arXiv subjects

D. J. Sullivan

Publications and source records attributed to D. J. Sullivan.

At least 109 records · Page 6Linked to original sources

arXiv2008

Microlensing Event MOA-2007-BLG-400: Exhuming the Buried Signature of a Cool, Jovian-Mass Planet

We report the detection of the cool, Jovian-mass planet MOA-2007-BLG-400Lb. The planet was detected in a high-magnification microlensing event (with peak magnification A_max = 628) in which the primary lens transited the source, resulting in a dramatic smoothing of the peak of the event. The angular extent of the region of perturbation due to the planet is significantly smaller than the angular size of the source, and as a result the planetary signature is also smoothed out by the finite source size. Thus the deviation from a single-lens fit is broad and relatively weak (~ few percent). Nevertheless, we demonstrate that the planetary nature of the deviation can be unambiguously ascertained from the gross features of the residuals, and detailed analysis yields a fairly precise planet/star mass ratio of q = 0.0026+/-0.0004, in accord with the large significance (Δχ^2=1070) of the detection. The planet/star projected separation is subject to a strong close/wide degeneracy, leading to two indistinguishable solutions that differ in separation by a factor of ~8.5. Upper limits on flux from the lens constrain its mass to be M < 0.75 M_Sun (assuming it is a main-sequence star). A Bayesian analysis that includes all available observational constraints indicates a primary in the Galactic bulge with a mass of ~0.2-0.5 M_Sun and thus a planet mass of ~ 0.5-1.3 M_Jupiter. The separation and equilibrium temperature are ~0.6-1.1AU (~5.3-9.7AU) and ~103K (~34K) for the close (wide) solution. If the primary is a main-sequence star, follow-up observations would enable the detection of its light and so a measurement of its mass and distance.

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arXiv2008

A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192

We report the detection of an extrasolar planet of mass ratio q ~ 2 x 10^(-4) in microlensing event MOA-2007-BLG-192. The best fit microlensing model shows both the microlensing parallax and finite source effects, and these can be combined to obtain the lens masses of M = 0.060 (+0.028 -0.021) M_sun for the primary and m = 3.3 (+4.9 -1.6) M_earth for the planet. However, the observational coverage of the planetary deviation is sparse and incomplete, and the radius of the source was estimated without the benefit of a source star color measurement. As a result, the 2-sigma limits on the mass ratio and finite source measurements are weak. Nevertheless, the microlensing parallax signal clearly favors a sub-stellar mass planetary host, and the measurement of finite source effects in the light curve supports this conclusion. Adaptive optics images taken with the Very Large Telescope (VLT) NACO instrument are consistent with a lens star that is either a brown dwarf or a star at the bottom of the main sequence. Follow-up VLT and/or Hubble Space Telescope (HST) observations will either confirm that the primary is a brown dwarf or detect the low-mass lens star and enable a precise determination of its mass. In either case, the lens star, MOA-2007-BLG-192L, is the lowest mass primary known to have a companion with a planetary mass ratio, and the planet, MOA-2007-BLG-192Lb, is probably the lowest mass exoplanet found to date, aside from the lowest mass pulsar planet.

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arXiv2008

MOA-cam3: a wide-field mosaic CCD camera for a gravitational microlensing survey in New Zealand

We have developed a wide-field mosaic CCD camera, MOA-cam3, mounted at the prime focus of the Microlensing Observations in Astrophysics (MOA) 1.8-m telescope. The camera consists of ten E2V CCD4482 chips, each having 2kx4k pixels, and covers a 2.2 deg^2 field of view with a single exposure. The optical system is well optimized to realize uniform image quality over this wide field. The chips are constantly cooled by a cryocooler at -80C, at which temperature dark current noise is negligible for a typical 1-3 minute exposure. The CCD output charge is converted to a 16-bit digital signal by the GenIII system (Astronomical Research Cameras Inc.) and readout is within 25 seconds. Readout noise of 2--3 ADU (rms) is also negligible. We prepared a wide-band red filter for an effective microlensing survey and also Bessell V, I filters for standard astronomical studies. Microlensing studies have entered into a new era, which requires more statistics, and more rapid alerts to catch exotic light curves. Our new system is a powerful tool to realize both these requirements.

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arXiv2008

Discovery of a Jupiter/Saturn Analog with Gravitational Microlensing

Searches for extrasolar planets have uncovered an astonishing diversity of planetary systems, yet the frequency of solar system analogs remains unknown. The gravitational microlensing planet search method is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury. We report the detection of a multiple-planet system with microlensing. We identify two planets with masses of ~0.71 and ~0.27 times the mass of Jupiter and orbital separations of ~2.3 and ~4.6 astronomical units orbiting a primary star of mass ~0.50 solar masses at a distance of ~1.5 kiloparsecs. This system resembles a scaled version of our solar system in that the mass ratio, separation ratio, and equilibrium temperatures of the planets are similar to those of Jupiter and Saturn. These planets could not have been detected with other techniques; their discovery from only six confirmed microlensing planet detections suggests that solar system analogs may be common.

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arXiv2008

Whole Earth Telescope observations of the hot helium atmosphere pulsating white dwarf EC 20058-5234

We present the analysis of a total of 177h of high-quality optical time-series photometry of the helium atmosphere pulsating white dwarf (DBV) EC 20058-5234. The bulk of the observations (135h) were obtained during a WET campaign (XCOV15) in July 1997 that featured coordinated observing from 4 southern observatory sites over an 8-day period. The remaining data (42h) were obtained in June 2004 at Mt John Observatory in NZ over a one-week observing period. This work significantly extends the discovery observations of this low-amplitude (few percent) pulsator by increasing the number of detected frequencies from 8 to 18, and employs a simulation procedure to confirm the reality of these frequencies to a high level of significance (1 in 1000). The nature of the observed pulsation spectrum precludes identification of unique pulsation mode properties using any clearly discernable trends. However, we have used a global modelling procedure employing genetic algorithm techniques to identify the n, l values of 8 pulsation modes, and thereby obtain asteroseismic measurements of several model parameters, including the stellar mass (0.55 M_sun) and T_eff (~28200 K). These values are consistent with those derived from published spectral fitting: T_eff ~ 28400 K and log g ~ 7.86. We also present persuasive evidence from apparent rotational mode splitting for two of the modes that indicates this compact object is a relatively rapid rotator with a period of 2h. In direct analogy with the corresponding properties of the hydrogen (DAV) atmosphere pulsators, the stable low-amplitude pulsation behaviour of EC 20058 is entirely consistent with its inferred effective temperature, which indicates it is close to the blue edge of the DBV instability strip. (abridged)

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arXiv2005

The MOA 1.8-metre alt-az wide-field survey telescope and the MOA project

A new 1.8-m wide-field alt-az survey telescope was installed at Mt John University Observatory in New Zealand in October 2004. The telescope will be dedicated to the MOA (Microlensing Observations in Astrophysics) project. The instrument is equipped with a large 10-chip mosaic CCD camera with 80 Mpixels covering about 2 square degrees of sky. It is mounted at the f/3 prime focus. The telescope will be used for finding and following microlensing events in the galactic bulge and elsewhere, with an emphasis on the analysis of microlensing light curves for the detection of extrasolar planets. The MOA project is a Japan-New Zealand collaboration, with the participation of Nagoya University and four universities in New Zealand.

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arXiv2005

Determination of stellar shape in microlensing event MOA 2002-BLG-33

We report a measurement of the shape of the source star in microlensing event MOA 2002-BLG-33. The lens for this event was a close binary whose centre-of-mass passed almost directly in front of the source star. At this time, the source star was closely bounded on all sides by a caustic of the lens. This allowed the oblateness of the source star to be constrained. We found that a/b = 1.02^{+0.04}_{-0.02} where a and b are its semi-major and semi-minor axes respectively. The angular resolution of this measurement is approximately 0.04 microarcsec. We also report HST images of the event that confirm a previous identification of the source star as an F8-G2 turn-off main-sequence star.

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arXiv2005

Multiple Outbursts of a Cataclysmic Variable in the Globular Cluster M22

We present a 4 year light curve of a cataclysmic variable in M22, based on an analysis of accumulated data from the MOA microlensing survey. The position of the star coincides with that of a transient event observed by HST in 1999, originally attributed to microlensing but later suspected to be a dwarf nova outburst. Two outburst episodes, one in 2002 and one in 2003, with $ΔI\sim3$ are seen in the MOA data thus confirming that the HST event was a dwarf nova outburst. The MOA and HST data show that this dwarf nova underwent at least three outburst episodes during 1999--2004. Further close monitoring of this event is encouraged as future outburst episodes are expected.

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arXiv2004

Search for Low-Mass Exoplanets by Gravitational Microlensing at High Magnification

Observations of the gravitational microlensing event MOA 2003-BLG-32/OGLE 2003-BLG-219 are presented for which the peak magnification was over 500, the highest yet reported. Continuous observations around the peak enabled a sensitive search for planets orbiting the lens star. No planets were detected. Planets 1.3 times heavier than Earth were excluded from more than 50 % of the projected annular region from approximately 2.3 to 3.6 astronomical units surrounding the lens star, Uranus-mass planets from 0.9 to 8.7 astronomical units, and planets 1.3 times heavier than Saturn from 0.2 to 60 astronomical units. These are the largest regions of sensitivity yet achieved in searches for extrasolar planets orbiting any star.

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arXiv2004

OGLE 2003-BLG-235/MOA 2003-BLG-53: A planetary microlensing event

We present observations of the unusual microlensing event OGLE 2003-BLG-235/MOA 2003-BLG-53. In this event a short duration (~7 days) low amplitude deviation in the light curve due a single lens profile was observed in both the MOA and OGLE survey observations. We find that the observed features of the light curve can only be reproduced using a binary microlensing model with an extreme (planetary) mass ratio of 0.0039 +/- (11, 07) for the lensing system. If the lens system comprises a main sequence primary, we infer that the secondary is a planet of about 1.5 Jupiter masses with an orbital radius of ~3 AU.

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arXiv2004

MOA 2003-BLG-37: A Bulge Jerk-Parallax Microlens Degeneracy

We analyze the Galactic bulge microlensing event MOA-2003-BLG-37. Although the Einstein timescale is relatively short, t_e=43 days, the lightcurve displays deviations consistent with parallax effects due to the Earth's accelerated motion. We show that the chi^2 surface has four distinct local minima that are induced by the ``jerk-parallax'' degeneracy, with pairs of solutions having projected Einstein radii, \tilde r_e = 1.76 AU and 1.28 AU, respectively. This is the second event displaying such a degeneracy and the first toward the Galactic bulge. For both events, the jerk-parallax formalism accurately describes the offsets between the different solutions, giving hope that when extra solutions exist in future events, they can easily be found. However, the morphologies of the chi^2 surfaces for the two events are quite different, implying that much remains to be understood about this degeneracy.

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arXiv2004

A Strong Test of Electro-Weak Theory using Pulsating DB White Dwarf Stars as Plasmon Neutrino Detectors

We demonstrate that plasmon neutrinos are the dominant form of energy loss in model white dwarf stars down to T_eff ~ 25,000 K, depending on the stellar mass. The lower end of this range overlaps the observed temperatures for the V777 Her star (DBV) instability strip. The evolution of white dwarfs at these temperatures is driven predominantly by cooling, so this directly affects the stellar evolutionary timescale in proportion to the ratio of the neutrino energy loss to the photon energy loss. This evolutionary timescale is observable through the time rate of change of the pulsation periods. Although the unified electro-weak theory of lepton interactions that is crucial for understanding neutrino production has been well tested in the high energy regime, the approach presented here should result in an interesting low-energy test of the theory. We discuss observational strategies to achieve this goal.

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arXiv2003

Probing the atmosphere of a solar-like star by galactic microlensing at high magnification

We report a measurement of limb darkening of a solar-like star in the very high magnification microlensing event MOA 2002-BLG-33. A 15 hour deviation from the light curve profile expected for a single lens was monitored intensively in V and I passbands by five telescopes spanning the globe. Our modelling of the light curve showed the lens to be a close binary system whose centre-of-mass passed almost directly in front of the source star. The source star was identified as an F8-G2 main sequence turn-off star. The measured stellar profiles agree with current stellar atmosphere theory to within ~4% in two passbands. The effective angular resolution of the measurements is <1 micro-arcsec. These are the first limb darkening measurements obtained by microlensing for a Solar-like star.

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arXiv2003

Microlensing optical depth towards the Galactic bulge from MOA observations during 2000 with Difference Image Analysis

We analyze the data of the gravitational microlensing survey carried out by by the MOA group during 2000 towards the Galactic Bulge (GB). Our observations are designed to detect efficiently high magnification events with faint source stars and short timescale events, by increasing the the sampling rate up to 6 times per night and using Difference Image Analysis (DIA). We detect 28 microlensing candidates in 12 GB fields corresponding to 16 deg^2. We use Monte Carlo simulations to estimate our microlensing event detection efficiency, where we construct the I-band extinction map of our GB fields in order to find dereddened magnitudes. We find a systematic bias and large uncertainty in the measured value of the timescale $t_{\rm Eout}$ in our simulations. They are associated with blending and unresolved sources, and are allowed for in our measurements. We compute an optical depth tau = 2.59_{-0.64}^{+0.84} \times 10^{-6} towards the GB for events with timescales 0.3 10). These events are useful for studies of extra-solar planets.

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arXiv2002

Study by MOA of extra-solar planets in gravitational microlensing events of high magnification

A search for extra-solar planets was carried out in three gravitational microlensing events of high magnification, MACHO 98-BLG-35, MACHO 99-LMC-2, and OGLE 00-BUL-12. Photometry was derived from observational images by the MOA and OGLE groups using an image subtraction technique. For MACHO 98-BLG-35, additional photometry derived from the MPS and PLANET groups was included. Planetary modeling of the three events was carried out in a super-cluster computing environment. The estimated probability for explaining the data on MACHO 98-BLG-35 without a planet is <1%. The best planetary model has a planet of mass ~(0.4-1.5) X 10^-5 M_Earth at a projected radius of either ~1.5 or ~2.3 AU. We show how multi-planet models can be applied to the data. We calculated exclusion regions for the three events and found that Jupiter-mass planets can be excluded with projected radii from as wide as about 30 AU to as close as around 0.5 AU for MACHO 98-BLG-35 and OGLE 00-BUL-12. For MACHO 99-LMC-2, the exclusion region extends out to around 10 AU and constitutes the first limit placed on a planetary companion to an extragalactic star. We derive a particularly high peak magnification of ~160 for OGLE 00-BUL-12. We discuss the detectability of planets with masses as low as Mercury in this and similar events.

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arXiv2002

Improving the Prospects for Detecting Extrasolar Planets in Gravitational Microlensing in 2002

Gravitational microlensing events of high magnification have been shown to be promising targets for detecting extrasolar planets. However, only a few events of high magnification have been found using conventional survey techniques. Here we demonstrate that high magnification events can be readily found in microlensing surveys using a strategy that combines high frequency sampling of target fields with online difference imaging analysis. We present 10 microlensing events with peak magnifications greater than 40 that were detected in real-time towards the Galactic Bulge during 2001 by MOA. We show that Earth mass planets can be detected in future events such as these through intensive follow-up observations around the event peaks. We report this result with urgency as a similar number of such events are expected in 2002.

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arXiv2001

Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud

One hundred and forty six long-period red variable stars in the Large Magellanic Cloud (LMC) from the three year MOA project database were analysed. A careful periodic analysis was performed on these stars and a catalogue of their magnitudes, colours, periods and amplitudes is presented. We convert our blue and red magnitudes to $K$ band values using 19 oxygen-rich stars. A group of red short-period stars separated from the Mira sequence has been found on a (log P, K) diagram. They are located at the short period side of the Mira sequence consistent with the work of Wood and Sebo (1996). There are two interpretations for such stars; a difference in pulsation mode or a difference in chemical composition. We investigated the properties of these stars together with their colour, amplitude and periodicity. We conclude that they have small amplitudes and less regular variability. They are likely to be higher mode pulsators. A large scatter has been also found on the long period side of the (log P, K) diagram. This is possibly a systematic spread given that the blue band of our photometric system covers both standard B and V bands and affects carbon-rich stars.

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arXiv2001

Real-Time Difference Imaging Analysis of MOA Galactic Bulge Observations During 2000

We describe observations carried out by the MOA group of the Galactic Bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration. These events are particularly useful for studies of extra-solar planets and faint stars. Approximately 17 degrees square were monitored at a sampling rate of up to 6 times per night. The images were analysed in real-time using a difference imaging technique. Twenty microlensing candidates were detected, of which 8 were alerted to the microlensing community whilst in progress. Approximately half of the candidates had high magnifications (>~10), at least one had very high magnification (>~50), and one exhibited a clear parallax effect. The details of these events are reported here, together with details of the on-line difference imaging technique. Some nova-like events were also observed and these are described, together with one asteroid.

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