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  • 1. Bacchus, E.
    et al.
    Parry, I. R.
    Oppenheimer, R.
    Aguilar, J.
    Beichman, C.
    Brenner, D.
    Burruss, R.
    Cady, E.
    Luszcz-Cook, S.
    Crepp, J.
    Dekany, R.
    Gianninas, A.
    Hillenbrand, L.
    Kilic, M.
    King, D.
    Lockhart, T. G.
    Matthews, C. T.
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy. American Museum of Natural History, USA; California Institute of Technology, USA.
    Pueyo, L.
    Rice, E. L.
    Roberts, L. C.
    Sivaramakrishnan, A.
    Soummer, R.
    Vasisht, G.
    Veicht, A.
    Zhai, C.
    Zimmerman, N. T.
    Project 1640 observations of the white dwarf HD 114174 B2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, no 4, p. 4796-4805Article in journal (Refereed)
    Abstract [en]

    We present the first near infrared spectrum of the faint white dwarf companion HD 114174 B, obtained with Project 1640. Our spectrum, covering the Y, J and H bands, combined with previous TaRgetting bENchmark-objects with Doppler Spectroscopy (TRENDS) photometry measurements, allows us to place further constraints on this companion. We suggest two possible scenarios; either this object is an old, low-mass, cool H atmosphere white dwarf with T-eff similar to 3800 K or a high-mass white dwarf with T-eff > 6000 K, potentially with an associated cool (T-eff similar to 700 K) brown dwarf or debris disc resulting in an infrared excess in the L' band. We also provide an additional astrometry point for 2014 June 12 and use the modelled companion mass combined with the radial velocity and direct imaging data to place constraints on the orbital parameters for this companion.

  • 2. Boettcher, Thomas
    et al.
    Huber, Liliane
    Le Corre, Lucille
    Leitner, Johannes
    McCarthy, David
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy.
    Teixeira, Carlos
    Vaquer Araujo, Sergi
    Wilson, Rebecca C.
    Adjali, Fatah
    Altenburg, Martin
    Briani, Giacomo
    Buchas, Peter
    Le Postollec, Aurelie
    Meier, Teresa
    The HADES mission concept: astrobiological survey of Jupiter's icy moon Europa2009In: International Journal of Astrobiology, ISSN 1473-5504, E-ISSN 1475-3006, Vol. 8, no 4, p. 321-329Article in journal (Refereed)
    Abstract [en]

    The HADES Europa mission concept aims to provide a framework for an astrobiological in-depth investigation of the Jupiter moon Europa, relying on existing technologies and feasibility. This mission study proposes a system consisting of an orbiter, lander and cryobot as a platform for detailed exploration of Europa. While the orbiter will investigate the presence of a liquid ocean and characterize Europa's internal structure, the lander will survey local dynamics of the ice layer and the surface environment. The lander releases a cryobot, that melts into the ice, will sample the pristine subsurface and is expected to provide data on organic and gaseous content and putative bio-signatures. In summary, we present the scientific objectives for an astrobiological investigation of Europa, resulting in a mission concept with a detailed evaluation of scientific instrumentation, mission sequences, basic design of the spacecraft, technology needs and cost estimations.

  • 3. Crepp, Justin R.
    et al.
    Rice, Emily L.
    Veicht, Aaron
    Aguilar, Jonathan
    Pueyo, Laurent
    Giorla, Paige
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy.
    Luszcz-Cook, Statia H.
    Oppenheimer, Rebecca
    Hinkley, Sasha
    Brenner, Douglas
    Vasisht, Gautam
    Cady, Eric
    Beichman, Charles A.
    Hillenbrand, Lynne A.
    Lockhart, Thomas
    Matthews, Christopher T.
    Roberts, Lewis C., Jr.
    Sivaramakrishnan, Anand
    Soummer, Remi
    Zhai, Chengxing
    DIRECT SPECTRUM OF THE BENCHMARK T DWARF HD 19467 B2015In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 798, no 2, article id L43Article in journal (Refereed)
    Abstract [en]

    HD 19467 B is presently the only directly imaged T dwarf companion known to induce a measurable Doppler acceleration around a solar-type star. We present spectroscopy measurements of this important benchmark object taken with the Project 1640 integral field unit at Palomar Observatory. Our high-contrast R approximate to 30 observations obtained simultaneously across the JH bands confirm the cold nature of the companion as reported from the discovery article and determine its spectral type for the first time. Fitting the measured spectral energy distribution to SpeX/IRTF T dwarf standards and synthetic spectra from BT-Settl atmospheric models, we find that HD 19467 B is a T5.5 +/- 1 dwarf with effective temperature T-eff = 978(-43)(+20) K. Our observations reveal significant methane absorption affirming its substellar nature. HD 19467 B shows promise to become the first T dwarf that simultaneously reveals its mass, age, and metallicity independent from the spectrum of light that it emits.

  • 4. Liseau, Rene
    et al.
    Risacher, Christophe
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Eiroa, Carlos
    Fridlund, Malcolm
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pilbratt, Göran L.
    Thébault, Philippe
    q1 Eridani: a solar-type star with a planet and a dust belt2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 480, no 3, p. L47-L50Article in journal (Refereed)
    Abstract [en]

    Context. Far-infrared excess emission from main-sequence stars is due to dust produced by orbiting minor bodies. In these disks, larger bodies, such as planets, may also be present and the understanding of their incidence and influence currently presents a challenge.

    Aims. Only very few solar-type stars exhibiting an infrared excess and harbouring planets are known to date. Indeed, merely a single case of a star-planet-disk system has previously been detected at submillimeter (submm) wavelengths. Consequently, one of our aims is to understand the reasons for these poor statistics, i.e., whether these results reflected the composition and/or the physics of the planetary disks or were simply due to observational bias and selection effects. Finding more examples would be very significant.

    Methods. The selected target, q(1) Eri, is a solar-type star, which was known to possess a planet, q(1) Eri b, and to exhibit excess emission at IRAS wavelengths, but had remained undetected in the millimeter regime. Therefore, submm flux densities would be needed to better constrain the physical characteristics of the planetary disk. Consequently, we performed submm imaging observations of q(1) Eri.

    Results. The detected dust toward q(1) Eri at 870 mu m exhibits the remarkable fact that the entire SED, from the IR to mm-wavelengths, is fit by a single-temperature blackbody function (60 K). This would imply that the emitting regions are confined to a narrow region (ring) at radial distances much larger than the orbital distance of q(1) Eri b, and that the emitting particles are considerably larger than some hundred micron. However, the 870 mu m source is extended, with a full-width-half-maximum of roughly 600AU. Therefore, a physically more compelling model also invokes a belt of cold dust (17 K), located at 300AU from the star and about 60AU wide.

    Conclusions. The minimum mass of 0.04 M-circle plus (3 M-Moon) of 1 mm-size icy ring-particles is considerable, given the stellar age of >= 1Gyr. These big grains form an inner edge at about 25 AU, which may suggest the presence of an unseen outer planet (q(1) Eri c).

  • 5.
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy.
    Circumstellar Debris Disks: Observational Studies of Cold Dust and Gas Emission in Planetary Nurseries2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Planetary systems form in disks of gas and dust surrounding newborn stars. The young circumstellar environment is characterized by frequent collisions between rocky bodies, leading to a continuous production of small dust grains. Such collisional processing persists in leftover debris disks or belts akin to the Solar System's asteroid and Kuiper belts, during the star's entire main-sequence lifetime. This thesis presents observations of thermal emission from cold dust in extended debris disks, in addition to spatially resolved observations of dust scattered light and gas emission in nearby debris disk systems. A total of 30 debris disk candidates identified from infrared excess were observed at submillimeter (submm) wavelengths with the Atacama Pathfinder EXperiment (APEX) telescope in search for dust at radial distances corresponding to the Kuiper belt or beyond. Two observing campaigns with the PolCor instrument at the Nordic Optical Telescope (NOT), performing optical polarimetric coronagraphy to resolve scattered light from nearby disks were also carried out. The edge-on debris disk surrounding the star β Pictoris was explored using integral-field spectroscopy with the Very Large Telescope (VLT) in an attempt to map the spatial distribution of previously detected gas. The APEX observations detected 14 exo-Kuiper belts, out of which 7 were new discoveries in the submm region. Modeling of the spectral energy distribution from available photometry and detected submm fluxes allowed us to study the dependence of the fractional dust luminosity and characteristic radial dust distance on stellar spectral type and age. The results indicate a decrease in fractional dust luminosity as t-α, where t is the age of the system and α = 0.8–2.0. From the VLT data we retrieved the first complete image of Ca II and Fe I emission in the disk of β Pictoris. Subsequent modeling demonstrated that the anomalous vertical structure of the observed Ca II emission can be explained by an optically thick disk midplane.

  • 6.
    Nilsson, Ricky
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Fathi, Kambiz
    Stockholm University, Faculty of Science, Department of Astronomy.
    Thébault, Philippe
    Liseau, René
    VLT imaging of the β Pictoris gas diskArticle in journal (Refereed)
  • 7.
    Nilsson, Ricky
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Liseau, René
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pilbratt, Göran
    Risacher, Christophe
    Rodmann, Jens
    Augereau, Jean-Charles
    Bergman, Per
    Eiroa, Carlos
    Fridlund, Malcolm
    Thébault, Philippe
    White, Glenn
    Kuiper belts around nearby stars2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 518, p. A40-Article in journal (Refereed)
    Abstract [en]

    Context. The existence of dusty debris disks around a large fraction of solar type main-sequence stars, inferred from excess far-IR and submillimetre emission compared to that expected from stellar photospheres, suggests that leftover planetesimal belts analogous to the asteroid-and comet reservoirs of the solar system are common.

    Aims. Sensitive submillimetre observations are essential to detect and characterise cold extended dust originating from collisions of small bodies in disks, belts, or rings at Kuiper-belt distances (30-50 AU or beyond). Measurements of the flux densities at these wavelengths will extend existing IR photometry and permit more detailed modelling of the Rayleigh-Jeans tail of the disks spectral energy distribution (SED), effectively constraining dust properties and disk extensions. By observing stars spanning from a few up to several hundred Myr, the evolution of debris disks during crucial phases of planet formation can be studied.

    Methods. We observed 22 exo-Kuiper-belt candidates at 870 mu m, as part of a large programme with the LABOCA bolometer at the APEX telescope. Dust masses (or upper limits) were calculated from integrated 870 mu m fluxes, and fits to the SED of detected sources revealed the fractional dust luminosities f(dust), dust temperatures T(dust), and power-law exponents beta of the opacity law.

    Results. A total of 10 detections with at least 3 sigma significance were made, out of which five (HD95086, HD131835, HD161868, HD170773, and HD207129) have previously never been detected at submillimetre wavelengths. Three additional sources are marginally detected with > 2.5 sigma significance. The best-fit beta parameters all lie between 0.1 and 0.8, in agreement with previous results indicating the presence of significantly larger grains than those in the ISM. From our relatively small sample we estimate f(dust) proportional to t(-alpha), with a similar to 0.8-2.0, and identify an evolution of the characteristic radial dust distance R(dust) that is consistent with the t(1/3) increase predicted from models of self-stirred collisions in debris disks.

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    Nilsson2010
  • 8.
    Nilsson, Ricky
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Liseau, René
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Risacher, Christophe
    Fridlund, Malcolm
    Pilbratt, Göran
    A submillimetre search for cold extended debris disks in the β Pictoris moving group2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 508, no 2, p. 1057-1065Article in journal (Refereed)
    Abstract [en]

    Context. Previous observations with the Infrared Astronomical Satellite and the Infrared Space Observatory, and ongoing observations with Spitzer and AKARI, have led to the discovery of over 200 debris disks, based on detected mid-and far infrared excess emission, indicating warm circumstellar dust. To constrain the properties of these systems, e.g., to more accurately determine the dust mass, temperature and radial extent, follow-up observations in the submillimetre wavelength region are needed.

    Aims. The beta Pictoris moving group is a nearby stellar association of young (similar to 12 Myr) co-moving stars including the classical debris disk star beta Pictoris. Due to their proximity and youth, they are excellent targets when searching for submillimetre emission from cold, extended, dust components produced by collisions in Kuiper-Belt-like disks. They also allow an age independent study of debris disk properties as a function of other stellar parameters.

    Methods. We observed 7 infrared-excess stars in the beta Pictoris moving group with the LABOCA bolometer array, operating at a central wavelength of 870 mu m at the 12-m submillimetre telescope APEX. The main emission at these wavelengths comes from large, cold dust grains, which constitute the main part of the total dust mass, and hence, for an optically thin case, make better estimates on the total dust mass than earlier infrared observations. Fitting the spectral energy distribution with combined optical and infrared photometry gives information on the temperature and radial extent of the disk.

    Results. From our sample, beta Pic, HD181327, and HD172555 were detected with at least 3 sigma certainty, while all others are below 2 sigma and considered non-detections. The image of beta Pic shows an offset flux density peak located near the south-west extension of the disk, similar to the one previously found by SCUBA at the JCMT. We present SED fits for detected sources and give an upper limit on the dust mass for undetected ones.

    Conclusions. We find a mean fractional dust luminosity (f) over bar (dust) = 1.1 x 10(-3) at t approximate to 12 Myr, which together with recent data at 100 Myr suggests an f(dust) proportional to t(-alpha) a decline of the emitting dust, with alpha > 0.8.

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  • 9.
    Nilsson, Ricky
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. American Museum of Natural History, USA; California Institute of Technology, USA.
    Veicht, A.
    Godfrey, P. A. Giorla
    Rice, E. L.
    Aguilar, J.
    Pueyo, L.
    Roberts, L. C.
    Oppenheimer, R.
    Brenner, D.
    Luszcz-Cook, S. H.
    Bacchus, E.
    Beichman, C.
    Burruss, R.
    Cady, E.
    Dekany, R.
    Fergus, R.
    Hillenbrand, L.
    Hinkley, S.
    King, D.
    Lockhart, T.
    Parry, I. R.
    Sivaramakrishnan, A.
    Soummer, R.
    Vasisht, G.
    Zhai, C.
    Zimmerman, N. T.
    Project 1640 Observations of Brown Dwarf GJ 758 B: Near-infrared Spectrum and Atmospheric Modeling2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 838, no 1, article id 64Article in journal (Refereed)
    Abstract [en]

    The nearby Sun-like star GJ 758 hosts a cold substellar companion, GJ 758 B, at a projected separation of l less than or similar to 30 au, previously detected in high-contrast multi-band photometric observations. In order to better constrain the companion's physical characteristics, we acquired the first low-resolution (R similar to 50) near-infrared spectrum of it using the high-contrast hyperspectral imaging instrument Project 1640 on Palomar Observatory's 5 m Hale telescope. We obtained simultaneous images in 32 wavelength channels covering the Y, J, and H bands (similar to 9521770 nm), and used data processing techniques based on principal component analysis to efficiently subtract chromatic background speckle-noise. GJ 758 B was detected in four epochs during 2013 and 2014. Basic astrometric measurements confirm its apparent northwest trajectory relative to the primary star, with no clear signs of orbital curvature. Spectra of SpeX/IRTF observed T dwarfs were compared to the combined spectrum of GJ 758 B, with chi(2) minimization suggesting a best fit for spectral type T7.0 +/- 1.0, but with a shallow minimum over T5T8. Fitting of synthetic spectra from the BT-Settl13 model atmospheres gives an effective temperature T-eff = 741 +/- 25 K and surface gravity log g=4.3 +/- 0.5 dex (cgs). Our derived best-fit spectral type and effective temperature from modeling of the low-resolution spectrum suggest a slightly earlier and hotter companion than previous findings from photometric data, but do not rule out current results, and confirm GJ 758 B as one of the coolest sub-stellar companions to a Sun-like star to date.

  • 10.
    Olofsson, Göran
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy.
    Florén, Hans-Gustav
    Stockholm University, Faculty of Science, Department of Astronomy.
    Djupvik, Amanda
    Aberasturi, M.
    Polarimetric coronagraphy of BD+31°643Article in journal (Refereed)
  • 11. Roberts, Lewis C.
    et al.
    Mason, Brian D.
    Aguilar, Jonathan
    Carson, Joseph
    Crepp, Justin
    Beichman, Charles
    Brenner, Douglas
    Burruss, Rick
    Cady, Eric
    Luszcz-Cook, Statia
    Dekany, Richard
    Hillenbrand, Lynne
    Hinkley, Sasha
    King, David
    Lockhart, Thomas G.
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy. American Museum of Natural History, USA.
    Oppenheimer, Rebecca
    Parry, Ian R.
    Pueyo, Laurent
    Rice, Emily L.
    Sivaramakrishnan, Anand
    Soummer, Remi
    Vasisht, Gautam
    Veicht, Aaron
    Wang, Ji
    Zhai, Chengxing
    Zimmerman, Neil T.
    CHARACTERIZATION OF THE COMPANION mu HER2016In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 151, no 6, article id 169Article in journal (Refereed)
    Abstract [en]

    mu Her is a nearby quadruple system with a G-subgiant primary and several low-mass companions arranged in a 2+2 architecture. While the BC components have been well characterized, the Ab component has been detected astrometrically and with direct imaging but there has been some confusion over its nature, in particular, whether the companion is stellar or substellar. Using near-infrared spectroscopy, we are able to estimate the spectral type of the companion as an M4 +/- 1V star. In addition, we have measured the astrometry of the system for over a decade. We combined the astrometry with archival radial velocity measurements to compute an orbit of the system. From the combined orbit, we are able to compute the mass sum of the system. Using the estimated mass of the primary, we estimate the mass of the secondary as 0.32 MG, which agrees with the estimated spectral type. Our computed orbit is preliminary due to the incomplete orbital phase coverage, but it should be sufficient to predict ephemerides over the next decade.

  • 12. Roberts, Lewis C.
    et al.
    Oppenheimer, Rebecca
    Crepp, Justin R.
    Baranec, Christoph
    Beichman, Charles
    Brenner, Douglas
    Burruss, Rick
    Cady, Eric
    Luszcz-Cook, Statia
    Dekany, Richard
    Hillenbrand, Lynne
    Hinkley, Sasha
    King, David
    Lockhart, Thomas G.
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy. American Museum of Natural History, New York, USA.
    Parry, Ian R.
    Pueyo, Laurent
    Sivaramakrishnan, Anand
    Stockholm University, Faculty of Science, Department of Astronomy.
    Soummer, Rémi
    Stockholm University, Faculty of Science, Department of Astronomy.
    Rice, Emily L.
    Veicht, Aaron
    Vasisht, Gautam
    Zhai, Chengxing
    Zimmerman, Neil T.
    KNOW THE STAR, KNOW THE PLANET. V. CHARACTERIZATION OF THE STELLAR COMPANION TO THE EXOPLANET HOST STAR HD 1778302015In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 150, no 4, article id 103Article in journal (Refereed)
    Abstract [en]

    HD 177830 is an evolved K0IV star with two known exoplanets. In addition to the planetary companions it has a late-type stellar companion discovered with adaptive optics imagery. We observed the binary star system with the PHARO near-IR camera and the Project 1640 coronagraph. Using the Project 1640 coronagraph and integral field spectrograph we extracted a spectrum of the stellar companion. This allowed us to determine that the spectral type of the stellar companion is a M4 +/- 1 V. We used both instruments to measure the astrometry of the binary system. Combining these data with published data, we determined that the binary star has a likely period of approximately 800 years with a semimajor axis of 100-200 AU. This implies that the stellar companion has had little or no impact on the dynamics of the exoplanets. The astrometry of the system should continue to be monitored, but due to the slow nature of the system, observations can be made once every 5-10 years.

  • 13. Wilson, T. L.
    et al.
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy. American Museum of Natural History, USA.
    Chen, C. H.
    Lisse, C. M.
    Moerchen, M.
    Käufl, H. -U.
    Banzatti, A.
    CONSTRAINTS ON THE PRESENCE OF SiO GAS IN THE DEBRIS DISK OF HD 1725552016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 826, no 2, article id 165Article in journal (Refereed)
    Abstract [en]

    We have carried out two sets of observations to quantify the properties of SiO gas in the unusual HD 172555 debris disk: (1) a search for the J = 8-7 rotational transition from the vibrational ground state, carried out with the Atacama Pathfinder EXperiment (APEX) submillimeter telescope and heterodyne receiver at 863 mu m. and (2) a search at 8.3 mu m for the P(17) ro-vibrational transition of gas phase SiO, carried out with the Very Large Telescope (VLT)/VISIR with a resolution, lambda/Delta lambda, of 30,000. The APEX measurement resulted in a 3.3 sigma detection of an interstellar feature, but only an upper limit to emission at the radial velocity and line width expected from HD 172555. The VLT/VISIR result was also an upper limit. These were used to provide limits for the abundance of gas phase SiO. for a range of temperatures. The upper limit from our APEX detection, assuming an 8000 K primary star photospheric excitation, falls more than an order of magnitude below the self-shielding stability threshold derived by Johnson et al. (2012). Our results thus favor a solid-state origin for the 8.3 mu m feature seen in the Spitzer IRS spectrum of the circumstellar excess emission. and the production of circumstellar O I and Si I by SiO UV photolysis. The implications of these estimates are explored in the framework of models of the HD 172555 circumstellar disk.

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