arXiv subjects

M. Tamura

Publications and source records attributed to M. Tamura.

At least 127 records · Page 7Linked to original sources

arXiv2009

Exoplanet Characterization and the Search for Life

Over 300 extrasolar planets (exoplanets) have been detected orbiting nearby stars. We now hope to conduct a census of all planets around nearby stars and to characterize their atmospheres and surfaces with spectroscopy. Rocky planets within their star's habitable zones have the highest priority, as these have the potential to harbor life. Our science goal is to find and characterize all nearby exoplanets; this requires that we measure the mass, orbit, and spectroscopic signature of each one at visible and infrared wavelengths. The techniques for doing this are at hand today. Within the decade we could answer long-standing questions about the evolution and nature of other planetary systems, and we could search for clues as to whether life exists elsewhere in our galactic neighborhood.

astro-ph.EP↗

arXiv2009

A deep look into the core of young clusters. II. lambda-Orionis

Over the past years, the lambda-Orionis cluster has been a prime location for the study of young very low mass stars, substellar and isolated planetary mass objects and the determination of the initial mass function and other properties of low mass cluster members. In the continuity of our previous studies of young associations cores, we search for ultracool members and new multiple systems within the central 5.3' (~0.6pc) of the cluster. We obtained deep seeing limited J, Ks-band images of the 5.3' central part of the cluster with NTT/SofI and H-band images with CAHA/Omega2000. These images were complemented by multi-conjugate adaptive optics (MCAO) H and Ks images of the 1.5' central region of the lambda-Orionis cluster obtained with the prototype MCAO facility MAD at the VLT. The direct vicinity of the massive lambda-Ori O8III-star was probed using NACO/SDI at the VLT. Finally, we also retrieved Spitzer IRAC images of the same area and used archival Subaru Suprime-Cam and CFHT CFHT12K i-band images. We report the detection of 9 new member candidates selected from optical and near-IR color-color and color-magnitude diagrams and 7 previously known members. The high spatial resolution images resolve 3 new visual multiple systems. Two of them are most likely not members of the association. The third one is made of a brown dwarf candidate companion to the F8V star HD36861C. The simultaneous differential images allow us to rule out the presence of visual companions more massive than M>0.07Msun in the range 1-2.5", and M>0.25Msun in the range 0.5-2.5"

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arXiv2009

The discovery of an M4+T8.5 binary system

We report the discovery of a T8.5 dwarf, which is a companion to the M4 dwarf Wolf 940. At a distance of 12.50 (+0.75,-0.67) pc, the angular separation of 32arcsec corresponds to a projected separation of 400 AU. The M4 primary displays no Halpha emission, and we apply the age-activity relations of West et al. to place a lower limit on the age of the system of 3.5 Gyr. Weak Halpha absorption suggests some residual activity and we estimate an upper age limit of 6 Gyr. We apply the relations of Bonfils et al for V-Ks and M_Ks to determine the metallicity, [Fe/H] = -0.06 +/- 0.20 for Wolf~940A, and by extension the T8.5 secondary, Wolf 940B. We have obtained JHK NIRI spectroscopy and JHKL' photometry of Wolf 940B, and use these data, in combination with theoretical extensions, to determine its bolometric flux, Fbol = 1.75 +/- 0.18 E-16 Wm^-2 and thus its luminosity log(L*/Lsun) = -6.07 +/- 0.04. Using the age constraints for the system, and evolutionary structural models of Baraffe et al. we determine Teff = 570 +/- 25K and log g = 4.75-5.00 for Wolf940B, based on its bolometric luminosity. This represents the first determination of these properties for a T8+ dwarf that does not rely on the fitting of T-dwarf spectral models. This object represents the first system containing a T8+ dwarf for which fiducial constraints on its properties are available, and we compare its spectra with those of the latest very cool BT-Settl models. This clearly demonstrates that the use of the (W_J,K/J) spectral ratios (used previously to constrain Teff and log g) would have over-estimated Teff by ~100K.

astro-ph.SR↗

arXiv2009

The Physical Properties of Four ~600K T Dwarfs

We present Spitzer 7.6-14.5um spectra of ULAS J003402.77-005206.7 and ULAS J133553.45+113005.2, two T9 dwarfs with the latest spectral types currently known. We fit synthetic spectra and photometry to the near- through mid-infrared energy distributions of these dwarfs and that of the T8 dwarf 2MASS J09393548-2448279. We also analyse near-infrared data for another T9, CFBD J005910.82-011401.3. We find that the ratio of the mid- to near-infrared fluxes is very sensitive to effective temperature at these low temperatures, and that the 2.2 and 4.5um fluxes are sensitive to metallicity and gravity; there is a degeneracy between these parameters. The 4.5 and 10um fluxes are also sensitive to vertical transport of gas through the atmosphere, which we find to be significant for these dwarfs. The full near- through mid-infrared spectral energy distribution allows us to constrain the effective temperature (K)/gravity (m/s2)/metallicity ([m/H] dex) of ULAS J0034-00 and ULAS J1335+11 to 550-600/ 100-300/ 0.0-0.3 and 500-550/ 100-300/ 0.0-0.3, respectively. These fits imply low masses and young ages for the dwarfs of 5-20 M(Jup) and 0.1-2 Gyr. The fits to 2MASS J0939-24 are in good agreement with the measured distance, the observational data, and the earlier T8 near-infrared spectral type if it is a slightly metal-poor 4-10 Gyr old system consisting of a 500 and 700K, ~25 and ~40 M(Jup), pair, although it is also possible that it is an identical pair of 600K, 30 M(Jup), dwarfs. As no mid-infrared data are available for CFBD J0059-01 its properties are less well constrained; nevertheless it appears to be a 550-600K dwarf with g= 300-2000 m/s2 and [m/H]= 0-0.3 dex. These properties correspond to mass and age ranges of 10-50 M(Jup) and 0.5-10 Gyr for this dwarf.

astro-ph.SR↗

arXiv2009

Near-Infrared Circular Polarimetry and Correlation Diagrams in the Orion BN/KL Region: Contribution of Dichroic Extinction

We present a deep circular polarization image of the Orion BN/KL nebula in the Ks band and correlations of circular polarization, linear polarization, and H-Ks color representing extinction. The image of circular polarization clearly reveals the quadrupolar structure around the massive star IRc2, rather than BN. H-Ks color is well correlated with circular polarization. A simple relation between dichroic extinction, color excess, circular and linear polarization is derived. The observed correlation between the Stokes parameters and the color excess agrees with the derived relation, and suggests a major contribution of dichroic extinction to the production of circular polarization in this region, indicating the wide existence of aligned grains.

astro-ph.GA↗

arXiv2008

Young Brown Dwarfs in the Core of the W3 Main Star-Forming Region

We present the results of deep and high-resolution (FWHM ~ 0".35) JHK NIR observations with the Subaru telescope, to search for very low mass young stellar objects (YSOs) in the W3 Main star-forming region. The NIR survey covers an area of ~ 2.6 arcmin^2 with 10-sigma limiting magnitude exceeding 20 mag in the JHK bands. The survey is sensitive enough to provide unprecedented details in W3 IRS 5 region and reveals a census of the stellar population down to objects below the hydrogen-burning limit. We construct JHK color-color (CC) and J-H/J and H-K/K color-magnitude (CM) diagrams to identify very low luminosity YSOs and to estimate their masses. Based on these CC and CM diagrams, we identified a rich population of embedded YSO candidates with infrared excesses (Class I and Class II), associated with the W3 Main region. A large number of red sources (H-K > 2) have also been detected around W3 Main. We argue that these red stars are most probably pre-main-sequence (PMS) stars with intrinsic color excesses. Based on the comparison between theoretical evolutionary models of very low-mass PMS objects with the observed CM diagram, we find there exists a substantial substellar population in the observed region. The mass function (MF) does not show the presence of cutoff and sharp turnover around the substellar limit, at least at the hydrogen-burning limit. Furthermore, the MF slope indicates that the number ratio of young brown dwarfs and hydrogen-burning stars in the W3 Main is probably higher than those in Trapezium and IC 348. The presence of mass segregation, in the sense that relatively massive YSOs lie near the cluster center, is seen. The estimated dynamical evolution time indicates that the observed mass segregation in the W3 Main may be the imprint of the star formation process.

astro-ph↗

arXiv2008

Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo

We present very sensitive polarimetry of 55 Cnc and tau Boo in an attempt to detect the partially polarised reflected light from the planets orbiting these two stars. 55 Cnc is orbited by a hot Neptune planet (55 Cnc e) at 0.038 AU, a hot Jupiter planet (55 Cnc b) at 0.11 AU, and at least 3 more distant planets. The polarisation of this system is very stable, showing no sign of the periodic variations that would be expected if a short period planet were detected. The measured standard deviation of the night averaged Stokes Q/I and U/I parameters is 2.2x10^{-6}. We derive upper limits on the geometric albedo, A_G and planetary radius using Monte Carlo multiple scattering simulations of a simple model atmosphere. We assume Rayleigh-like scattering (scaled by the maximum polarisation, p_m at 90 degrees) and pressure insensitive extinction. Atmospheres in which multiple scattering plays only a small role have an almost linear relation between polarisation and A_G. In this case, the 4 sigma upper limits is A_G<0.13(R/1.2 R_{Jup})^{-2}p_m^{-1} for 55 Cnc e. This suggests that 55 Cnc e is relatively small and not a pure H-He planet. The data do not give a useful upper limit for 55 Cnc b. tau Boo is orbited by an unusually massive hot Jupiter planet. The standard deviation in the night averaged Stokes Q/I and U/I polarisation parameters is 5.1x10^{-6}. The 4 sigma upper limit is A_G<0.37(R/1.2 R_{Jup})^{-2}p_m^{-1} for tau Boo b. This extends the similar upper limits reported previously for this planet to longer wavelengths. The larger scatter in the tau Boo data may be due to the spot activity detected photometrically by the MOST satellite. These results contrast markedly with the recent claim of a 3 sigma detection of a periodic polarisation signal from HD189733 with amplitude P=2x10^{-4}, attributed to the planet HD189733 b.

astro-ph↗

arXiv2008

The UKIDSS Galactic Plane Survey

The UKIDSS Galactic Plane Survey (GPS) is one of the five near infrared Public Legacy Surveys that are being undertaken by the UKIDSS consortium, using the Wide Field Camera on the United Kingdom Infrared Telescope. It is surveying 1868 sq.deg. of the northern and equatorial Galactic plane at Galactic latitudes -5<b<5 in the J, H and K filters and a ~200 sq.deg. area of the Taurus-Auriga-Perseus molecular cloud complex in these three filters and the 2.12 um (1-0) H_2 filter. It will provide data on ~2 billion sources. Here we describe the properties of the dataset and provide a user's guide for its exploitation. We also present brief Demonstration Science results from DR2 and from the Science Verification programme. These results illustrate how GPS data will frequently be combined with data taken in other wavebands to produce scientific results. The Demonstration Science includes studies of: (i) the star formation region G28.983-0.603, cross matching with Spitzer-GLIMPSE data to identify YSOs; (ii) the M17 nebula; (iii) H_2 emission in the rho Ophiuchi dark cloud; (iv) X-ray sources in the Galactic Centre; (v) external galaxies in the Zone of Avoidance; (vi) IPHAS-GPS optical-infrared spectrophotometric typing. (abridged).

astro-ph↗

arXiv2008

Using SPICA Space Telescope to characterize Exoplanets

We present the 3.5m SPICA space telescope, a proposed Japanese-led JAXA-ESA mission scheduled for launch around 2017. The actively cooled (<5 K), single aperture telescope and monolithic mirror will operate from ~3.5 to ~210 um and will provide superb sensitivity in the mid- and far-IR spectral domain (better than JWST at lambda > 18 um). SPICA is one of the few space missions selected to go to the next stage of ESA's Cosmic Vision 2015-2025 selection process. In this White Paper we present the main specifications of the three instruments currently baselined for SPICA: a mid-infrared (MIR) coronagraph (~3.5 to ~27 um) with photometric and spectral capabilities (R~200), a MIR wide-field camera and high resolution spectrometer (R~30,000), and a far-infrared (FIR ~30 to ~210 um) imaging spectrometer - SAFARI - led by a European consortium. We discuss their capabilities in the context of MIR direct observations of exo-planets (EPs) and multiband photometry/high resolution spectroscopy observations of transiting exo-planets. We conclude that SPICA will be able to characterize the atmospheres of transiting exo-planets down to the super-Earth size previously detected by ground- or space-based observatories. It will also directly detect and characterize Jupiter/Neptune-size planets orbiting at larger separation from their parent star (>5-10 AU), by performing quantitative atmospheric spectroscopy and studying proto-planetary and debris disks. In addition, SPICA will be a scientific and technological precursor for future, more ambitious, IR space missions for exo-planet direct detection as it will, for example, quantify the prevalence exo-zodiacal clouds in planetary systems and test coronographic techniques, cryogenic systems and lightweight, high quality telescopes. (abridged)

astro-ph↗

arXiv2008

An Eight-Octant Phase-Mask Coronagraph

We present numerical simulations and laboratory experiments on an eight-octant phase-mask (EOPM) coronagraph. The numerical simulations suggest that an achievable contrast for the EOPM coronagraph can be greatly improved as compared to that of a four-quadrant phase-mask (FQPM) coronagraph for a partially resolved star. On-sky transmission maps reveal that the EOPM coronagraph has relatively high optical throughput, a small inner working angle and large discovery space. We have manufactured an eight-segment phase mask utilizing a nematic liquid-crystal device, which can be easily switched between the FQPM and the EOPM modes. The laboratory experiments demonstrate that the EOPM coronagraph has a better tolerance of the tip-tilt error than the FQPM one. We also discuss feasibility of a fully achromatic and high-throughput EOPM coronagraph utilizing a polarization interferometric technique.

astro-ph↗

arXiv2008

Exploring the substellar temperature regime down to ~550K

We report the discovery of three very late T dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS) Third Data Release: ULAS J101721.40+011817.9 (ULAS1017), ULAS J123828.51+095351.3 (ULAS1238) and ULAS J133553.45+113005.2 (ULAS1335).We detail optical and near-infrared photometry for all three sources, and mid-infrared photometry for ULAS1335. We use near-infrared spectra of each source to assign spectral types T8p (ULAS1017), T8.5 (ULAS1228) and T9 (ULAS1335) to these objects. We estimate that ULAS1017 has 750 < Teff < 850K, and 5.0 < log g < 5.5, assuming solar metallicity, an age of 1.6-15 Gyr, a mass of 33-70 MJ and lies at a distance of 31-54 pc. We extend the unified scheme of Burgasser et al. (2006) to the type T9 and suggest the inclusion of the WJ index to replace the now saturated J-band indices. ULAS1335 is the same spectral type as ULAS J003402.77-005206.7 and CFBDS J005910.90-011401.3. Comparison of model spectra with that of ULAS1335 suggest a temperature below 600K. We find ULAS1335 to be extremely red in near to mid-infrared colours, with H-[4.49]=4.34+/-0.04. This is the reddest near to mid-infrared colour yet observed for a T dwarf, which supports Teff < 600K, and we estimate Teff ~550-600K for ULAS1335. We estimate that ULAS1335 has an age of 0.6-5.3 Gyr, a mass of 15-31 MJ and lies at a distance of 8-12 pc.

astro-ph↗

arXiv2008

Optical to Near-IR Spectrum of a Massive Evolved Galaxy at z = 1.26

We present the optical to near-infrared (IR) spectrum of the galaxy TSPS J1329-0957, a red and bright member of the class of extremely red objects (EROs) at z = 1.26. This galaxy was found in the course of the Tokyo-Stromlo Photometry Survey (TSPS) which we are conducting in the southern sky. The spectroscopic observations were carried out with the Gemini Multi-Object Spectrograph (GMOS) and the Gemini Near Infra-Red Spectrograph (GNIRS) mounted on the Gemini-South telescope. The wide wavelength coverage of 0.6 - 2.3 um provides useful clues as to the nature of EROs while most published spectra are limited to a narrower spectral range which is dictated by the need for efficient redshift determination in a large survey. We compare our spectrum with several optical composite spectra obtained in recent large surveys, and with stellar population synthesis models. The effectiveness of using near-IR broad-band data, instead of the spectral data, in deriving the galaxy properties are also investigated. We find that TSPS J1329-0957 formed when the universe was 2 - 3 Gyr old, and subsequently evolved passively to become one of the most massive galaxies found in the z = 1 - 2 universe. Its early type and estimated stellar mass of M* = 10^{11.5} Msun clearly point to this galaxy being a direct ancestor of the brightest elliptical and spheroidal galaxies in the local universe.

astro-ph↗

arXiv2008

Fifteen new T dwarfs discovered in the UKIDSS Large Area Survey

We present the discovery of fifteen new T2.5-T7.5 dwarfs (with estimated distances between ~24-93pc, identified in the first three main data releases of the UKIRT Infrared Deep Sky Survey. This brings the total number of T dwarfs discovered in the Large Area Survey (to date) to 28. These discoveries are confirmed by near infrared spectroscopy, from which we derive spectral types on the unified scheme of Burgasser et al. (2006). Seven of the new T dwarfs have spectral types of T2.5-T4.5, five have spectral types of T5-T5.5, one is a T6.5p, and two are T7-7.5. We assess spectral morphology and colours to identify T dwarfs in our sample that may have non-typical physical properties (by comparison to solar neighbourhood populations). The colours of the full sample of LAS T dwarfs show a possible trend to bluer Y-J with decreasing effective temperature beyond T8. By accounting for the main sources of incompleteness (selection, follow-up and spatial) as well as the effects of unresolved binarity and Malmquist bias, we estimate that there are 17+-4 >=T4 dwarfs in the J<=19 volume of the LAS second data release. Comparing this to theoretical predictions is most consistent with a sub-stellar mass function exponent alpha between -1.0 and 0. This is consistent with the latest 2MASS/SDSS constraint (which is based on lower number statistics), and is significantly lower than the alpha~1.0 suggested by L dwarf field populations, possibly a result of the lower mass range probed by the T dwarf class.

astro-ph↗

arXiv2008

DARWIN - A Mission to Detect, and Search for Life on, Extrasolar Planets

The discovery of extra-solar planets is one of the greatest achievements of modern astronomy. The detection of planets with a wide range of masses demonstrates that extra-solar planets of low mass exist. In this paper we describe a mission, called Darwin, whose primary goal is the search for, and characterization of, terrestrial extrasolar planets and the search for life. Accomplishing the mission objectives will require collaborative science across disciplines including astrophysics, planetary sciences, chemistry and microbiology. Darwin is designed to detect and perform spectroscopic analysis of rocky planets similar to the Earth at mid-infrared wavelengths (6 - 20 micron), where an advantageous contrast ratio between star and planet occurs. The baseline mission lasts 5 years and consists of approximately 200 individual target stars. Among these, 25 to 50 planetary systems can be studied spectroscopically, searching for gases such as CO2, H2O, CH4 and O3. Many of the key technologies required for the construction of Darwin have already been demonstrated and the remainder are estimated to be mature in the near future. Darwin is a mission that will ignite intense interest in both the research community and the wider public.

astro-ph↗

arXiv2008

Wide-Field Infrared Imaging Polarimetry of the NGC 6334 Region: A Nest of Infrared Reflection Nebulae

We report the detection of eighteen infrared reflection nebulae (IRNe) in the $J$, $H$, & $Ks$ linear polarimetric observations of the NGC 6334 massive star-formation complex, of which 16 IRNe are new discoveries. Our images cover $\sim$180 square arcminutes, one of the widest near-infrared polarization data in star-formation regions so far. These IRNe are most likely associated with embedded young OB stars at different evolutionary phases, showing a variety of sizes, morphologies, and polarization properties, which can be divided into four categories. We argue the different nebula characteristics to be a possible evolutionary sequence of circumstellar structures around young massive stars.

astro-ph↗

arXiv2007

An X-ray Imaging Study of the Stellar Population in RCW49

We present the results of a high-resolution X-ray imaging study of the stellar population in the Galactic massive star-forming region RCW49 and its central OB association Westerlund 2. We obtained a 40 ks X-ray image of a 17'x17' field using the Chandra X-ray Observatory and deep NIR images using the Infrared Survey Facility in a concentric 8'3x8'3 region. We detected 468 X-ray sources and identified optical, NIR, and Spitzer Space Telescope MIR counterparts for 379 of them. The unprecedented spatial resolution and sensitivity of the X-ray image, enhanced by optical and infrared imaging data, yielded the following results: (1) The central OB association Westerlund 2 is resolved for the first time in the X-ray band. X-ray emission is detected from all spectroscopically-identified early-type stars in this region. (2) Most (86%) X-ray sources with optical or infrared identifications are cluster members in comparison with a control field in the Galactic Plane. (3) A loose constraint (2--5 kpc) for the distance to RCW49 is derived from the mean X-ray luminosity of T Tauri stars. (4) The cluster X-ray population consists of low-mass pre--main-sequence and early-type stars as obtained from X-ray and NIR photometry. About 30 new OB star candidates are identified. (5) We estimate a cluster radius of 6'--7' based on the X-ray surface number density profiles. (6) A large fraction (90%) of cluster members are identified individually using complimentary X-ray and MIR excess emission. (7) The brightest five X-ray sources, two Wolf-Rayet stars and three O stars, have hard thermal spectra.

astro-ph↗

arXiv2007

Simultaneous X-ray, radio, near-infrared, and optical monitoring of Young Stellar Objects in the Coronet cluster

Multi-wavelength (X-ray to radio) monitoring of Young Stellar Objects (YSOs) can provide important information about physical processes at the stellar surface, in the stellar corona, and/or in the inner circumstellar disk regions. While coronal processes should mainly cause variations in the X-ray and radio bands, accretion processes may be traced by time-correlated variability in the X-ray and optical/infrared bands. Several multi-wavelength studies have been successfully performed for field stars and approx. 1-10 Myr old T Tauri stars, but so far no such study succeeded in detecting simultaneous X-ray to radio variability in extremely young objects like class I and class 0 protostars. Here we present the first simultaneous X-ray, radio, near-infrared, and optical monitoring of YSOs, targeting the Coronet cluster in the Corona Australis star-forming region, which harbors at least one class 0 protostar, several class I objects, numerous T Tauri stars, and a few Herbig AeBe stars. [...] Seven objects are detected simultaneously in the X-ray, radio, and optical/infrared bands; they constitute our core sample. While most of these sources exhibit clear variability in the X-ray regime and several also display optical/infrared variability, none of them shows significant radio variability on the timescales probed. We also do not find any case of clearly time-correlated optical/infrared and X-ray variability. [...] The absence of time-correlated multi-wavelength variability suggests that there is no direct link between the X-ray and optical/infrared emission and supports the notion that accretion is not an important source for the X-ray emission of these YSOs. No significant radio variability was found on timescales of days.

astro-ph↗

arXiv2007

Planetary Formation Scenarios Revistied: Core-Accretion Versus Disk Instability

The core-accretion and disk instability models have so far been used to explain planetary formation. These models have different conditions, such as planet mass, disk mass, and metallicity for formation of gas giants. The core-accretion model has a metallicity condition ([Fe/H] > −1.17 in the case of G-type stars), and the mass of planets formed is less than 6 times that of the Jupiter mass MJ. On the other hand, the disk instability model does not have the metallicity condition, but requires the disk to be 15 times more massive compared to the minimum mass solar nebulae model. The mass of planets formed is more than 2MJ. These results are compared to the 161 detected planets for each spectral type of the central stars. The results show that 90% of the detected planets are consistent with the core-accretion model regardless of the spectral type. The remaining 10% are not in the region explained by the core-accretion model, but are explained by the disk instability model. We derived the metallicity dependence of the formation probability of gas giants for the core-accretion model. Comparing the result with the observed fraction having gas giants, they are found to be consistent. On the other hand, the observation cannot be explained by the disk instability model, because the condition for gas giant formation is independent of the metallicity. Consequently, most of planets detected so far are thought to have been formed by the core-accretion process, and the rest by the disk instability process.

astro-ph↗