| 1. |
- Andreoni, I., et al.
(författare)
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Follow Up of GW170817 and Its Electromagnetic Counterpart by Australian-Led Observing Programmes
- 2017
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Ingår i: Publications Astronomical Society of Australia. - : Cambridge University Press (CUP). - 1323-3580 .- 1448-6083. ; 34
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Forskningsöversikt (refereegranskat)abstract
- The discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. In this paper, we present follow-up observations of the gravitational wave event GW170817 and its electromagnetic counterpart SSS17a/DLT17ck (IAU label AT2017gfo) by 14 Australian telescopes and partner observatories as part of Australian-based and Australian-led research programs. We report early- to late-time multi-wavelength observations, including optical imaging and spectroscopy, mid-infrared imaging, radio imaging, and searches for fast radio bursts. Our optical spectra reveal that the transient source emission cooled from approximately 6 400 K to 2 100 K over a 7-d period and produced no significant optical emission lines. The spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. Star formation in the host galaxy probably ceased at least a Gyr ago, although there is evidence for a galaxy merger. Binary pulsars with short (100 Myr) decay times are therefore unlikely progenitors, but pulsars like PSR B1534+12 with its 2.7 Gyr coalescence time could produce such a merger. The displacement (similar to 2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor.
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| 2. |
- Brandeker, Alexis, et al.
(författare)
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Contrast sensitivities in the Gaia Data Release 2
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 621
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Tidskriftsartikel (refereegranskat)abstract
- The source detection sensitivity of Gaia is reduced near sources. To characterise this contrast sensitivity is important for understanding the completeness of the Gaia data products, in particular when evaluating source confusion in lower resolution surveys such as photometric monitoring for transits. Here, we statistically evaluate the catalogue source density to determine the Gaia Data Release 2 source detection sensitivity as a function of angular separation and brightness ratio from a bright source. The contrast sensitivity from similar to 0.4 '' out to 12 '' ranges in Delta G = 0-14 mag. We find the derived contrast sensitivity to be robust with respect to target brightness, colour, source density, and Gaia scan coverage.
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| 3. |
- Brandeker, Alexis, et al.
(författare)
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Herschel detects oxygen in the beta Pictoris debris disk
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 591
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Tidskriftsartikel (refereegranskat)abstract
- The young star beta Pictoris is well known for its dusty debris disk produced through collisional grinding of planetesimals, kilometre-sized bodies in orbit around the star. In addition to dust, small amounts of gas are also known to orbit the star; this gas is likely the result of vaporisation of violently colliding dust grains. The disk is seen edge on and from previous absorption spectroscopy we know that the gas is very rich in carbon relative to other elements. The oxygen content has been more difficult to assess, however, with early estimates finding very little oxygen in the gas at a C/O ratio that is 20x higher than the cosmic value. A C/O ratio that high is difficult to explain and would have far-reaching consequences for planet formation. Here we report on observations by the far-infrared space telescope Herschel, using PACS, of emission lines from ionised carbon and neutral oxygen. The detected emission from C+ is consistent with that previously reported observed by the HIFI instrument on Herschel, while the emission from O is hard to explain without assuming a higher density region in the disk, perhaps in the shape of a clump or a dense torus required to sufficiently excite the O atoms. A possible scenario is that the C/O gas is produced by the same process responsible for the CO clump recently observed by the Atacama Large Millimeter/submillimeter Array in the disk and that the redistribution of the gas takes longer than previously assumed. A more detailed estimate of the C/O ratio and the mass of O will have to await better constraints on the C/O gas spatial distribution.
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| 4. |
- Cataldi, Gianni, et al.
(författare)
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ALMA Resolves CI Emission from the beta Pictoris Debris Disk
- 2018
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 861:1
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Tidskriftsartikel (refereegranskat)abstract
- The debris disk around beta Pictoris is known to contain gas. Previous ALMA observations revealed a CO belt at similar to 85 au with a distinct clump, interpreted as a location of enhanced gas production. Photodissociation converts CO into C and O within similar to 50 a. We resolve C I emission at 492 GHz using ALMA and study its spatial distribution. C I shows the same clump as seen for CO. This is surprising, as C is expected to quickly spread in azimuth. We derive a low C mass (between 5 x 10(-4) and 3.1 x 10(-3) MA(circle plus)), indicating that gas production started only recently (within similar to 5000 a). No evidence is seen for an atomic accretion disk inward of the CO belt, perhaps because the gas did not yet have time to spread radially. The fact that C and CO share the same asymmetry argues against a previously proposed scenario where the clump is due to an outward-migrating planet trapping planetesimals in a resonance, nor can the observations be explained by an eccentric planetesimal belt secularly forced by a planet. Instead, we suggest that the dust and gas disks should be eccentric. Such a configuration, we further speculate, might be produced by a recent tidal disruption event. Assuming that the disrupted body has had a CO mass fraction of 10%, its total mass would be greater than or similar to 3M(Moon).
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| 5. |
- Cataldi, Gianni, et al.
(författare)
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Constraints on the gas content of the Fomalhaut debris belt Can gas-dust interactions explain the belt's morphology?
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574
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Tidskriftsartikel (refereegranskat)abstract
- Context. The 440 Myr old main-sequence A-star Fomalhaut is surrounded by an eccentric debris belt with sharp edges. This sort of a morphology is usually attributed to planetary perturbations, but the orbit of the only planetary candidate detected so far, Fomalhaut b, is too eccentric to efficiently shape the belt. Alternative models that could account for the morphology without invoking a planet are stellar encounters and gas-dust interactions. Aims. We aim to test the possibility of gas-dust interactions as the origin of the observed morphology by putting upper limits on the total gas content of the Fomalhaut belt. Methods. We derive upper limits on the CII 158 mu m and 01 63 pint emission by using non detections from the Photocletector Array Camera and Spectrometer (PACS) onboard the Herschel Space Observatory. Line fluxes are converted into total gas mass using the non-local thermodynamic equilibrium (non-LTE) code RADEX. We consider two different cases for the elemental abundances of the gas: solar abundances and abundances similar to those observed for the gas in the beta Pictoris debris disc. Results. The gas mass is shown to be below the millimetre dust mass by a factor of at least similar to 3 (for solar abundances) respectively similar to 300 (for beta Pic-like abundances). Conclusions. The lack of gas co-spatial with the dust implies that gas-dust interactions cannot efficiently shape the Fomalhaut debris belt. The morphology is therefore more likely due to a yet unseen planet (Fomalhaut c) or stellar encounters.
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| 6. |
- Cataldi, Gianni, 1986-
(författare)
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Debris disks and the search for life in the universe
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Circumstellar debris disks are the extrasolar analogues of the asteroid belt and the Kuiper belt. These disks consist of comets and leftover planetesimals that continuously collide to produce copious amounts of circumstellar dust that can be observed as infrared excess or in resolved imaging. As an obvious outcome of the planet formation process, debris disks can help us constrain planet formation theories and learn about the history of our own solar system. Structures in the disks such as gaps or warps can hint at the presence of planets. Thus, the study of debris disks is an important branch of exoplanetary science. In this thesis, some aspects of debris disks are considered in detail.A handful of debris disks show observable amounts of gas besides the dust. One such case is the edge-on debris disk around the young A-type star β Pictoris, where the gas is thought to be of secondary origin, i.e. derived from the dust itself. By observing this gas, we can thus learn something about the dust, and therefore about the building blocks of planets. In paper I, spectrally resolved observations of C II emission with Herschel/HIFI are presented. The line profile is used to constrain the spatial distribution of carbon gas in the disk, which helps understanding the gas producing mechanism. In paper II, we analyse C II and O I emission detected with Herschel/PACS and find that the oxygen must be located in a relatively dense region, possibly similar to the CO clump seen by ALMA. An upcoming analysis of our ALMA C I observations will give us a clearer picture of the system.Another famous debris disk is found around the nearby, 440 Myr old A-star Fomalhaut. Its morphology is that of an eccentric debris belt with sharp edges, suggesting shaping by a planet. However, gas-dust interactions may result in a similar morphology without the need to invoke planets. We test this possibility in paper III by analysing non-detections of C II and O I emission by Herschel/PACS. We find that there is not enough gas present to efficiently sustain gas-dust interactions, implying that the morphology of the Fomalhaut belt is due to a yet unseen planet or alternatively stellar encounters.One of the biggest challenges in exoplanetary research is to answer the question whether there are inhabited worlds other than the Earth. With the number of known rocky exoplanets in the habitable zone increasing rapidly, we might actually be able to answer this question in the coming decades. Different approaches exist to detect the presence of life remotely, for example by studying exoplanetary atmospheres or by analysing light reflected off the surface of an exoplanet. In paper IV, we study whether biosignatures (for example, certain minerals or microorganisms) ejected into a circumstellar debris disk by an impact event could be detected. We consider an impact similar to the Chicxulub event and model the collisional evolution of the ejected debris. Dust from such an event can potentially be detected by current telescopes, but analysis of the debris composition has to wait for future, advanced instruments.
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| 7. |
- Cataldi, Gianni, et al.
(författare)
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Searching for Biosignatures in Exoplanetary Impact Ejecta
- 2017
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Ingår i: Astrobiology. - : Mary Ann Liebert Inc. - 1531-1074 .- 1557-8070. ; 17:8, s. 721-746
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Tidskriftsartikel (refereegranskat)abstract
- With the number of confirmed rocky exoplanets increasing steadily, their characterization and the search for exoplanetary biospheres are becoming increasingly urgent issues in astrobiology. To date, most efforts have concentrated on the study of exoplanetary atmospheres. Instead, we aim to investigate the possibility of characterizing an exoplanet (in terms of habitability, geology, presence of life, etc.) by studying material ejected from the surface during an impact event. For a number of impact scenarios, we estimate the escaping mass and assess its subsequent collisional evolution in a circumstellar orbit, assuming a Sun-like host star. We calculate the fractional luminosity of the dust as a function of time after the impact event and study its detectability with current and future instrumentation. We consider the possibility to constrain the dust composition, giving information on the geology or the presence of a biosphere. As examples, we investigate whether calcite, silica, or ejected microorganisms could be detected. For a 20km diameter impactor, we find that the dust mass escaping the exoplanet is roughly comparable to the zodiacal dust, depending on the exoplanet's size. The collisional evolution is best modeled by considering two independent dust populations, a spalled population consisting of nonmelted ejecta evolving on timescales of millions of years, and dust recondensed from melt or vapor evolving on much shorter timescales. While the presence of dust can potentially be inferred with current telescopes, studying its composition requires advanced instrumentation not yet available. The direct detection of biological matter turns out to be extremely challenging. Despite considerable difficulties (small dust masses, noise such as exozodiacal dust, etc.), studying dusty material ejected from an exoplanetary surface might become an interesting complement to atmospheric studies in the future.
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| 8. |
- Cavallius, Maria, et al.
(författare)
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Upper limits on the water vapour content of the beta Pictoris debris disk
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 628
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Tidskriftsartikel (refereegranskat)abstract
- Context. The debris disk surrounding beta Pictoris has been observed with ALMA to contain a belt of CO gas with a distinct peak at similar to 85 au. This CO clump is thought to be the result of a region of enhanced density of solids that collide and release CO through vaporisation. The parent bodies are thought to be comparable to solar system comets, in which CO is trapped inside a water ice matrix. Aims. Since H2O should be released along with CO, we aim to put an upper limit on the H2O gas mass in the disk of beta Pictoris. Methods. We used archival data from the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory to study the ortho-H2O 1(10)-1(01) emission line. The line is undetected. Using a python implementation of the radiative transfer code RADEX, we converted upper limits on the line flux to H2O gas masses. The resulting lower limits on the CO/H2O mass ratio are compared to the composition of solar system comets. Results. Depending on the assumed gas spatial distribution, we find a 95% upper limit on the ortho-H2O line flux of 7.5 x 10(-20) W m(-2) or 1.2 x 10(-19) W m(-2). These translate into an upper limit on the H2O mass of 7.4 x 10(16)-1.1 x 10(18) kg depending on both the electron density and gas kinetic temperature. The range of derived gas-phase CO/H2O ratios is marginally consistent with low-ratio solar system comets.
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| 9. |
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| 10. |
- Matthews, Brenda C., et al.
(författare)
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THE AU MIC DEBRIS DISK : FAR-INFRARED AND SUBMILLIMETER RESOLVED IMAGING
- 2015
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 811:2
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Tidskriftsartikel (refereegranskat)abstract
- We present far-infrared and submillimeter maps from the Herschel Space Observatory and the James Clerk Maxwell Telescope of the debris disk host star AU Microscopii. Disk emission is detected at 70, 160, 250, 350, 450, 500, and 850 mu m. The disk is resolved at 70, 160, and 450 mu m. In addition to the planetesimal belt, we detect thermal emission from AU Mic's halo for the first time. In contrast to the scattered light images, no asymmetries are evident in the disk. The fractional luminosity of the disk is 3.9 x 10(-4) and its milimeter-grain dust mass is 0.01 M-circle dot (+/- 20%). We create a simple spatial model that reconciles the disk spectral energy distribution as a blackbody of 53 +/- 2K (a composite of 39 and 50 K components) and the presence of small (non-blackbody) grains which populate the extended halo. The best-fit model is consistent with the birth ring model explored in earlier works, i.e., an edge-on dust belt extending from 8.8 to 40 AU, but with an additional halo component with an r(-1.5) surface density profile extending to the limits of sensitivity (140 AU). We confirm that AU Mic does not exert enough radiation force to blow out grains. For stellar mass-loss rates of 10-100 times solar, compact (zero porosity) grains can only be removed if they are very small; consistently with previous work, if the porosity is 0.9, then grains approaching 0.1 mu m can be removed via corpuscular forces (i.e., the stellar wind).
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