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Register a new userDeep LMT/AzTEC millimeter observations of Epsilon Eridani and its surroundings
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Epsilon Eridani is a nearby, young Sun-like star that hosts a ring of cool debris analogous to the solar systems Edgeworth-Kuiper belt. Early observations at (sub-)mm wavelengths gave tentative evidence of the presence of inhomogeneities in the ring, which have been ascribed to the effect of a putative low eccentricity planet, orbiting close to the ring. The existence of these structures have been recently challenged by high resolution interferometric millimeter observations. Here we present the deepest single-dish image of Epsilon Eridani at millimeter wavelengths, obtained with the Large Millimeter Telescope Alfonso Serrano (LMT). The main goal of these LMT observations is to confirm (or refute) the presence of non-axisymmetric structure in the disk. The dusty ring is detected for the first time along its full projected elliptical shape. The radial extent of the ring is not spatially resolved and shows no evidence, to within the uncertainties, of dust density enhancements. Additional features of the 1.1 mm map are: (i) the presence of significant flux in the gap between the ring and the star, probably providing the first exo-solar evidence of Poynting-Robertson drag, (ii) an unambiguous detection of emission at the stellar position with a flux significantly above that expected from Epsilon Eridanis photosphere, and (iii) the identification of numerous unresolved sources which could correspond to background dusty star-forming galaxies.
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We present observations of Epsilon Eridani from the Submillimeter Array (SMA) at 1.3 millimeters and from the Australia Telescope Compact Array (ATCA) at 7 millimeters that reach an angular resolution of ~4 (13 AU). These first millimeter interferometer observations of Epsilon Eridani, which hosts the closest debris disk to the Sun, reveal two distinct emission components: (1) the well-known outer dust belt, which, although patchy, is clearly resolved in the radial direction, and (2) an unresolved source coincident with the position of the star. We use direct model-fitting of the millimeter visibilities to constrain the basic properties of these two components. A simple Gaussian shape for the outer belt fit to the SMA data results in a radial location of $64.4^{+2.4}_{-3.0}$ AU and FWHM of $20.2^{+6.0}_{-8.2}$ AU (fractional width $Delta R/R = 0.3$. Similar results are obtained taking a power law radial emission profile for the belt, though the power law index cannot be usefully constrained. Within the noise obtained (0.2 mJy/beam), these data are consistent with an axisymmetric belt model and show no significant azimuthal structure that might be introduced by unseen planets in the system. These data also limit any stellocentric offset of the belt to $<9$ AU, which disfavors the presence of giant planets on highly eccentric ($>0.1$) and wide (10s of AU) orbits. The flux density of the unresolved central component exceeds predictions for the stellar photosphere at these long wavelengths, by a marginally significant amount at 1.3 millimeters but by a factor of a few at 7 millimeters (with brightness temperature $13000 pm 1600$ K for a source size of the optical stellar radius). We attribute this excess emission to ionized plasma from a stellar corona or chromosphere.
We present 1.3 millimeter observations of the debris disk surrounding the HR 8799 multi-planet system from the Submillimeter Array to complement archival ALMA observations that spatially filtered away the bulk of the emission. The image morphology at $3.8$ arcsecond (150 AU) resolution indicates an optically thin circumstellar belt, which we associate with a population of dust-producing planetesimals within the debris disk. The interferometric visibilities are fit well by an axisymmetric radial power-law model characterized by a broad width, $Delta R/Rgtrsim 1$. The belt inclination and orientation parameters are consistent with the planet orbital parameters within the mutual uncertainties. The models constrain the radial location of the inner edge of the belt to $R_text{in}= 104_{-12}^{+8}$ AU. In a simple scenario where the chaotic zone of the outermost planet b truncates the planetesimal distribution, this inner edge location translates into a constraint on the planet~b mass of $M_text{pl} = 5.8_{-3.1}^{+7.9}$ M$_{rm Jup}$. This mass estimate is consistent with infrared observations of the planet luminosity and standard hot-start evolutionary models, with the uncertainties allowing for a range of initial conditions. We also present new 9 millimeter observations of the debris disk from the Very Large Array and determine a millimeter spectral index of $2.41pm0.17$. This value is typical of debris disks and indicates a power-law index of the grain size distribution $q=3.27pm0.10$, close to predictions for a classical collisional cascade.
Debris disks often take the form of eccentric rings with azimuthal asymmetries in surface brightness. Such disks are often described as showing pericenter glow, an enhancement of the disk brightness in regions nearest the central star. At long wavelengths, however, the disk apocenters should appear brighter than their pericenters: in the long wavelength limit, we find the apocenter/pericenter flux ratio scales as 1+e for disk eccentricity e. We produce new models of this apocenter glow to explore its causes and wavelength dependence and study its potential as a probe of dust grain properties. Based on our models, we argue that several far-infrared and (sub)millimeter images of the Fomalhaut and epsilon Eridani debris rings obtained with Herschel, JCMT, SHARC II, ALMA, and ATCA should be reinterpreted as suggestions or examples of apocenter glow. This reinterpretation yields new constraints on the disks dust grain properties and size distributions.
Potential signatures of proto-planets embedded in their natal protoplanetary disk are radial gaps or cavities in the continuum emission in the IR-mm wavelength range. ALMA observations are now probing spatially resolved rotational line emission of CO and other chemical species. These observations can provide complementary information on the mechanism carving the gaps in dust and additional constraints on the purported planet mass. We post-process 2D hydrodynamical simulations of planet-disk models, where the dust densities and grain size distributions are computed with a dust evolution code. The simulations explore different planet masses ($1,M_{rm J}leq M_{rm p}leq15,M_{rm J}$) and turbulent parameters. The outputs are post-processed with the thermo-chemical code DALI, accounting for the radially and vertically varying dust properties as in Facchini et al. (2017). We obtain the gas and dust temperature structures, chemical abundances, and synthetic emission maps of both thermal continuum and CO rotational lines. This is the first study combining hydro simulations, dust evolution and chemistry to predict gas emission of disks hosting massive planets. All radial intensity profiles of the CO main isotopologues show a gap at the planet location. The ratio between the location of the gap as seen in CO and the peak in the mm continuum at the pressure maximum outside the orbit of the planet shows a clear dependence on planet mass. Due to the low dust density in the gaps, the dust and gas components can become thermally decoupled, with the gas being colder than the dust. The gaps seen in CO are due to a combination of gas temperature dropping at the location of the planet, and of the underlying surface density profile. In none of the models is a CO cavity observed, only CO gaps, indicating that one single massive planet is not able to explain the CO cavities observed in transition disks.
The nearby star $rm epsilon Eridani$ has been a frequent target of radio surveys for stellar emission and extraterrestial intelligence. Using deep $rm 2-4 GHz$ observations with the Very Large Array, we have uncovered a $29 mu {rm Jy}$ compact, steady continuum radio source coincident with $rm epsilon Eridani$ to within 0.06 arcseconds ($lesssim 2sigma$; 0.2 au at the distance of the star). Combining our data with previous high frequency continuum detections of $rm epsilon Eridani$, our observations reveal a spectral turnover at $rm 6 GHz$. We ascribe the $rm 2-6 GHz$ emission to optically thick, thermal gyroresonance radiation from the stellar corona, with thermal free-free opacity likely becoming relevant at frequencies below $rm 1 GHz$. The steep spectral index ($alpha simeq 2$) of the $rm 2-6 GHz$ spectrum strongly disfavors its interpretation as stellar wind-associated thermal bremsstrahlung ($alpha simeq 0.6$). Attributing the entire observed $rm 2-4 GHz$ flux density to thermal free-free wind emission, we thus, derive a stringent upper limit of $3 times 10^{-11} M_{odot} {rm yr}^{-1}$ on the mass loss rate from $rm epsilon Eridani$. Finally, we report the non-detection of flares in our data above a $5sigma$ threshold of $rm 95 mu Jy$. Together with the optical non-detection of the most recent stellar maximum expected in 2019, our observations postulate a likely evolution of the internal dynamo of $rm epsilon Eridani$.
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