC and a model in which the luminosity stems from a cooled synchrotron spectrum can explain the flare. In order to describe the mean SED, both models require specific values of the maximum Lorentz factor $γ_{max}$, which however differ by roughly two orders of magnitude: the SSC model suggests that electrons are accelerated to $γ_{max}\sim 500$, while cooled synchrotron model requires acceleration up to $γ_{max}\sim5\times 10^{4}$. The SSC scenario requires electron densities of $10^{10}~\mathrm{cm^{-3}}$ much larger than typical ambient densities in the accretion flow, and thus require in an extraordinary accretion event. In contrast, assuming a source size of $1R_s$, the cooled synchrotron scenario can be realized with densities and magnetic fields comparable with the ambient accretion flow. For both models, the temporal evolution is regulated through the maximum acceleration factor $γ_{max}$, implying that sustained particle acceleration is required to explain at least a part of the temporal evolution of the flare.

astro-ph.HE↗

arXiv2021

The Galaxy Activity, Torus and Outflow Survey (GATOS): II. Torus and polar dust emission in nearby Seyfert galaxies

We compare mid-IR and ALMA far-IR images of 12 nearby Seyferts selected from GATOS. The mid-IR unresolved emission contributes more than 60% of the nuclear emission in most galaxies. By contrast, the ALMA 870micron continuum emission is mostly resolved and typically along the torus equatorial direction (Paper I, Garcia-Burillo et al. 2021). The Eddington ratios and nuclear hydrogen column densities NH of half the sample are favorable to launching polar and/or equatorial dusty winds, according to simulations. Six show mid-IR extended emission in the polar direction as traced by the NLR and perpendicular to the ALMA emission. In a few, the nuclear NH might be too high to uplift large quantities of dusty material along the polar direction. Five galaxies have low NH and/or Eddington ratios and thus polar dusty winds are not likely. We generate new CAT3D-WIND disk-wind model images. At low wind-to-disk cloud ratios the far-IR model images have disk- and ring-like morphologies. The X-shape associated with dusty winds is seen better in the far-IR at intermediate-high inclinations for the extended-wind configurations. In most models, the mid-IR emission comes from the inner part of the disk/cone. Extended bi-conical and one-sided polar mid-IR emission is seen in extended-wind configurations and high wind-to-disk cloud ratios. When convolved to our resolution, the model images reproduce qualitative aspects of the observed morphologies. Low-intermediate wind-to-disk ratios are required to account for the large fractions of unresolved mid-IR emission. This work and Paper I provide observational support for the torus+wind scenario. The wind component is more relevant at high Eddington ratios and/or AGN luminosities, and polar dust emission is predicted at NH of up to $10^{24}$cm$^{-2}$. The torus/disk component, on the other hand, prevails at low luminosities and/or Eddington ratios. (Abridged)

astro-ph.GA↗

arXiv2021

The Galaxy Activity, Torus and Outflow Survey (GATOS) I. ALMA images of dusty molecular tori in Seyfert galaxies

We present the first results of the Galaxy Activity, Torus and Outflow Survey (GATOS), a project aimed at understanding the properties of the dusty molecular tori and their connection to the host galaxy in nearby Seyfert galaxies. Our project expands the range of AGN luminosities and Eddington ratios covered by previous surveys of Seyferts conducted by ALMA and allows us to study the gas feeding and feedback cycle in a combined sample of 19 Seyferts. We used ALMA to obtain new images of the emission of molecular gas and dust using the CO(3-2) and HCO+(4-3) lines as well as their underlying continuum emission at 870 microns with high spatial resolutions (0.1'' ~ 7 - 13 pc) in the CND of 10 nearby (D < 28 Mpc) Seyfert galaxies. Our new ALMA observations detect 870 micron continuum and CO line emission from spatially resolved disks located around the AGN in all the sources. The bulk of the continuum flux can be accounted for by thermal emission from dust in the majority of the targets. For most of the sources the disks show a preponderant orientation perpendicular to the AGN wind axes, as expected for dusty molecular tori. The median diameters and molecular gas masses of the tori are ~ 42 pc, and ~ 6 x 10**5 Msun, respectively. We find a positive correlation between the line-of-sight gas column densities responsible for the absorption of X-rays and the molecular gas column densities derived from CO towards the AGN in our sources. The radial distributions of molecular gas in the CND of our combined sample show signs of nuclear-scale molecular gas deficits. We also detect molecular outflows in the sources that show the most extreme nuclear-scale gas deficits in our sample. These observations find for the first time supporting evidence that the imprint of AGN feedback is more extreme in higher luminosity and/or higher Eddington ratio Seyfert galaxies.

astro-ph.GA↗

arXiv2021

GRAVITY K-band spectroscopy of HD 206893 B: brown dwarf or exoplanet

We aim to reveal the nature of the reddest known substellar companion HD 206893 B by studying its near-infrared colors and spectral morphology and by investigating its orbital motion. We fit atmospheric models for giant planets and brown dwarfs and perform spectral retrievals with petitRADTRANS and ATMO on the observed GRAVITY, SPHERE, and GPI spectra of HD 206893 B. To recover its unusual spectral features, we include additional extinction by high-altitude dust clouds made of enstatite grains in the atmospheric model fits. We also infer the orbital parameters of HD 206893 B by combining the $\sim 100~μ\text{as}$ precision astrometry from GRAVITY with data from the literature and constrain the mass and position of HD 206893 C based on the Gaia proper motion anomaly of the system. The extremely red color and the very shallow $1.4~μ\text{m}$ water absorption feature of HD 206893 B can be fit well with the adapted atmospheric models and spectral retrievals. Altogether, our analysis suggests an age of $\sim 3$-$300~\text{Myr}$ and a mass of $\sim 5$-$30~\text{M}_\text{Jup}$ for HD 206893 B, which is consistent with previous estimates but extends the parameter space to younger and lower-mass objects. The GRAVITY astrometry points to an eccentric orbit ($e = 0.29^{+0.06}_{-0.11}$) with a mutual inclination of $< 34.4~\text{deg}$ with respect to the debris disk of the system. While HD 206893 B could in principle be a planetary-mass companion, this possibility hinges on the unknown influence of the inner companion on the mass estimate of $10^{+5}_{-4}~\text{M}_\text{Jup}$ from radial velocity and Gaia as well as a relatively small but significant Argus moving group membership probability of $\sim 61\%$. However, we find that if the mass of HD 206893 B is $< 30~\text{M}_\text{Jup}$, then the inner companion HD 206893 C should have a mass between $\sim 8$-$15~\text{M}_\text{Jup}$.

astro-ph.EP↗

arXiv2021

MOLsphere and pulsations of the Galactic Center's red supergiant GCIRS 7 from VLTI/GRAVITY

GCIRS 7, the brightest star in the Galactic central parsec, formed $6\pm2$ Myr ago together with dozens of massive stars in a disk orbiting the central black-hole. It has been argued that GCIRS 7 is a pulsating body, on the basis of photometric variability. We present the first medium-resolution ($R=500$), K-band spectro-interferometric observations of GCIRS 7, using the GRAVITY instrument with the four auxiliary telescopes of the ESO VLTI. We looked for variations using two epochs, namely 2017 and 2019. We find GCIRS 7 to be moderately resolved with a uniform-disk photospheric diameter of $θ^*_\text{UD}=1.55 \pm 0.03$ mas ($R^*_\text{UD}=1368 \pm 26$ $R_\odot$) in the K-band continuum. The narrow-band uniform-disk diameter increases above 2.3 $μ$m, with a clear correlation with the CO band heads in the spectrum. This correlation is aptly modeled by a hot ($T_\text{L}=2368\pm37$ K), geometrically thin molecular shell with a diameter of $θ_\text{L}=1.74\pm0.03$ mas, as measured in 2017. The shell diameter increased ($θ_\text{L}=1.89\pm0.03$ mas), while its temperature decreased ($T_\text{L}=2140\pm42$ K) in 2019. In contrast, the photospheric diameter $θ^*_\text{UD}$ and the extinction up to the photosphere of GCIRS 7 ($A_{\mathrm{K}_\mathrm{S}}=3.18 \pm 0.16$) have the same value within uncertainties at the two epochs. In the context of previous interferometric and photo-spectrometric measurements, the GRAVITY data allow for an interpretation in terms of photospheric pulsations. The photospheric diameter measured in 2017 and 2019 is significantly larger than previously reported using the PIONIER instrument ($θ_*=1.076 \pm 0.093$ mas in 2013 in the H band). The parameters of the photosphere and molecular shell of GCIRS 7 are comparable to those of other red supergiants that have previously been studied using interferometry.

astro-ph.SR↗