Search for: All records

We present new Very Large Telescope Interferometer (VLTI)/GRAVITY near-infrared interferometric measurements of the angular size of the innermost hot dust continuum for 14 type 1 active galactic nuclei (AGNs). The angular sizes are resolved on scales of ∼0.7 mas and the inferred ring radii range from 0.028 to 1.33 pc, comparable to those reported previously and a factor of 10−20 smaller than the mid-infrared sizes in the literature. Combining our new data with previously published values, we compiled a sample of 25 AGNs with bolometric luminosity ranging from 10⁴²to 10⁴⁷erg s⁻¹, with which we studied the radius-luminosity (R − L) relation for the hot dust structure. Our interferometric measurements of radius are offset by a factor of 2 from the equivalent relation derived through reverberation mapping. Using a simple model to explore the dust structure’s geometry, we conclude that this offset can be explained if the 2 μm emitting surface has a concave shape. Our data show that the slope of the relation is in line with the canonicalR ∝ L^0.5when using an appropriately non-linear correction for bolometric luminosity. In contrast, using optical luminosity or applying a constant bolometric correction to it results in a significant deviation in the slope, suggesting a potential luminosity dependence on the spectral energy distribution. Over four orders of magnitude in luminosity, the intrinsic scatter around theR − Lrelation is 0.2 dex, suggesting a tight correlation between the innermost hot dust structure size and the AGN luminosity. 

Context . The study of the multiplicity of massive stars gives hints on their formation processes and their evolutionary paths, which are still not fully understood. Large separation binaries (>50 milliseconds of arc, mas) can be probed by adaptive-optics-assisted direct imaging and sparse aperture masking, while close binaries can be resolved by photometry and spectroscopy. However, optical long baseline interferometry is mandatory to establish the multiplicity of Galactic massive stars at the separation gap between 1 and 50 mas. Aims . In this paper, we aim to demonstrate the capability of the new interferometric instrument MIRC-X, located at the CHARA Array, to study the multiplicity of O-type stars and therefore probe the full range of separation for more than 120 massive stars ( H < 7 . 5 mag). Methods . We initiated a pilot survey of bright O-type stars ( H < 6.5 mag) observable with MIRC-X. We observed 29 O-type stars, including two systems in average atmospheric conditions around a magnitude of H = 7.5 mag. We systematically reduced the obtained data with the public reduction pipeline of the instrument. We analyzed the reduced data using the dedicated python software CANDID to detect companions. Results . Out of these 29 systems, we resolved 19 companions in 17 different systems with angular separations between ~0.5 and 50 mas. This results in a multiplicity fraction ƒ m = 17/29 = 0.59 ± 0.09, and an average number of companions ƒ c = 19/29 = 0.66 ± 0.13. Those results are in agreement with the results of the SMASH+ survey in the Southern Hemisphere. Thirteen of these companions have been resolved for the first time, including the companion responsible for the nonthermal emission in Cyg OB2-5 A and the confirmation of the candidate companion of HD 47129 suggested by SMASH+. Conclusions . A large survey on more than 120 northern O-type stars ( H < 7.5) is possible with MIRC-X and will be fruitful. 

Context. Magnetic fields influence the formation and evolution of stars and impact the observed stellar properties. magnetic A-type stars (Ap stars) are a prime example of this. Access to precise and accurate determinations of their stellar fundamental properties, such as masses and ages, is crucial to understand the origin and evolution of fossil magnetic fields. Aims. We propose using the radii and luminosities determined from interferometric measurements, in addition to seismic constraints when available, to infer fundamental properties of 14 Ap stars préviously characterised. Methods. We used a grid-based modelling approach, employing stellar models computed with the CESTAM stellar evolution code, and the parameter search performed with the AIMS optimisation method. The stellar model grid was built using a wide range of initial helium abundances and metallicities in order to avoid any bias originating from the initial chemical composition. The large frequency separations (Δ ν ) of HR 1217 (HD 24712) and α Cir (HD 128898), two rapidly oscillating Ap stars of the sample, were used as seismic constraints. Results. We inferred the fundamental properties of the 14 stars in the sample. The overall results are consistent within 1 σ with previous studies, however, the stellar masses inferred in this study are higher. This trend likely originates from the broader range of chemical compositions considered in this work. We show that the use of Δ ν in the modelling significantly improves our inferences, allowing us to set reasonable constraints on the initial metallicity which is, otherwise, unconstrained. This gives an indication of the efficiency of atomic diffusion in the atmospheres of roAp stars and opens the possibility of characterising the transport of chemical elements in their interiors. 

By using the GRAVITY instrument with the near-infrared (NIR) Very Large Telescope Interferometer (VLTI), the structure of the broad (emission-)line region (BLR) in active galactic nuclei (AGNs) can be spatially resolved, allowing the central black hole (BH) mass to be determined. This work reports new NIR VLTI/GRAVITY interferometric spectra for four type 1 AGNs (Mrk 509, PDS 456, Mrk 1239, and IC 4329A) with resolved broad-line emission. Dynamical modelling of interferometric data constrains the BLR radius and central BH mass measurements for our targets and reveals outflow-dominated BLRs for Mrk 509 and PDS 456. We present an updated radius-luminosity (R-L) relation independent of that derived with reverberation mapping (RM) measurements using all the GRAVITY-observed AGNs. We find our R-L relation to be largely consistent with that derived from RM measurements except at high luminosity, where BLR radii seem to be smaller than predicted. This is consistent with RM-based claims that high Eddington ratio AGNs show consistently smaller BLR sizes. The BH masses of our targets are also consistent with the standardM_BH-σ_*relation. Model-independent photocentre fitting shows spatial offsets between the hot dust continuum and the BLR photocentres (ranging from ∼17 μas to 140 μas) that are generally perpendicular to the alignment of the red- and blueshifted BLR photocentres. These offsets are found to be related to the AGN luminosity and could be caused by asymmetricK-band emission of the hot dust, shifting the dust photocentre. We discuss various possible scenarios that can explain this phenomenon. 

Context. Surface brightness-color relations (SBCRs) are widely used for estimating angular diameters and deriving stellar properties. They are critical to derive extragalactic distances of early-type and late-type eclipsing binaries or, potentially, for extracting planetary parameters of late-type stars hosting planets. Various SBCRs have been implemented so far, but strong discrepancies in terms of precision and accuracy still exist in the literature. Aims. We aim to develop a precise SBCR for early-type B and A stars using selection criteria, based on stellar characteristics, and combined with homogeneous interferometric angular diameter measurements. We also improve SBCRs for late-type stars, in particular in the Gaia photometric band. Methods. We observed 18 early-type stars with the VEGA interferometric instrument, installed on the CHARA array. We then applied additional criteria on the photometric measurements, together with stellar characteristics diagnostics in order to build the SBCRs. Results. We calibrated a SBCR for subgiant and dwarf early-type stars. The RMS of the relation is σ F V 0  = 0.0051 mag, leading to an average precision of 2.3% on the estimation of angular diameters, with 3.1% for V − K < −0.2 mag and 1.8% for V − K > −0.2 mag. We found that the conversion between Johnson- K and 2MASS- K s photometries is a key issue for early-type stars. Following this result, we have revisited our previous SBCRs for late-type stars by calibrating them with either converted Johnson- K or 2MASS- K s photometries. We also improve the calibration of these SBCRs based on the Gaia photometry. The expected precision on the angular diameter using our SBCRs for late-type stars ranges from 1.0 to 2.7%. Conclusions. By reaching a precision of 2.3% on the estimation of angular diameters for early-type stars, significant progress has been made to determine extragalactic distances, such as M31 and M33 galaxies, using early-type eclipsing binaries. 

Context. A low-mass companion potentially in the brown dwarf mass regime was discovered on a ~12 yr orbit (~5.5 au) around HD 167665 using radial velocity (RV) monitoring. Joint RV–astrometry analyses confirmed that HD 167665B is a brown dwarf with precisions on the measured mass of ~4–9%. Brown dwarf companions with measured mass and luminosity are valuable for testing formation and evolutionary models. However, its atmospheric properties and luminosity are still unconstrained, preventing detailed tests of evolutionary models. Aims. We further characterize the HD 167665 system by measuring the luminosity and refining the mass of its companion and reassessing the stellar age. Methods. We present new high-contrast imaging data of the star and of its close-in environment from SPHERE and GRAVITY, which we combined with RV data from CORALIE and HIRES and astrometry from HIPPARCOSandGaia. Results. The analysis of the host star properties indicates an age of 6.20 ± 1.13 Gyr. GRAVITY reveals a point source near the position predicted from a joint fit of RV data and HIPPARCOS–Gaiaproper motion anomalies. Subsequent SPHERE imaging confirms the detection and reveals a faint point source of contrast of ∆H2= 10.95 ± 0.33 mag at a projected angular separation of ~180 mas. A joint fit of the high-contrast imaging, RV, and HIPPARCOSintermediate astrometric data together with theGaiaastrometric parameters constrains the mass of HD 167665B to ~1.2%, 60.3 ± 0.7M_J. The SPHERE colors and spectrum point to an early or mid-T brown dwarf of spectral type T4₋₂⁺¹. Fitting the SPHERE spectrophotometry and GRAVITY spectrum with synthetic spectra suggests an effective temperature of ~1000–1150 K, a surface gravity of ~5.0–5.4 dex, and a bolometric luminosity log(L/L_⊙)=−4.892_−0.028^+0.024dex. The mass, luminosity, and age of the companion can only be reproduced within 3σby the hybrid cloudy evolutionary models of Saumon & Marley (2008, ApJ, 689, 1327), whereas cloudless evolutionary models underpredict its luminosity. 

Context. The variety of physical processes at play in chemically peculiar stars makes it difficult to determine their fundamental parameters. In particular, for the magnetic ones, called Ap stars, the strong magnetic fields and the induced spotted stellar surfaces may lead to biased effective temperatures when these values are derived through spectro-photometry. Aims. We propose to benefit from the exquisite angular resolution provided by long-baseline interferometry in the visible to determine the accurate angular diameters of a number of Ap stars, and thus estimate their radii by a method that is as independent as possible of atmospheric models. Methods. We used the visible spectrograph VEGA at the CHARA interferometric array to complete the sample of Ap stars currently observable with this technique. We estimated the angular diameter and radius of six new targets. We estimated their bolometric flux based solely on observational spectroscopic and photometric data to derive nearly model-independent luminosities and effective temperatures. Results. We extend to 14 the number of Ap stars for which interferometric angular diameters have been measured. The fundamental parameters we derived for the complete Ap sample are compared with those obtained through a self-consistent spectroscopic analysis. Based on a model fitting approach of high-resolution spectra and spectro-photometric observations over a wide wavelength range, this method takes into account the anomalous chemical composition of the atmospheres and the inhomogeneous vertical distribution for different chemical elements. Regarding both the radii and the effective temperatures, the derived values from our interferometric observations and from self-consistent modelling are consistent within better than 2 σ for nine targets out of ten. We thus benchmark nine Ap stars for effective temperatures ranging from 7200 and 9100 K, and luminosities ranging between 7 L ⊙ and 86 L ⊙ . Conclusions. These results will be key for the future derivation of accurate radii and other fundamental parameters of fainter peculiar stars for which both the sensitivity and the angular resolution of the current interferometers are not sufficient. Within the context of the observations of Ap stars with the Transiting Exoplanet Survey Satellite (TESS), these interferometric measurements are crucial for testing the mechanism of pulsation excitation at work in these peculiar stars. In particular, our interferometric measurements provide accurate locations in the Hertzsprung-Russell diagram for hot Ap stars among which pulsations may be searched for with TESS, putting to test the blue edge of the theoretical instability strip. These accurate locations could be used to derive masses and ages of these stars through a specific grid of models, and to test correlations between the properties of these peculiar stars and their evolutionary state. 

ABSTRACT The determination of fundamental parameters of stars is one of the main tasks of astrophysics. For magnetic chemically peculiar stars, this problem is complicated by the anomalous chemical composition of their atmospheres, which requires special analysis methods. We present the results of the effective temperature, surface gravity, abundance, and radius determinations for three CP stars HD 188041, HD 111133, and HD 204411. Our analysis is based on a self-consistent model fitting of high-resolution spectra and spectrophotometric observations over a wide wavelength range, taking into account the anomalous chemical composition of atmospheres and the inhomogeneous vertical distribution for three chemical elements: Ca, Cr, and Fe. For two stars, HD 188041 and HD 204411, we also performed interferometric observations that provided us with the direct estimates of stellar radii. Comparison of the radii determined from the analysis of spectroscopic/spectrophotometric observations with direct measurements of the radii by interferometry methods for seven CP stars shows that the radii agree within the limits of measurement errors, which proves indirect spectroscopic analysis capable of proving reliable determinations of the fundamental parameters of fainter Ap stars that are not possible to study with modern interferometric facilities. 

This work focuses on active galactic nuclei (AGNs) and on the relation between the sizes of the hot dust continuum and the broad-line region (BLR). We find that the continuum size measured using optical/near-infrared interferometry (OI) is roughly twice that measured by reverberation mapping (RM). Both OI and RM continuum sizes show a tight relation with the H β BLR size, with only an intrinsic scatter of 0.25 dex. The masses of supermassive black holes (BHs) can hence simply be derived from a dust size in combination with a broad line width and virial factor. Since the primary uncertainty of these BH masses comes from the virial factor, the accuracy of the continuum-based BH masses is close to those based on the RM measurement of the broad emission line. Moreover, the necessary continuum measurements can be obtained on a much shorter timescale than those required monitoring for RM, and they are also more time efficient than those needed to resolve the BLR with OI. The primary goal of this work is to demonstrate a measuring of the BH mass based on the dust-continuum size with our first calibration of the R BLR – R d relation. The current limitation and caveats are discussed in detail. Future GRAVITY observations are expected to improve the continuum-based method and have the potential of measuring BH masses for a large sample of AGNs in the low-redshift Universe. 

Context . We implement an electron avalanche photodiode (e-APD) in the MIRC-X instrument, which is an upgrade of the six-telescope near-infrared imager MIRC, at the CHARA array. This technology should improve the sensitivity of near-infrared interferometry. Aims . We aim to characterize a near-infrared C-RED ONE camera from First Light Imaging (FLI) using an e-APD from Leonardo (previously SELEX). Methods . We first used the classical mean-variance analysis to measure the system gain and the amplification gain. We then developed a physical model of the statistical distribution of the camera output signal. This model is based on multiple convolutions of the Poisson statistic, the intrinsic avalanche gain distribution, and the observed distribution of the background signal. At low flux level, this model independently constrains the incident illumination level, the total gain, and the excess noise factor of the amplification. Results . We measure a total transmission of 48 ± 3% including the cold filter and the Quantum Efficiency. We measure a system gain of 0.49 ADU/e, a readout noise of 10 ADU, and amplification gains as high as 200. These results are consistent between the two methods and therefore validate our modeling approach. The measured excess noise factor based on the modeling is 1.47 ± 0.03, with no obvious dependency with flux level or amplification gain. Conclusions . The presented model allows the characteristics of the e-APD array to be measured at low flux level independently of a preexisting calibration. With < 0.3 electron equivalent readout noise at kilohertz frame rates, we confirm the revolutionary performances of the camera with respect to the PICNIC or HAWAII technologies. However, the measured excess noise factor is significantly higher than what is claimed in the literature (< 1.25), and explains why counting multiple photons remains challenging with this camera.