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Register a new userALMA imaging of the M-dwarf Fomalhaut Cs debris disc
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Added by
Patrick Cronin-Coltsmann
Publication date
2021
fields
Physics
and research's language is
English
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Fomalhaut C (LP 876-10) is a low mass M4V star in the intriguing Fomalhaut triple system and, like Fomalhaut A, possesses a debris disc. It is one of very few nearby M-dwarfs known to host a debris disc and of these has by far the lowest stellar mass. We present new resolved observations of the debris disc around Fomalhaut C with the Atacama Large Millimetre Array which allow us to model its properties and investigate the systems unique history. The ring has a radius of 26 au and a narrow full width at half maximum of at most 4.2 au. We find a 3$sigma$ upper limit on the eccentricity of 0.14, neither confirming nor ruling out previous dynamic interactions with Fomalhaut A that could have affected Fomalhaut Cs disc. We detect no $^{12}$CO J=3-2 emission in the system and do not detect the disc in scattered light with HST/STIS or VLT/SPHERE. We find the original Herschel detection to be consistent with our ALMA models radial size. We place the disc in the context of the wider debris disc population and find that its radius is as expected from previous disc radius-host luminosity trends. Higher signal-to-noise observations of the system would be required to further constrain the disc properties and provide further insight to the history of the Fomalhaut triple system as a whole.
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We present ALMA Band 6 observations (1.3 mm/233 GHz) of Fomalhaut and its debris disc. The observations achieve a sensitivity of 17 $mu$Jy and a resolution of 0.28 arcsec (2.1 au at a distance of 7.66 pc), which are the highest resolution observations to date of the millimetre grains in Fomalhauts main debris ring. The ring is tightly constrained to $139^{+2}_{-3}$ au with a FWHM of $13pm3$ au, following a Gaussian profile. The millimetre spectral index is constrained to $alpha_{mm} = -2.62pm0.12$. We explore fitting debris disc models in the image plane, as well as fitting models using visibility data directly. The results are compared and the potential advantages/disadvantages of each approach are discussed. The detected central emission is indistinguishable from a point source, with a most probable flux of $0.90pm 0.12$ mJy (including calibration uncertainties). This implies that any inner debris structure, as was inferred from far-Infrared observations, must contribute little to the total central emission. Moreover, the stellar flux is less than 70% of that predicted by extrapolating a black body from the constrained stellar photosphere temperature. This result emphasizes that unresolved inner debris components cannot be fully characterized until the behaviour of the host stars intrinsic stellar emission at millimetre wavelengths is properly understood.
We present ALMA mosaic observations at 1.3 mm (223 GHz) of the Fomalhaut system with a sensitivity of 14 $mu$Jy/beam. These observations provide the first millimeter map of the continuum dust emission from the complete outer debris disk with uniform sensitivity, enabling the first conclusive detection of apocenter glow. We adopt a MCMC modeling approach that accounts for the eccentric orbital parameters of a collection of particles within the disk. The outer belt is radially confined with an inner edge of $136.3pm0.9$ AU and width of $13.5pm1.8$ AU. We determine a best-fit eccentricity of $0.12pm0.01$. Assuming a size distribution power law index of $q=3.46pm 0.09$, we constrain the dust absorptivity power law index $beta$ to be $0.9<beta<1.5$. The geometry of the disk is robustly constrained with inclination $65.!!^circ6pm0.!!^circ3$, position angle $337.!!^circ9pm0.!!^circ3$, and argument of periastron $22.!!^circ5pm4.!!^circ3$. Our observations do not confirm any of the azimuthal features found in previous imaging studies of the disk with HST, SCUBA, and ALMA. However, we cannot rule out structures $leq10$ AU in size or which only affect smaller grains. The central star is clearly detected with a flux density of $0.75pm0.02$ mJy, significantly lower than predicted by current photospheric models. We discuss the implications of these observations for the directly imaged Fomalhaut b and the inner dust belt detected at infrared wavelengths.
We present the first spatially resolved mid-infrared (37.1 $mu$m) image of the Fomalhaut debris disk. We use PSF fitting and subtraction to distinctly measure the flux from the unresolved component and the debris disk. We measure an infrared excess in the point source of $0.9 pm 0.2$ Jy, consistent with emission from warm dust in an inner disk structure (Su et al. 2016), and inconsistent with a stellar wind origin. We cannot confirm or rule out the presence of a pileup ring (Su et al. 2016) near the star. In the cold region, the 37 $mu$m imaging is sensitive to emission from small, blowout grains, which is an excellent probe of the dust production rate from planetesimal collisions. Under the assumptions that the dust grains are icy aggregates and the debris disk is in steady state, this result is consistent with the dust production rates predicted by Kenyon & Bromley (2008) from theoretical models of icy planet formation. We find a dust luminosity of $(7.9 pm 0.8) times 10^{-4}$ L$_odot$ and a dust mass of 8 -- 16 lunar masses, depending on grain porosity, with $sim 1$ lunar mass in grains with radius 1 $mu$m -- 1 mm. If the grains are icy and highly porous, meter-sized objects must be invoked to explain the far-IR, submm, and mm emission. If the grains are composed of astronomical silicates, there is a dearth of blowout grains (Pawellek et al. 2014) and the mass loss rate is well below the predicted dust production values.
One of the defining properties of debris discs compared to protoplanetary discs used to be their lack of gas, yet small amounts of gas have been found around an increasing number of debris discs in recent years. These debris discs found to have gas tend to be both young and bright. In this paper we conduct a deep search for CO gas in the system HD 95086 - a 17 Myr old, known planet host that also has a debris disc with a high fractional luminosity of $1.5times10^{-3}$. Using the Atacama Large Millimeter/submillimeter Array (ALMA) we search for CO emission lines in bands 3, 6 and 7. By implementing a spectro-spatial filtering technique, we find tentative evidence for CO $J$=2-1 emission in the disc located at a velocity, 8.5$pm$0.2 km s$^{-1}$, consistent with the radial velocity of the star. The tentative detection suggests that the gas on the East side of the disc is moving towards us. In the same region where continuum emission is detected, we find an integrated line flux of 9.5$pm$3.6 mJy km s$^{-1}$, corresponding to a CO mass of (1.4-13)$times10^{-6}$ M$_oplus$. Our analysis confirms that the level of gas present in the disc is inconsistent with the presence of primordial gas in the system and is consistent with second generation production through the collisional cascade.
Fomalhaut plays an important role in the study of debris disks and small bodies in other planetary systems. The proximity and luminosity of the star make key features of its debris, like the water ice-line, accessible. Here we present ALMA cycle 1, 870 mu m (345 GHz) observations targeted at the inner part of the Fomalhaut system with a synthesized beam of 0.45x0.37 (~3 AU linear resolution at the distance of Fomalhaut) and a rms of 26 mu Jy/beam. The high angular resolution and sensitivity of the ALMA data enable us to place strong constraints on the nature of the warm excess revealed by Spitzer and Herschel observations. We detect a point source at the star position with a total flux consistent with thermal emission from the stellar photosphere. No structures that are brighter than 3sigma are detected in the central 15 AU x 15 AU region. Modeling the spectral energy distribution using parameters expected for a dust-producing planetesimal belt indicates a radial location in the range ~8-15 AU. This is consistent with the location where ice sublimates in Fomalhaut, i.e., an asteroid-belt analog. The 3sigma upper limit for such a belt is <1.3 mJy at 870 mu m. We also interpret the 2 and 8-13 mu m interferometric measurements to reveal the structure in the inner 10 AU region as dust naturally connected to this proposed asteroid belt by Poynting-Robertson drag, dust sublimation, and magnetically trapped nano grains.
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