Poster Abstract
Directly observing the innermost astronomical units of the disks that surround stars as they form is crucial to our understanding of star and planet formation. Unless large-scale planetary migration is invoked, these regions are understood to be the main sites of planet formation. In addition, the structure and dynamics of the gas and dust in this region impacts stellar accretion and its associated outflows. For the closest young stellar objects (YSOs), the inner disc regions subtend an angle on the order of 10s of milliarcseconds at most - a distance scale only accessible to optical long baseline interferometry. I provide an overview of the challenges involved with this technique and how we have overcome them using our MIRC-X instrument which we have successfully commissioned at the CHARA Array. In particular, I focus on results from our surveys of YSOs with the CHARA Array from before and after MIRC-X commissioning, featuring the intermediate-to-low mass YSOs GW Ori and RY Tau. With the 330 m baselines of the CHARA Array and the sensitivity and image plane coverage of MIRC-X, we have opened up the time domain for imaging studies of protoplanetary discs.
Plain text summary
The first page of this poster outlines the scientific motivation for our instrumentation and engineering developments. A flow chart illustrates proto-stellar evolution from a centrally-focussed spherical collapse stage to a circum-stellar accretion disc stage as conservation of angular momentum acts to direct the infalling material onto a plane perpendicular to the axis of rotation. Secondly, I use the full moon (which itself subtends an area of sky equal to half a square degree) and an imaginary cricket pitch on its surface to illustrate how small an angular size of 1-10 milli-arcseconds is. An angular resolution of this scale is required to observe the main planet forming regions of discs. Finally, an illustration taken from Dullemond & Monnier (2010) illustrates how the temperature in the main planet forming disc regions means we require near-infrared optical long baseline interferometry to study them. Slide two provides some background to optical long baseline interferometry as an observational technique. In particular, you are reminded that the angular resolution of an interferometer is inversely proportional to the separation between the telescopes in the interferometric array. This telescope separation is known as the baseline length. Images of two world-leading optical interferometric facilities are shown: (i) the 4 telescopes of the Very Large Telescope Interferometer (or VLTI, for short) with maximum baseline lengths of 130 metres; (ii) the 6 telescopes of the Center for High Angular Resolution Astronomy (or CHARA, for short) with maximum baseline lengths of 330 metres. With interferometers, you don't take an image of the sky. Instead, you sample a portion of the image plane. How much of the image plane you sample is dependent on the number of telescopes in your array. This is akin to viewing a picture through a comb: having more telescopes in your array would be akin to having larger spacings between the teeth of the comb, allowing you to recover more of the full image. In addition, we make use of the Earth's rotation to sample more of the image plane as we rotate below it. Slide three presents an overview of our successful commissioning of the MIRC-X instrument. This is an upgrade to the previous six-telescope beam combiner, MIRC, at CHARA. An image shows a few of the commissioning team enjoying a glass of sparkling wine after observing our first on-sky interferometric fringes following commissioning. Our sensitivity improvements provide us with visibility precision of 0.5 percent, closure phase accuracy of 0.5 degrees and a limiting H-band magnitude of 8.2 (nearly 2 magnitudes better than previously achieved with MIRC). We've increased the wavelength range to cover the J as well as the H band and the installation of half wave plates and a Wollaston prism has made near-infrared polarimetric interferometry possible at CHARA. The final slide shows a couple of science highlights so far. The first of these was published in Science in early September 2020 as part of an 11 year multi-wavelength and multi-instrument survey of the protoplanetary disk-hosting triple system, GW Ori. The increased sensitivity of MIRC-X enabled us to marginally resolve the circum-primary and circum-secondary discs in this spectacular system. Secondly, the time domain for imaging studies of discs has been made accessible by MIRC-X. Preliminary data for RY Tau from two observation dates are shown to demonstrate apparent differences detected in the brightness distribution between these epochs.
Poster Title
Studying disc-hosting young stars at high angular resolution
