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High-contrast imaging of low-mass companions and debris disks
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

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.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Astronomy, Stockholm University , 2019. , s. 74
Nyckelord [en]
Direct imaging, extrasolar planets, debris disks, planet formation, high angular resolution
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
URN: urn:nbn:se:su:diva-163784ISBN: 978-91-7797-558-8 (tryckt)ISBN: 978-91-7797-559-5 (digital)OAI: oai:DiVA.org:su-163784DiVA, id: diva2:1276753
Disputation
2019-03-15, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (Engelska)
Opponent
Handledare
Anmärkning

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: In press.

Tillgänglig från: 2019-02-20 Skapad: 2019-01-08 Senast uppdaterad: 2019-02-06Bibliografiskt granskad
Delarbeten
1. Polarimetry and flux distribution in the debris disk around HD 32297
Öppna denna publikation i ny flik eller fönster >>Polarimetry and flux distribution in the debris disk around HD 32297
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2016 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 593, artikel-id A73Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

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.

Nyckelord
protoplanetary disks, techniques: high angular resolution, stars: individual: HD 32297
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
urn:nbn:se:su:diva-136939 (URN)10.1051/0004-6361/201628543 (DOI)000385820100081 ()
Tillgänglig från: 2016-12-29 Skapad: 2016-12-19 Senast uppdaterad: 2019-01-21Bibliografiskt granskad
2. SPOTS: The Search for Planets Orbiting Two Stars III. Complete sample and statistical analysis
Öppna denna publikation i ny flik eller fönster >>SPOTS: The Search for Planets Orbiting Two Stars III. Complete sample and statistical analysis
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2018 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, artikel-id A43Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

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.

Nyckelord
instrumentation: high angular resolution, binaries: general, planets and satellites: gaseous planets, astrometry, brown dwarfs, stars: imaging
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
urn:nbn:se:su:diva-161910 (URN)10.1051/0004-6361/201833349 (DOI)000448959600001 ()
Tillgänglig från: 2018-11-13 Skapad: 2018-11-13 Senast uppdaterad: 2019-01-21Bibliografiskt granskad
3. Isochronal age-mass discrepancy of young stars: SCExAO/CHARIS integral field spectroscopy of the HIP 79124 triple system
Öppna denna publikation i ny flik eller fönster >>Isochronal age-mass discrepancy of young stars: SCExAO/CHARIS integral field spectroscopy of the HIP 79124 triple system
2019 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 622, artikel-id A42Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

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.

Nyckelord
Stars: pre-main sequence, Stars: low-mass, Techniques: imaging spectroscopy, Planets and satellites: detection
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Forskningsämne
astronomi
Identifikatorer
urn:nbn:se:su:diva-163467 (URN)10.1051/0004-6361/201834688 (DOI)000456679300005 ()
Tillgänglig från: 2019-01-05 Skapad: 2019-01-05 Senast uppdaterad: 2019-03-04Bibliografiskt granskad

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