After decades of fast-paced technical advances, optical/infrared (O/IR) interferometry has seen a revolution in recent years. ▪ The GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) with four 8-m telescopes reaches thousand-times-fainter objects than possible with earlier interferometers, and the Center for High Angular Resolution Astronomy (CHARA) array routinely offers up to 330-m baselines and aperture-synthesis with six 1-m telescopes. ▪ The observed objects are fainter than 19 mag, the images have submilliarcsecond resolution, and the astrometry reaches microarcsecond precision. ▪ This led to breakthrough results on the Galactic Center, exoplanets, active galactic nuclei, young stellar objects, and stellar physics. Following a primer in interferometry, we summarize the advances that led to the performance boost of modern interferometers: ▪ Single-mode beam combiners now combine up to six telescopes, and image reconstruction software has advanced over earlier developments for radio interferometry. ▪ With a combination of large telescopes, adaptive optics (AO), fringe tracking, and especially dual-beam interferometry, GRAVITY has boosted the sensitivity by many orders of magnitudes. Another order-of-magnitude improvement will come from laser guide star AO. In combination with large separation fringe tracking, O/IR interferometry will then provide complete sky coverage for observations in the Galactic plane and substantial coverage for extragalactic targets. Expected final online publication date for the Annual Review of Astronomy and Astrophysics, Volume 61 is August 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Advances in Optical/Infrared Interferometry


 The study of protoplanetary disks has become increasingly important with the Kepler satellite finding that exoplanets are ubiquitous around stars in our galaxy and the discovery of enormous diversity in planetary system architectures and planet properties. High-resolution near-IR and ALMA images show strong evidence for ongoing planet formation in young disks. The JWST MIRI mid-INfrared Disk Survey (MINDS) aims to (1) investigate the chemical inventory in the terrestrial planet-forming zone across stellar spectral type, (2) follow the gas evolution into the disk dispersal stage, and (3) study the structure of protoplanetary and debris disks in the thermal mid-IR. The MINDS survey will thus build a bridge between the chemical inventory of disks and the properties of exoplanets. The survey comprises 52 targets (Herbig Ae stars, T Tauri stars, very low-mass stars and young debris disks). We primarily obtain MIRI/MRS spectra with high signal-to-noise ratio (∼100–500) covering the complete wavelength range from 4.9 to 27.9 μm. For a handful of selected targets we also obtain NIRSpec IFU high resolution spectroscopy (2.87–5.27 μm). We will search for signposts of planet formation in thermal emission of micron-sized dust—information complementary to near-IR scattered light emission from small dust grains and emission from large dust in the submillimeter wavelength domain. We will also study the spatial structure of disks in three key systems that have shown signposts for planet formation, TW Hya and HD 169142 using the MIRI coronagraph at 15.5 μm and 10.65 μm respectively and PDS 70 using NIRCam imaging in the 1.87 μm narrow and the 4.8 μm medium band filter. We provide here an overview of the MINDS survey and showcase the power of the new JWST mid-IR molecular spectroscopy with the TW Hya disk spectrum where we report the detection of the molecular ion 
 


 
 
 
 
 
 CH
 
 
 3
 
 
 
 
 +
 
 
 
 
 and the robust confirmation of HCO+ earlier detected with Spitzer.

pdf/minds-the-jwst-miri-mid-infrared-disk-survey-1g45uh4w6s.pdf

MINDS: The JWST MIRI Mid-INfrared Disk Survey

Abstract The accretion of material from protoplanetary disks onto their central stars is a fundamental process in the evolution of these systems and a key diagnostic in constraining the disk lifetime. We analyze the relationship between the stellar accretion rate and the disk mass in 32 intermediate-mass Herbig Ae/Be systems and compare them to their lower-mass counterparts, T Tauri stars. We find that the <?CDATA $\dot{M}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> – M disk relationship for Herbig Ae/Be stars is largely flat at ∼10 −7 M ☉ yr −1 over 3 orders of magnitude in dust mass. While most of the sample follows the T Tauri trend, a subset of objects with high accretion rates and low dust masses are identified. These outliers (12 out of 32 sources) have an inferred disk lifetime of less than 0.01 Myr and are dominated by objects with low infrared excess. This outlier sample is likely identified in part by the bias in classifying Herbig Ae/Be stars, which requires evidence of accretion that can only be reliably measured above a rate of ∼10 −9 M ☉ yr −1 for these spectral types. If the disk masses are not underestimated and the accretion rates are not overestimated, this implies that these disks may be on the verge of dispersal, which may be due to efficient radial drift of material or outer disk depletion by photoevaporation and/or truncation by companions. This outlier sample likely represents a small subset of the larger young, intermediate-mass stellar population, the majority of which would have already stopped accreting and cleared their disks.

The Ṁ –M disk Relationship for Herbig Ae/Be Stars: A Lifetime Problem for Disks with Low Masses?

To date, the proposed observation of young stellar objects (YSOs) in the Galactic center still raises the question of where and how these objects could have formed due to the violent vicinity of Sgr A*. Here, we report the multiwavelength detection of a highly dynamic YSO close to Sgr A* that might be a member of the IRS 13 cluster. We observe the beforehand known coreless bow-shock source X3 in the near- and mid-infrared (NIR/MIR) with SINFONI (VLT), NACO (VLT), ISAAC (VLT), VISIR (VLT), SHARP (NTT), and NIRCAM2 (KECK). In the radio domain, we use CO continuum and H30α ALMA observations to identify system components at different temperatures and locations concerning the central stellar source. It is suggested that these radio/submillimeter observations in combination with the NIR Brγ line can be associated with a protoplanetary disk of the YSO, which is consistent with manifold VISIR observations that reveal complex molecules and elements such as PAH, S iv, Ne ii, and Ar iii in a dense and compact region. Based on the photometric multiwavelength analysis, we infer the mass of 15−5+10M⊙ for the YSO with a related age of a few 104 yr. Due to this age estimate and the required relaxation timescales for high-mass stars, this finding is an indication of ongoing star formation in the inner parsec. The proper motion and 3D distance imply a relation between X3 and IRS 13. We argue that IRS 13 may serve as a birthplace for young stars that are ejected due to the evaporation of the cluster.

X3: A High-mass Young Stellar Object Close to the Supermassive Black Hole Sgr A*

Methods. We use the code MCFOST to solve the non-LTE problem of line formation in non-axisymmetric accreting magnetospheres. We compute the Br{\gamma} line profile originating from accretion columns for models with different magnetic obliquities. We also derive monochromatic synthetic images of the Br{\gamma} line emitting region across the line profile. This spectral line is a prime diagnostics of magnetospheric accretion in young stars and is accessible with the long baseline near-infrared interferometer GRAVITY installed at the ESO Very Large Telescope Interferometer. Results. We derive Br{\gamma} line profiles as a function of rotational phase and compute interferometric observables, visibilities and phases, from synthetic images. The line profile shape is modulated along the rotational cycle, exhibiting inverse P Cygni profiles at the time the accretion shock faces the observer. The size of the line's emission region decreases as the magnetic obliquity increases, which is reflected in a lower line flux. We apply interferometric models to the synthetic visibilities in order to derive the size of the line-emitting region. We find the derived interferometric size to be more compact than the actual size of the magnetosphere, ranging from 50 to 90\% of the truncation radius. Additionally, we show that the rotation of the non-axisymmetric magnetosphere is recovered from the rotational modulation of the Br{\gamma}-to-continuum photo-centre shifts, as measured by the differential phase of interferometric visibilities.

/pdf/spectroscopic-and-interferometric-signatures-of-kr9q03sv.pdf

Spectroscopic and interferometric signatures of magnetospheric accretion in young stars

Context. Hot atomic hydrogen emission lines in pre-main sequence stars serve as tracers for physical processes in the innermost regions of circumstellar accretion disks, where the interaction between a star and disk is the dominant inﬂuence on the formation of infalls and outﬂows. In the highly magnetically active T Tauri stars, this interaction region is particularly shaped by the stellar magnetic ﬁeld and the associated magnetosphere, covering the inner ﬁve stellar radii around the central star. Even for the closest T Tauri stars, a region as compact as this is only observed on the sky plane at sub-mas scales. To resolve it spatially, the capabilities of optical long baseline interferometry are required. Aims. We aim to spatially and spectrally resolve the Br γ hydrogen emission line with the methods of interferometry in order to examine the kinematics of the hydrogen gas emission region in the inner accretion disk of a sample of solar-like young stellar objects. The goal is to identify trends and categories among the sources of our sample and to discuss whether or not they can be tied to di ﬀ erent origin mechanisms associated with Br γ emission in T Tauri stars, chieﬂy and most prominently magnetospheric accretion. Methods. We observed a sample of seven T Tauri stars for the ﬁrst time with VLTI GRAVITY, recording spectra and spectrally dispersed inter- ferometric quantities across the Br γ line at 2.16 µ m in the near-infrared K-band. We used the visibilities and di ﬀ erential phases to extract the size of the Br γ emission region and the photocentre shifts on a channel-by-channel basis, probing the variation of spatial extent at di ﬀ erent radial velocities. To assist in the interpretation, we also made use of radiative transfer models of magnetospheric accretion to establish a baseline of expected interferometric signatures if accretion is the primary driver of Br γ emission. Results. From among our sample, we ﬁnd that ﬁve of the seven T Tauri stars show an emission region with a half-ﬂux radius in the four to seven stellar radii range that is broadly expected for magnetospheric truncation. Two of the ﬁve objects also show Br γ emission primarily originating from within the co-rotation radius, which is an important criterion for magnetospheric accretion. Two objects exhibit extended emission on a scale beyond 10 R ∗ , one of them is even beyond the K-band continuum half-ﬂux radius of 11.3 R ∗ . The observed photocentre shifts across the line can be either similar to what is expected for disks in rotation or show patterns of higher complexity. Conclusions. Based on the observational ﬁndings and the comparison with the radiative transfer models, we ﬁnd strong evidence to suggest that for the two weakest accretors in the sample, magnetospheric accretion is the primary driver of Br γ radiation. The results for the remaining sources imply either partial or strong contributions coming from additional, spatially extended emission components in the form of outﬂows, such as stellar or disk winds. We expect that in actively accreting T Tauri stars, these phenomena typically occur simultaneously on di ﬀ erent spatial scales. Through more advanced modelling, interferometry will be a key factor in disentangling their distinct contributions to the total Br γ ﬂux arising from the innermost disk regions.

/pdf/the-gravity-young-stellar-object-survey-ix-spatially-1azmj7m8.pdf

The GRAVITY Young Stellar Object survey. IX. Spatially resolved kinematics of hot hydrogen gas in the star-disk interaction region of T Tauri stars

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The GRAVITY Young Stellar Object survey. IX. Spatially resolved kinematics of hot hydrogen gas in the star-disk interaction region of T Tauri stars