| 1. |
- Cataldi, Gianni, et al.
(författare)
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The Surprisingly Low Carbon Mass in the Debris Disk around HD 32297
- 2020
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 892:2
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Tidskriftsartikel (refereegranskat)abstract
- Gas has been detected in a number of debris disks. It is likely secondary, i.e., produced by colliding solids. Here, we report ALMA Band 8 observations of neutral carbon in the CO-rich debris disk around the 15-30 Myr old A-type star HD 32297. We find that C-0 is located in a ring at similar to 110 au with an FWHM of similar to 80 au and has a mass of (3.5 0.2) x 10(-3) M-circle plus. Naively, such a surprisingly small mass can be accumulated from CO photodissociation in a time as short as similar to 10(4) yr. We develop a simple model for gas production and destruction in this system, properly accounting for CO self-shielding and shielding by neutral carbon, and introducing a removal mechanism for carbon gas. We find that the most likely scenario to explain both C-0 and CO observations is one where the carbon gas is rapidly removed on a timescale of order a thousand years and the system maintains a very high CO production rate of similar to 15 M-circle plus Myr(-1), much higher than the rate of dust grind-down. We propose a possible scenario to meet these peculiar conditions: the capture of carbon onto dust grains, followed by rapid CO re-formation and rerelease. In steady state, CO would continuously be recycled, producing a CO-rich gas ring that shows no appreciable spreading over time. This picture might be extended to explain other gas-rich debris disks.
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| 2. |
- Janson, Markus, et al.
(författare)
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The B-Star Exoplanet Abundance Study : a co-moving 16-25 M-Jup companion to the young binary system HIP 79098
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 626
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Tidskriftsartikel (refereegranskat)abstract
- Wide low-mass substellar companions are known to be very rare among low-mass stars, but appear to become increasingly common with increasing stellar mass. However, B-type stars, which are the most massive stars within similar to 150 pc of the Sun, have not yet been examined to the same extent as AFGKM-type stars in that regard. In order to address this issue, we launched the ongoing B-star Exoplanet Abundance Study (BEAST) to examine the frequency and properties of planets, brown dwarfs, and disks around B-type stars in the Scorpius-Centaurus (Sco-Cen) association; we also analyzed archival data of B-type stars in Sco-Cen. During this process, we identified a candidate substellar companion to the B9-type spectroscopic binary HIP 79098 AB, which we refer to as HIP 79098 (AB)b. The candidate had been previously reported in the literature, but was classified as a background contaminant on the basis of its peculiar colors. Here we demonstrate that the colors of HIP 79098 (AB)b are consistent with several recently discovered young and low-mass brown dwarfs, including other companions to stars in Sco-Cen. Furthermore, we show unambiguous common proper motion over a 15-yr baseline, robustly identifying HIP 79098 (AB)b as a bona fide substellar circumbinary companion at a 345 +/- 6 AU projected separation to the B9-type stellar pair. With a model-dependent mass of 16-25 M-Jup yielding a mass ratio of <1%, HIP 79098 (AB)b joins a growing number of substellar companions with planet-like mass ratios around massive stars. Our observations underline the importance of common proper motion analysis in the identification of physical companionship, and imply that additional companions could potentially remain hidden in the archives of purely photometric surveys.
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| 3. |
- Lawson, Kellen, et al.
(författare)
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SCExAO/CHARIS Near-infrared Integral Field Spectroscopy of the HD 15115 Debris Disk
- 2020
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 0004-6256 .- 1538-3881. ; 160:4
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Tidskriftsartikel (refereegranskat)abstract
- We present new, near-infrared (1.1-2.4 mu m) high-contrast imaging of the debris disk around HD 15115 with the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system coupled with the Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS). The SCExAO/CHARIS resolves the disk down to rho similar to 02 (r(proj) similar to 10 au), a factor of similar to 3-5 smaller than previous recent studies. We derive a disk position angle of PA similar to 2794-2805 and an inclination ofi similar to 853-86.2. While recent SPHERE/IRDIS imagery of the system could suggest a significantly misaligned two-ring disk geometry, CHARIS imagery does not reveal conclusive evidence for this hypothesis. Moreover, optimizing models of both one- and two-ring geometries using differential evolution, we find that a single ring having a Hong-like scattering phase function matches the data equally well within the CHARIS field of view (rho less than or similar to 1 ''). The disk's asymmetry, well evidenced at larger separations, is also recovered; the west side of the disk appears, on average, around 0.4 mag brighter across the CHARIS bandpass between 025 and 1 ''. Comparing Space Telescope Imaging Spectrograph (STIS) 50CCD optical photometry (2000-10500 A) with CHARIS near-infrared photometry, we find a red (STIS/50CCD-CHARIS broadband) color for both sides of the disk throughout the 04-1 '' region of overlap, in contrast to the blue color reported at similar wavelengths for regions exterior to similar to 2 ''. Further, this color may suggest a smaller minimum grain size than previously estimated at larger separations. Finally, we provide constraints on planetary companions and discuss possible mechanisms for the observed inner disk flux asymmetry and color.
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| 4. |
- Asensio Torres, Rubén, 1990-
(författare)
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High-contrast imaging of faint substellar companions and debris disks
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Star formation results in the accumulation of circumstellar material orbiting around their host stars, such as debris disks and planetary-mass objects. A relatively modern technique that has shown to be very valu- able for studying these faint companions is high-contrast imaging, which allows to distinguish the light emitted or scattered off these objects, otherwise hidden behind the brighter stellar halo. In this Licentiate thesis we review the high-contrast imaging technique and its capabilities, giving examples of the latest achievements reached by this method. The star and planet formation scenario within young and gas-rich protoplanetary disks is also briefly discussed. We show that direct imaging observations constrain the protoplanetary disk evolution and the different planet formation processes. We also discuss the debris disk formation scenario and how direct imaging observations can help to understand their morphology and composition. The presence of planetary-mass companions within the disk can be revealed directly via high-contrast imaging or otherwise inferred from their interactions with the debris disk. Finally, we present a recent result on the polarimetry and the flux distribution in the debris disk around the HD 32297 star, which appears to be dominated by micron-sized dust particles and might have indications of a double ring structure (Paper I).
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| 5. |
- Asensio Torres, Ruben, 1990-
(författare)
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High-contrast imaging of low-mass companions and debris disks
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The search for exoplanets, i.e., planets orbiting other stars than the Sun, is a relatively new research field, but has already established itself as one of the most prolific and intriguing areas of astronomy. By now we are in a situation where the focus is not only on finding companions to stars, but also on characterising their atmospheres and physical properties, which overall allows us to put our Solar System into context. In the near future, these efforts could potentially lead to the first confirmation of a life-bearing planet besides the Earth. The great majority of these exoplanet studies have been carried out indirectly, where the presence and characterisation of the companions are inferred solely from the observation of the host star. In the last decade, however, high-contrast direct imaging has been continuously developed to get rid of the starlight and reveal the existence of low-mass companions. Although this technique is currently limited to giant planets orbiting at large separations, it is able to directly detect the light emitted or scattered off the planet’s atmosphere at high signal to noise, which makes it the most promising planet-hunting method to characterise new worlds. Moreover, its capability to image faint objects close to the parent star allows for not only the detection of planetary-mass companions, but also low-mass stars, brown dwarfs, and circumstellar disks where planet formation takes place. This opens up a broad range of science cases where direct observations can be used to understand planet formation, atmospheric physics and stellar evolution. In this PhD thesis I provide an up-to-date introduction to the basis of the direct imaging technique, and explain the star and planet formation mechanisms. Three publications are attached to this introduction, each of them dealing with distinct science cases that can be assessed with high-contrast observations. In Paper I we resolve and model the aftermath of star formation, the so-called debris disk phase analogue to the asteroid and Kuiper belts in our Solar System, around the HD 32297 star with Subaru/HiCIAO. We reveal an edge-on disk and find the first indications of a double-ring scenario. We also present the first polarimetric study of this system, constraining the properties of the dust around the star. In Paper II we focus on the planetary-mass regime, and conduct the first direct imaging survey searching for circumbinary planets orbiting tight binary systems (SPOTS: Search for Planets Orbiting Two Stars). We present the results of the observations of 62 targets with VLT/NaCo and VLT/SPHERE, and perform a statistical analysis on the findings, placing constraints on the population of giant planets and brown dwarfs on wide orbits. Finally, in Paper III we resolve a triple stellar system with the newly-commissioned SCExAO/CHARIS integral field spectrograph. Taking advantage of the coeval nature of the system and the different range of masses involved, we use the data to reaffirm a previously suggested isochronal age discrepancy between the low- and the intermediate-mass population of stars.
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| 6. |
- Asensio-Torres, Ruben, et al.
(författare)
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Isochronal age-mass discrepancy of young stars : SCExAO/CHARIS integral field spectroscopy of the HIP 79124 triple system
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 622
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Tidskriftsartikel (refereegranskat)abstract
- We present SCExAO/CHARIS 1.1--2.4 micron integral field direct spectroscopy of the young HIP 79124 triple system. HIP 79124 is a member of the Scorpius-Centaurus association, consisting of an A0V primary with two low-mass companions at a projected separation of <1 arcsecond. Thanks to the high quality wavefront corrections provided by SCExAO, both companions are decisively detected without the employment of any PSF-subtraction algorithm to eliminate quasi-static noise. The spectrum of the outer C object is very well matched by Upper Scorpius M4 pm 0.5 standard spectra, with a Teff = 2945 pm 100 and a mass of 350 MJup. HIP 79124 B is detected at a separation of only 180 mas in a highly-correlated noise regime, and it falls in the spectral range M6 pm 0.5 with Teff = 2840 pm 190 and 100 MJup. Previous studies of stellar populations in Sco-Cen have highlighted a discrepancy in isochronal ages between the lower-mass and higher-mass populations. This could be explained either by an age spread in the region, or by conventional isochronal models failing to reproduce the evolution of low-mass stars. The HIP 79124 system should be coeval, and therefore it provides an ideal laboratory to test these scenarios. We place the three components in a color-magnitude diagram and find that the models predict a younger age for the two low-mass companions (3 Myr) than for the primary star (6 Myr). These results imply that the omission of magnetic effects in conventional isochronal models inhibit them from reproducing early low-mass stellar evolution, which is further supported by the fact that new models that include such effects provide more consistent ages in the HIP 79124 system.
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| 7. |
- Asensio-Torres, Ruben, et al.
(författare)
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Polarimetry and flux distribution in the debris disk around HD 32297
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 593
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Tidskriftsartikel (refereegranskat)abstract
- We present high-contrast angular differential imaging (ADI) observations of the debris disk around HD32297 in H-band, as well as the first polarimetric images for this system in polarized differential imaging (PDI) mode with Subaru/HICIAO. In ADI, we detect the nearly edge-on disk at > 5 sigma levels from similar to 0.45 '' to similar to 1.7 '' (50-192AU) from the star and recover the spine deviation from the midplane already found in previous works. We also find for the first time imaging and surface brightness (SB) indications for the presence of a gapped structure on both sides of the disk at distances of similar to 0.75 '' (NE side) and similar to 0.65 '' (SW side). Global forward-modelling work delivers a best-fit model disk and well-fitting parameter intervals that essentially match previous results, with high-forward scattering grains and a ring located at 110AU. However, this single ring model cannot account for the gapped structure seen in our SB profiles. We create simple double ring models and achieve a satisfactory fit with two rings located at 60 and 95AU, respectively, low-forward scattering grains and very sharp inner slopes. In polarized light we retrieve the disk extending from similar to 0.25-1.6 '', although the central region is quite noisy and high S/N are only found in the range similar to 0.75-1.2 ''. The disk is polarized in the azimuthal direction, as expected, and the departure from the midplane is also clearly observed. Evidence for a gapped scenario is not found in the PDI data. We obtain a linear polarization degree of the grains that increases from similar to 10% at 0.55 '' to similar to 25% at 1.6 ''. The maximum is found at scattering angles of similar to 90 degrees, either from the main components of the disk or from dust grains blown out to larger radii.
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| 8. |
- Asensio-Torres, Ruben, et al.
(författare)
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SPOTS : The Search for Planets Orbiting Two Stars III. Complete sample and statistical analysis
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 619
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Tidskriftsartikel (refereegranskat)abstract
- Binary stars constitute a large percentage of the stellar population, yet relatively little is known about the planetary systems orbiting them. Most constraints on circumbinary planets (CBPs) so far come from transit observations with the Kepler telescope, which is sensitive to close-in exoplanets but does not constrain planets on wider orbits. However, with continuous developments in high-contrast imaging techniques, this population can now be addressed through direct imaging. We present the full survey results of the Search for Planets Orbiting Two Stars (SPOTS) survey, which is the first direct imaging survey targeting CBPs. The SPOTS observational program comprises 62 tight binaries that are young and nearby, and thus suitable for direct imaging studies, with VLT/NaCo and VLT/SPHERE. Results from SPOTS include the resolved circumbinary disk around AK Sco, the discovery of a low-mass stellar companion in a triple packed system, the relative astrometry of up to 9 resolved binaries, and possible indications of non-background planetary-mass candidates around HIP 77911. We did not find any CBP within 300 AU, which implies a frequency upper limit on CBPs (1-15 M-Jup) of 6-10% between 30-300 AU. Coupling these observations with an archival dataset for a total of 163 stellar pairs, we find a best-fit CBP frequency of 1.9% (2-15 M-Jup) between 1 and 300 AU with a 10.5% upper limit at a 95% confidence level. This result is consistent with the distribution of companions around single stars.
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| 9. |
- Calissendorff, Per, et al.
(författare)
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Spectral characterization of newly detected young substellar binaries with SINFONI
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 627
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Tidskriftsartikel (refereegranskat)abstract
- We observe 14 young low-mass substellar objects using the VLT/SINFONI integral field spectrograph with laser guide star adaptive optics to detect and characterize three candidate binary systems. All three binary candidates show strong signs of youth, with two of them likely belonging to young moving groups. Together with the adopted young-moving-group ages we employ isochrones from the BT-Settle CIFIST substellar evolutionary models to estimate individual masses for the binary components. We find 2MASS J15104786-2818174 to be part of the approximate to 30-50 Myr Argus moving group and to be composed of a 34-48 M-Jup primary brown dwarf with spectral type M9 gamma and a fainter 15 22 MJup companion, separated by approximate to 100 mas. 2MASS J22025794-5605087 is identified as an almost equal-mass binary in the AB Dor moving group, with a projected separation of approximate to 60 mas. Both components share spectral type M9 gamma/beta, which with the adopted age of 120 200 Myr yields masses in the range of 50 68 M-Jup for each component individually. The observations of 2MASS J15474719-2423493 are of lower quality and we obtain no spectral characterization for the target, but resolve two components separated by approximate to 170 mas which with the predicted young field age of 30 50 Myr yields individual masses below 20 M-Jup. Out of the three candidate binary systems, 2MASS J22025794-5605087 has unambiguous spectroscopic signs of being a bona-fide binary, while the other two will require second-epoch confirmation. The small projected separations between the binary components correspond to physical separations of approximate to 4-7AU, meaning that astrometric monitoring of just a few years would be adequate to generate constrained orbital fits and dynamical masses for the systems. In combination with their young ages, these binaries will prove to be excellent benchmarks for calibrating substellar evolutionary models down to a very low-mass regime.
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| 10. |
- Currie, Thayne, et al.
(författare)
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No Clear, Direct Evidence for Multiple Protoplanets Orbiting LkCa 15 : LkCa 15 bcd are Likely Inner Disk Signals
- 2019
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 877:1
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Tidskriftsartikel (refereegranskat)abstract
- Two studies utilizing sparse aperture-masking (SAM) interferometry and H-alpha differential imaging have reported multiple Jovian companions around the young solar-mass star, LkCa 15 (LkCa 15 bcd): the first claimed direct detection of infant, newly formed planets (protoplanets). We present new near-infrared direct imaging/spectroscopy from the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system coupled with Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS) integral field spectrograph and multi-epoch thermal infrared imaging from Keck/NIRC2 of LkCa 15 at high Strehl ratios. These data provide the first direct imaging look at the same wavelengths and in the same locations where previous studies identified the LkCa 15 protoplanets, and thus offer the first decisive test of their existence. The data do not reveal these planets. Instead, we resolve extended emission tracing a dust disk with a brightness and location comparable to that claimed for LkCa 15 bcd. Forward-models attributing this signal to orbiting planets are inconsistent with the combined SCExAO/CHARIS and Keck/NIRC2 data. An inner disk provides a more compelling explanation for the SAM detections and perhaps also the claimed H-alpha detection of LkCa 15 b. We conclude that there is currently no clear, direct evidence for multiple protoplanets orbiting LkCa 15, although the system likely contains at least one unseen Jovian companion. To identify Jovian companions around LkCa 15 from future observations, the inner disk should be detected and its effect modeled, removed, and shown to be distinguishable from planets. Protoplanet candidates identified from similar systems should likewise be clearly distinguished from disk emission through modeling.
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