宇宙物理学教室ランチセミナー

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1. 2020.06.23 Tuesday Lunch Seminar 1. Vedantham et al., Coherent radio emission from a quiescent red dwarf of star-planet interaction, Nature Astronomy, 4, 577-583 (2020) 2. Barnes et al., An ablating 2.6 M⊕ planet in an eccentric binary from the Dispersed Matter Planet Project, Nature Astronomy, 4, 419-426 (2020) Takanori Sasaki
2. How to detect Exoplanets ɾDoppler spectroscopy ɾTransit photometry ɾGravitational microlensing ɾPulsar timing ɾDirect imaging (c) Wikipedia
3. Doppler Spectroscopy 1995
   
   Mayor & Queloz (1995) An indirect method for finding exoplanets from radial velocity measurements via observation of Doppler shifts in the spectrum of the planet’s parents star. (c) Emmanuel Pécontal
4. Transit Photometry Charbonneau et al. (2000) As a planet transits its host star, it will block a portion of the light from the star. If the planet transits in-between the star and the observer, the change in light can be measured to construct a light curve. (c) ESA
5. Gravitational Microlensing Bond et al. (2004) In the case that the lensing object is a star with a planet orbiting it, if the source crosses a caustic, the deviations from a standard event can be large even for low mass planets. The deviations allow us to infer the existence and determine the mass and separation of the planet around the lens. (c) NASA
6. Pulsar Timing Wolszczan & Frail (1992) Planets orbiting the pulsar will cause regular changes in its pulsation. Since pulsars normally rotate at near- constant speed, any changes can easily be detected with the help of precise timing measurements.
7. Direct Imaging Wang et al. (2017) Planets orbiting far enough from stars to be resolved reflect very little starlight, so planets are detected through their thermal emission instead. It is easier to obtain images when the star system is relatively near to the Sun, and when the planet is especially large, widely separated from it parent star, and hot so that it emits intense infrared radiation.
8. And more… ɾDoppler spectroscopy ɾTransit photometry ɾGravitational microlensing ɾPulsar timing ɾDirect imaging ɾAstrometry ɾRelativistic beaming ɾEllipsoidal variations ɾVariable star timing ɾPolarimetry etc…
9. • Low-frequency ( MHz) stellar radio emission from other stars have detected only a single active flare star (AD Leo). • This paper report the detection of low- frequency emission from a quiescent star (GJ 1151, red M dwarf). • The characteristics of the emission are similar to those of planetary auroral emission such as Jupiter’s decametric emission. • The emission properties are consistent with theoretical expectations for interaction with an Earth-size planet in an approximately one- to five-day-long orbit. ν ≤ 150 Vedantham et al., Coherent radio emission from a quiescent red dwarf of star-planet interaction, Nat. Astron. (2020) news & views EXOPLANETS Stellar radio aurorae signal planetary systems A radio detection of an old red dwarf might reveal the presence of a planetary system, and open up the search for exoplanets to a new technique. J. Sebastian Pineda Exoplanet detection experiments have largely relied on either the radial velocity method or the transit method. For the radial velocity method, a planetary system is inferred based on the wavelength shifts in the starlight induced by the gravitational tug of the planet on the star. For the transit method, a planet is seen as a characteristic dip in the starlight as the planet passes in front of the star. By contrast, in their paper in this issue, Harish Vedantham and colleagues1 show how the detection of low-frequency radio waves might point to the presence of an exoplanetary system around some nearby red dwarf stars. Matching radio sources from a recent all-sky survey2 to the known positions of nearby stars in the sky, Vedantham et al. found an unusual coincidence between a radio detection and the low-mass star known as GJ 1151. Moreover, the properties of the emitted radio waves were curious. The radio emission was only seen once in the four times the survey looked at the star, it lasted for more than eight hours, and most importantly it was highly circularly polarized. The polarization, which refers to the particular orientation of the electromagnetic fields that make up the light wave, provides an important clue as to the processes responsible for the emission. This type of polarized radio emission those that generate aurorae in the Solar System (Fig. 1), as exemplified by Jupiter of the planetary properties necessary to produce the brightness of the radio waves. Foot point Flux tube Aurora Fig. 1 | The putative exoplanet generates a stellar aurora. The planet candidate around GJ 1151 is expected to be linked to its host star through a magnetic flux tube, causing a stellar aurora at its foot. (c) Nature Astronomy
10. LETTERS NATURE ASTRONOMY +48° 23′ 30″ 0.5 0.4 0.3 0.2 Flux density (mJy beam–1) 0.1 0 +48° 22′ 30″ +48° 21′ 30″ 1′ 11 h 51 m in 04.0 s 11 h 51 m in 0.0 s 11 h 50 m in 56.0 s 11 h 50 m in 52.0 s 11 h 50 m in 48.0 s +48° 23′ 00″ Declination (J2000) +48° 22′ 00″ +48° 21′ 00″ Right ascension (J2000) 1′ 11 h 51 m in 04.0 s 11 h 51 m in 0.0 s 11 h 50 m in 56.0 s 11 h 50 m in 52.0 s 11 h 50 m in 48.0 s Right ascension (J2000) Fig. 1 | Total intensity deconvolved images of the region around GJ 1151 for two different epochs. Left panel: 16 June 2014. Right: 28 May 2014. The cross- hairs point to the location of GJ 1151 (see Extended Fig. 1 for astrometric details). The inset in both panels displays the Stokes V (circular polarization) image for the respective epoch. The time–frequency-averaged Stokes I and V flux densities are 0.89(8)mJy and 0.57(4)mJy, respectively. The grey circle in top-left corner indicates the width of the point spread function. 0.75 1.00 1.25 1.50 0.75 1.00 1.25 1.50 ity (mJy) ity (mJy) a b The transient nature and high polarization fraction are inconsistent with known properties of extragalactic radio sources, but consistent with that of stellar and planetary emissions. ɹˠ They conclusively associate the radio source with GJ 1151.
11. bandwidths of Δν=ν  1 I and a duration of many hours. (2) Coherent emission (plasma or cyclotron emission), similar to solar radio bursts, characterized by a high degree of circular polarization (up to 100%), narrow instantaneous bandwidths (Δν=ν  1 I ) and a dura- tion ranging from seconds to minutes. The observed emission does not fit into either of these phenomenological classes. It is broad- band, has a duration of >8 h and is highly circularly polarized. The closest analogue of such emission is auroral radio emission from substellar objects such as planets and ultracool dwarfs3,17,18. While canonical stellar radio bursts are powered by impulsive heating of plasma trapped in compact coronal loops11,19 of size much smaller than the stellar radius, radio aurorae in substellar objects are driven by global current systems in a large-scale dipolar magnetic field. relativistic plasma are plasma and cyclotron emission, which lead to emission at harmonics of the plasma frequency ν p and the cyclotron frequency ν c , respectively. Stellar busts at centimetre wavelengths have previously been suc- cessfully modelled as fundamental plasma emission from coronal loops19. However, the emissivity of the fundamental emission drops nonlinearly with decreasing frequency. For typical coronal scale heights of quiescent red dwarfs, the height-integrated fundamen- tal emission is restricted to brightness temperatures of <1011 K at 150 MHz (Methods), which cannot account for the observed emis- sion with Tb  1012 K I . Second harmonic plasma emission has a higher emissivity at low frequencies but cannot attain the high observed level of fractional polarization (Methods). These inconsis- 0 1 2 3 4 5 6 7 8 Time since MJD 56823.6251 (h) –0.50 –0.25 0 0.25 0.50 0.75 1.00 1.25 1.50 –0.50 –0.25 0 0.25 0.50 0.75 1.00 1.25 1.50 Flux density (mJy) Flux density (mJy) a Stokes I Stokes V 120 130 140 150 160 Frequency (MHz) b Stokes I Stokes V Fig. 2 | The variability of the flux density. a,b, The temporal (a) and spectral (b) variability of the total flux density (Stokes I; black circles) and circular polarized flux density (Stokes V; magenta squares) of the radio source in GJ 1151. The spectrum is measured over the entire 8h exposure and the time series is measured over the entire bandwidth. The error bars span ±1σ. MJD is the modified Julian date. Stellar radio emission falls into two broad phenomenological categories: (1) Incoherent gyrosynchrotron emission: low degree of polarization, duration of many hours. (2) Coherent plasma/cyclotron emission: high degree of polarization, duration up to minutes. ɹˠ The observed emission does not fit into either of these phenomenological classes. The closest analogue of such emission is auroral radio emission from substellar objects such as planets. (c) NASA
12. weighting with a robustness parameter of −0.5 for the Stokes I images. is leads to higher noise level than naturally weighted images, but is more robust to systematic errors as it down-weights short baselines. Since the Stokes V sky is largely empty, we chose a Briggs’ robustness parameter of +0.5 for the Stokes V images, which being closer to natural weighting yields lower noise levels. specific scenarios30. However, if coronal loops in the entire stellar disk contribute to the emission, as required by the brightness temperature constraint, then the opposing handedness of emission from regions with oppositely directed magnetic fields must lead to a substantially lower degree of net polarization. Cyclotron maser from flaring coronal loop. We consider a compact magnetic 4 6 8 10 12 14 Distance/stellar radius 22.0 22.5 23.0 23.5 24.0 24.5 25.0 log 10 [Starward flux (ergs s–1)] B* = 100 G n 0 = 2 × 104 cm–3 ST model LZ model –3 –2 –1 0 a c d b log 10 (M A ) –3 –2 –1 0 log 10 (M A ) 0.25 Closed field Open field 0.5 1.0 Orbital period (d) 10 20 30 40 50 Distance/stellar radius 21.0 21.5 22.0 22.5 23.0 23.5 24.0 24.5 25.0 log 10 [Starward flux (ergs s–1)] B* = 100 G n base = 106 cm–3 ST model LZ model 0.5 1.0 2.0 3.0 5.0 7.0 Orbital period (d) Fig. 3 | A comparison of observationally inferred and theoretical values for the starward Poynting flux from sub-Alfvénic interaction with an Earth-size exoplanet. a–d, A closed dipolar geometry (a,b) and an open Parker spiral geometry (c,d) are assumed for the stellar magnetic field. a,c, Alfvén Mach number of the interaction (M A ). b,d, The cyan rectangle is the range allowed by the observed radio flux density. The blue and orange curves show the Poynting flux for two theoretical models of the interaction: the ST model proposed by refs. 5,6 and the LZ model proposed by ref. 7. B * is the assumed surface magnetic field of the star. n 0 is the plasma density at the location of the putative planet (closed-field case) and n base is the base density of the coronal wind (open-field case). Further details are given in Methods. A planet in a one- to five-day-long orbit can satisfy the total energy and brightness temperature requirements for the observed radio emission.
13. https://youtu.be/uQFESMYSrTE
14. • The star DMPP-3 (HD 42936) shows signs of circumstellar absorption, indicative of mass loss from ablating planets. • They report the radial velocity discovery of a highly eccentric 507 day binary companion and a hot super-Earth-mass planet in a 6.67 day orbit around the primary star. • The planet is possibly the residual core of a giant planet precursor, consistent with the inferred circumstellar gas shroud. • The binary is considerably tighter than others known to host planets orbiting only one of the component stars. • Barnes et al., An ablating 2.6 M⊕ planet in an eccentric binary from the Dispersed Matter Planet Project, Nat. Astron. (2020) (c) Mark A. Garlick
15. • Some highly irradiated close-in exoplanets orbit stars showing anomalously low stellar Ca II H and K chromospheric emission. • They attribute this deficit to absorption by circumstellar gas replenished by mass loss from ablating planets. • A priori inferences about the presence of short-period planets enable the efficient discovery of the close-in exoplanets. • They use the High Accuracy Radial velocity Planet Searcher (HARPS) to detect these exoplanets. Dispersed Matter Planet Project (DMPP) ARTICLES In addition to DMPP-3A b, the periodograms reveal further significant short-period peaks with P = 5.85, 10.40, 14.95 d that Normally RV if substa lack of corr of significan convective b emission fro compared w Activity and shift. For the modulation For later sp times small expected23. A root mean s suggesting D circumstella A strong is found in s S-index and 792.6 d betw FWHM valu number of p cycle, chang period plan cannot rule and are resp The proper Table 1 | HD 42936 (DMPP-3A) stellar parameters with 1σ uncertainties from various sources (see references indicated) Parameter Value Reference Spectral type K0V 8 Parallax (mas) 20.45±0.26 9 Distance (pc) 48.9±0.6 9 V (apparent magnitude) 9.09 SIMBAD B – V (apparent magnitude) 0.91 SIMBAD logR´ HK −5.14±0.05 11 T eff (K) 5,138±99 12 [Fe/H] 0.18±0.09 10 log[g (cm s−2)] 4.30±0.51 12 v sin i (km s−1) 1.97±0.14 12 v mac (km s−1) 1.50±0.14 12 R * (R ⊙ ) 0.91±0.02 12 M * (M ⊙ ) 0.87±0.05 12 Age (Gyr) 10.9±4.7 12 For SIMBAD see http://simbad.u-strasbg.fr. v mac , macroturbulent velocity.
16. ARTICLES NATURE ASTRONOMY however, be resolvable by state-of-the-art imaging systems38 and thus offers the prospect of measuring the emission from a binary companion at the very bottom of the main sequence. High- resolution spectroscopic techniques that make use of the many photospheric lines should be able to detect DMPP-3B in the pho- tometric K band39 where it is expected to be only ~800–1,000 times fainter than DMPP-3A. The estimated masses of DMPP-3A and DMPP-3B imply a DMPP-3B velocity amplitude of K ~ 30 ± 2 km s−1 (for orbital inclination, i = 90o), ensuring the spectroscopic signa- tures of each component are well resolved. Astrometric observa- tions by Gaia should enable us to determine the true mass and thus the orbital inclination40. Transit probability, eclipses and phase curve variations of the super-Earth-mass planet. The relatively low amplitude of the sig- nificant 6.6732 d Keplerian Signal 2 is reflected in the 18% uncer- tainty of the derived minimum mass. For this period, for random orientations the transit probability is 6.4%41; however, angular momentum considerations suggest the ablated planetary material is likely to remain concentrated in the orbital plane. Consequently, the transit probability for bodies in the DMPP-3 system is higher than for a randomly oriented system. For randomly oriented orbits, the probability that the DMPP- 3AB system is an eclipsing binary is small. The distance between the two stars at inferior conjunction of DMPP-3B is 1.09 au, imply- ing a probability of transit of only 0.4% if randomly oriented. Nonetheless, the possibility of an eclipsing binary containing a star at the hydrogen-burning limit is exciting, and worth exploring with high-quality photometry. The proximity of the DMPP-3 system, and the apparent lack of starspots on DMPP-3A makes DMPP-3A b an excellent prospect for detection of phase-dependent reflected light. This effect has less demanding alignment requirements than transits, and the exis- tence of absorbing material in the line of sight implies the system is likely to be more-or-less edge-on. For this reason, and to search for transits and eclipses DMPP-3 is an excellent prospective target for high-quality space-based photometry, but DMPP-3 is in a region of sky inaccessible to the European Space Agency’s CHaracterising ExOPlanet Satellite (CHEOPS) mission. Stability of DMPP-3A b and implications of orbital simulations –4,000 –2,000 0 2,000 a DMPP-3 observed RVs with fit and residuals RV (m s–1) –5 0 5 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Residual (m s–1) BJD – 2454579.56469 (d) –4,000 –3,000 –2,000 –1,000 0 1,000 2,000 0 100 200 300 400 500 RV (m s–1) Time (d) CHEPS (2008–2013) DMPP (2015–2018) CORALIE (2017) DMPP (nightly average) Phase-folded RVs and fit: P = 506.84 d, e = 0.596 0 0.2 0.4 0.6 0.8 1.0 Phase b –5 0 5 0 1 2 3 4 5 6 0 0.2 0.4 0.6 0.8 1.0 Phase RV (m s–1) Time (d) c Signal 2 folded: P = 6.6732 d, e = 0.140 are not particularly suitable for calibrating models of isolated ana- logues. With properly calibrated models, the fundamental param- eters of isolated objects can be inferred more reliably, or at least with better understood uncertainties, from the observations. DMPP-3B is a vitally interesting and important object for this reason, and because its orbital eccentricity means we can potentially probe how the atmospheric properties respond to changing proximity of the primary star. DMPP-3AB is not a particularly tight binary system: even with e = 0.597, tidal dissipation between the stars is negligible, with inspiral time τ B ~ 1.2 × 1017 yr (ref. 36). We estimate the age of DMPP-3 to be ≥6.2 Gyr (Table 1). For ages 2–10 Gyr, there is little change in luminosity expected for a 0.08 M ʘ object37; we expect the V-band contrast ratio between DMPP-3A 3AB system is an eclipsing bin the two stars at inferior conjunc ing a probability of transit of Nonetheless, the possibility of a at the hydrogen-burning limit is high-quality photometry. The proximity of the DMPP starspots on DMPP-3A makes for detection of phase-depend less demanding alignment requi tence of absorbing material in th likely to be more-or-less edge-o transits and eclipses DMPP-3 is high-quality space-based photo of sky inaccessible to the Europ ExOPlanet Satellite (CHEOPS) m Stability of DMPP-3A b and im for empirical RVs. With P orb = and eccentricity of HD 19176 cal to DMPP-3B’s, though M p s 191760B firmly in the brown candidates are identified, HD 1 configurations and stability for >0.17–0.18 au are not expected tigating the orbital stability of pl ent mass ratios42 predicts that in orbiting one binary component) planetary semi-major axes a p < we carried out orbital integratio Methods for further details). Th or equivalently >720,600 orbits acteristic quasi-periodic modula repeat throughout the simulatio is modified by the orbit of DMP e p,start = 0.14 results in modulatio ues per orbit of 0.00 < e p < 0.18. on the orbital timescale of DM with an r.m.s. of 0.02. In all orbital simulations the –4,000 0 100 200 300 400 500 Time (d) –5 0 5 0 1 2 3 4 5 6 0 0.2 0.4 0.6 0.8 1.0 Phase RV (m s–1) Time (d) c Signal 2 folded: P = 6.6732 d, e = 0.140 Fig. 1 | Observed and fitted RVs for DMPP-3. a, RV observations, maximum a posteriori fit20 (see Table 1) and residuals for the M p sin i ~ 80 M Jup companion DMPP-3B in a high-eccentricity orbit. Observation times are Barycentric Julian Date (BJD) relative to the first observation on BJD = 2454579.56469. b, RVs folded on the DMPP-3B orbit. c, Phase fold of Signal 2 (see Table 1), indicating a 2.6M ⊕ planet DMPP-3A. All RVs are plotted with 1σ uncertainties. The RVs suggest the presence of a low-mass stellar binary companion orbiting DMPP-3. ɹˠ DMPP-3B: very low mass (~80 ), very close (~507day or ~1.22 au) , highly eccentric ( ~0.59). Significant low-amplitude (Signal 2) indicates the presence of a low-mass planet around DMPP-3A. ɹˠ DMPP-3A b: super-Earth-mass (~2.58 ), very close (6.67 day), slightly eccentric ( ~0.14). MJup e M⊕ e
17. Formation and Evolution of the DMPP System 1. Conventional planet formation models (Core accretion models): Any circumprimary protoplanetary disk would be truncated by the tidal effect of DMPP-3B, which would both reduce the mass available for formation of planets and severely limit the lifetime of the disk. 2. Capture of circumbinary (P-type) planets into S-type orbits following scattering: Simulations suggest only ~1% of scattered P-type planets would be so captured in DMPP-3 so this seems intrinsically unlikely. 3. Eccentric Kozai-Lidov effect: The most promising mechanism for the current proximity of DMPP-3A b to its host star is the excitation of the eccentricity for the inner orbit in a triple system by DMPP-3B. In addition, the formation of very low mass stars such as DMPP-3B is poorly understood…
18. Please also see these articles: Haswell et al., Dispersed Matter Planet Project discoveries of ablating planets orbiting nearby bright stars, Nature Astronomy, 4, 408-418 (2020) Staab et al., A compact multi-planet system around a bright nearby star from the Dispersed Matter Planet Project, Nature Astronomy, 4, 399-407 (2020) https://www.eso.org/public/blog/hunting-hot-exoplanets/