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Register a new userHigh-resolution observations of gas and dust around Mira using ALMA and SPHERE/ZIMPOL
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The outflows of oxygen-rich asymptotic giant branch (AGB) stars are thought to be driven by radiation pressure by photon scattering on grains with sizes of tenths of microns. The details of the formation of dust in the extended atmospheres of these stars and the mass-loss process is still not well understood. We obtained quasi-simultaneous observations of the AGB star Mira using ALMA and ZIMPOL to probe the distribution of gas and large dust grains, respectively. The polarized light images show dust around Mira~A, the companion (Mira~B) and in a trail that connects the two sources. ALMA reveals that dust around Mira~A is contained in a high-gas-density region with a significant fraction of the polarized light arising from its edge. We constrained the gas density, temperature, and velocity within a few stellar radii from the star by modelling the CO $v=1, J=3-2$ line. We find a mass $(sim 3.8 pm 1.3) times 10^{-4}~M_odot$ to be contained between the stellar millimetre photosphere, $R^{rm 338~GHz}_star$, and $4~R^{rm 338~GHz}_star$. Our best-fit models with lower masses also reproduce the $^{13}$CO $v=0, J=3-2$ line emission from this region. We find TiO$_2$ and AlO abundances corresponding to 4.5% and $< 0.1$% of the total titanium and aluminium expected for a solar-composition gas. The low abundance of AlO allows for efficient Al depletion into dust already very close to the star, as expected from thermal dust emission observations and theoretical calculations of Mira variables. We constrain the presence of aluminium oxide grains based on the scattered light observations and our gas-phase model. We find that aluminium oxide grains can account for a significant fraction of the total aluminium atoms in this region only if the grains have sizes $lesssim 0.02~mu$m. This is an order of magnitude smaller than the maximum sizes predicted by dust-formation and wind-driving models.
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We have observed the protoplanetary disk of the well-known young Herbig star HD 142527 using ZIMPOL Polarimetric Differential Imaging with the VBB (Very Broad Band, ~600-900nm) filter. We obtained two datasets in May 2015 and March 2016. Our data allow us to explore dust scattering around the star down to a radius of ~0.025 (~4au). The well-known outer disk is clearly detected, at higher resolution than before, and shows previously unknown sub-structures, including spirals going inwards into the cavity. Close to the star, dust scattering is detected at high signal-to-noise ratio, but it is unclear whether the signal represents the inner disk, which has been linked to the two prominent local minima in the scattering of the outer disk, interpreted as shadows. An interpretation of an inclined inner disk combined with a dust halo is compatible with both our and previous observations, but other arrangements of the dust cannot be ruled out. Dust scattering is also present within the large gap between ~30 and ~140au. The comparison of the two datasets suggests rapid evolution of the inner regions of the disk, potentially driven by the interaction with the close-in M-dwarf companion, around which no polarimetric signal is detected.
MWC758 is a young star surrounded by a transitional disk. Recently, a protoplanet candidate has been detected around MWC758 through high-resolution $L$-band observations. The candidate is located inside the disk cavity at a separation of $sim$111 mas from the central star, and at an average position angle of $sim$165.5 degrees. We have performed simultaneous adaptive optics observations of MWC758 in the H$_{alpha}$ line and the adjacent continuum using SPHERE/ZIMPOL at the Very Large Telescope (VLT). We aim at detecting accreting protoplanet candidates through spectral angular differential imaging observations. The data analysis does not reveal any H$_{alpha}$ signal around the target. The derived contrast curve in the B_Ha filter allows us to derive a 5$sigma$ upper limit of $sim$7.6 mag at 111 mas, the separation of the previously detected planet candidate. This contrast translates into a H$_{alpha}$ line luminosity of $L_{rm H_{alpha}}lesssim$ 5$times$10$^{-5}$ $L_{odot}$ at 111 mas, and an accretion luminosity of $L_{acc} <$3.7$times$10$^{-4},L_{odot}$. For the predicted mass range of MWC758b, 0.5-5 $M_{rm Jup}$, this implies accretion rates of $dot M lesssim$ 3.4$times$(10$^{-8}$-10$^{-9}),M_{odot}/yr$, for an average planet radius of 1.1 $R_{rm Jup}$. Therefore, our estimates are consistent with the predictions of accreting circumplanetary accretion models for $R_{rm in} = 1 R_{rm Jup}$. In any case, the non-detection of any H$_{alpha}$ emitting source in the ZIMPOL images does not allow us to unveil the true nature of the $L$ detected source.
We analyze high angular resolution ALMA observations of the TW Hya disk to place constraints on the CO and dust properties. We present new, sensitive observations of the $^{12}$CO $J = 3-2$ line at a spatial resolution of 8 AU (0farcs14). The CO emission exhibits a bright inner core, a shoulder at $rapprox70$ AU, and a prominent break in slope at $rapprox90$ AU. Radiative transfer modeling is used to demonstrate that the emission morphology can be reasonably reproduced with a $^{12}$CO column density profile featuring a steep decrease at $rapprox15$ AU and a secondary bump peaking at $rapprox70$ AU. Similar features have been identified in observations of rarer CO isotopologues, which trace heights closer to the midplane. Substructure in the underlying gas distribution or radially varying CO depletion that affects much of the disks vertical extent may explain the shared emission features of the main CO isotopologues. We also combine archival 1.3 mm and 870 $mu$m continuum observations to produce a spectral index map at a spatial resolution of 2 AU. The spectral index rises sharply at the continuum emission gaps at radii of 25, 41, and 47 AU. This behavior suggests that the grains within the gaps are no larger than a few millimeters. Outside the continuum gaps, the low spectral index values of $alphaapprox 2$ indicate either that grains up to centimeter size are present, or that the bright continuum rings are marginally optically thick at millimeter wavelengths.
We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study the photosphere, the warm molecular layer, and the inner wind of the close-by oxygen-rich AGB star R Doradus. We present observations in filters V, cntH$alpha$, and cnt820 and investigate the surface brightness distribution of the star and of the polarised light produced in the inner envelope. Thanks to second-epoch observations in cntH$alpha$, we are able to see variability on the stellar photosphere. We find that in the first epoch the surface brightness of R Dor is asymmetric in V and cntH$alpha$, the filters where molecular opacity is stronger, while in cnt820 the surface brightness is closer to being axisymmetric. The second-epoch observations in cntH$alpha$ show that the morphology of R Dor changes completely in a timespan of 48 days to a more axisymmetric and compact configuration. The polarised intensity is asymmetric in all epochs and varies by between a factor of 2.3 and 3.7 with azimuth for the different images. We fit the radial profile of the polarised intensity using a spherically symmetric model and a parametric description of the dust density profile, $rho(r)=rho_circ r^{-n}$. On average, we find exponents of $- 4.5 pm 0.5$ that correspond to a much steeper density profile than that of a wind expanding at constant velocity. The dust densities we derive imply an upper limit for the dust-to-gas ratio of $sim 2times10^{-4}$ at 5.0 $R_star$. Given the uncertainties in observations and models, this value is consistent with the minimum values required by wind-driving models for the onset of a wind, of $sim 3.3times10^{-4}$. However, if the steep density profile we find extends to larger distances from the star, the dust-to-gas ratio will quickly become too small for the wind of R Dor to be driven by the grains that produce the scattered light.
We describe the Zurich Imaging Polarimeter (ZIMPOL), the visual focal plane subsystem of the SPHERE VLT planet finder, which pushes the limits of current AO systems to shorter wavelengths, higher spatial resolution, and much improved polarimetric performance. We provide new benchmarks for the performance of high contrast instruments, in particular for polarimetric differential imaging. We have analyzed SPHERE/ZIMPOL point spread functions and measure the peak surface brightness, the encircled energy, and the full width half maximum (FWHM) for different wavelengths, atmospheric conditions, star brightness, and instrument modes. Coronagraphic images are described and analized and the performance for different coronagraphs is compared with tests for the binary alpha Hyi with a separation of 92 mas and a contrast of 6 mag. For the polarimetric mode we made the instrument calibrations using zero polarization and high polarization standard stars and here we give a recipe for the absolute calibration of polarimetric data. The data show a small <1 mas but disturbing differential polarimetric beam shifts, which can be explained as Goos-Hahnchen shifts from the inclined mirrors, and we discuss how to correct this effect. The polarimetric sensitivity is investigated with non-coronagraphic and deep, coronagraphic observations of the dust scattering around the symbiotic Mira variable R Aqr. SPHERE/ZIMPOL achieves imaging performances in the visual range with unprecedented characteristics, in particular very high spatial resolution and very high polarimetric contrast. This instrument opens up many new research opportunities for the detailed investigation of circumstellar dust, in scattered and therefore polarized light, for the investigation of faint companions, and for the mapping of circumstellar Halpha emission.
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