Stellar Classification & Planet Detection Meteo 466.
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Transcript of Stellar Classification & Planet Detection Meteo 466.
Stellar Classification & Planet Detection
Meteo 466
Reading for this week
• “How to Find a Habitable Planet”, James Kasting. Chapter 10, 11 & 12.
• For this lecture, part of Chapter 10 & all of Chapter 11
• Cassan et al., Nature (2012)
• Udry & Santos, Ann. Rev. Astron. Astrophys. (2007))
How bright is a star ?• The brightness of a star is specified in magnitudes.
• Hipparchus (190 B.C – 120 B.C) based it on how bright a star appeared to the unaided eye.
• Brightest stars are Magnitude 1 & dimmest stars are Magnitude 6 (barely visible)
• Refined definition: Difference of 5 magnitudes (1 to 6) corresponds to a factor of 100 times in intensity (flux).
m1 – m2 = -2.5 log(flux2 / flux1)
Color- (apparent) magnitude
Apparent Magnitude: Brightness Measure as seen from Earth Color: The difference in apparent magnitudes in different filters
Color- (absolute) magnitudeIf the distance ‘d’ to the star is known: Absolute magnitude = app. Magnitude – 5 Log(d)
Mainsequence
White dwarfs
Giants
Hertzsprung-Russell (HR) Diagram
http://www.physics.howard.edu/students/Beth/bh_stellar.html
See also The Earth System,p. 194
G stars
O and Bstars
Main sequence
M-stars
Sun
O B A F G K M
Evolution of Sun like star
Is white-dwarf“The” end ??
Evolution of Sun like star
Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA),Northwestern University
Evolution of 10 MSun star
FusionFission
Evolution of 10 MSun star
Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA),Northwestern University
Stars in the solar neighborhood Within 12.5 light years, there are 33 stars. Most of themRed dwarfs.
• Ultimate goal: To find Earth-like planets, if they exist, and to search for evidence of life
– So, how do we do that?
Exoplanet detection methods(ordered by the number of detections )
Indirect• Radial velocity (Doppler method)• Transits• Gravitational microlensing• Pulsar planets• Astrometric
Direct• Optical imaging• Infrared interferometry
Pulsar planets (Doppler technique in time)
• Alex Wolszczan (1991): 3 planets around pulsar PSR PSR B1257+12B1257+12
• Arecibo radio telescope
http://www.astro.psu.edu/users/alex/pulsar_planets_text.html
Extrasolar planet geometry
• One must think about geometry at which the system is being observed• Let i = inclination of the planet’s orbital plane with respect to the plane of the sky = angle of the planet’s orbital planet with respect to the observer
Radial (Doppler) Velocity
http://www.eso.org/public/videos/eso1035g/
Radial (Doppler) Velocity
Jupiter: K = 12.6 m/sEarth : K = 0.1 m/s
http://www.astro.sunysb.edu/mzingale/software/astro/
First detection around sun like star :
51 Pegasi b ~ 0.5 Jupiter (Mayor & Queloz, 1995)
Velocity curve for 51 Pegasus(Mayor & Queloz, 1996)
• Mass of the planet is only a lower limit because the plane of the planet’s orbit is uncertain (Msin I = 0.47 MJ in this case)
http://obswww.unige.ch/~udry/planet/51peg.html
Radial (Doppler) Velocityelliptical orbit
Velocity curve for HD66428
• More often than not, the velocity curves are not symmetric orbit is eccentric (e = 0.5 in this case)
http://exoplanets.org/figures.html
RV around M-dwarfs
Mahadevan et al.(2011)
Currently known exoplanets
V
E
J
exoplanet.eu
Planet eccentricity vs. semi-major axis(Jan 27, 2012)
Extrasolar Planet Encyclopedia
88 Known Multi - Planet Systems
Kepler - 11
Planetary systems allow for more detailed analysis
• 2 massive planets orbiting HD 168443
• Planetary masses– 8 MJ
– 18 MJ
HD 69830b : 0.61 neptunec : 0.70 neptuned : 1.07 neptune
Gliese 581 system• Spectral type: M3V
(0.31 M, 0.0135 L)
• 4 planets discovered by radial velocity:
a (AU) Mass (M)
b 0.041 >15.6
c 0.073 >5.06d 0.253 >8.3e 0.028 >1.7
(Image from Wikkipedia)Ref.: S. Udry et al., A&A (2007)
Tentative conclusions for the Gliese 581 system*
• Gliese 581c (> 5.1 M) is probably not habitable– Stellar flux is 30% higher than that for Venus
• Gliese 581d (>8.3 M) could conceivably be habitable, but it is probably an ice giant– Near the (poorly determined) outer edge of the HZ
*Selsis et al., A&A (2007)*von Bloh et al., A&A (2007)
Gliese 581g ?• Gliese 581g (~ 3
M), “Zarmina’s world”, apparently exists in the HZ (Vogt et al 2010)
• The Swiss group with HARPS instrument found it doesn’t exist !
Planet Mass Distribution
Howard et al. (2011), Science
Occurrence rate α M-0.48
(for periods < 50 days)
Packed Planetary systemsPlanetary systems form in such a way that the system could not support additional planets between the orbits of the existing ones (gaps with stable orbits contain an unseen planet)
HD 74156 (Barnes et al.2005)HD 47186 (Kopparapu et al. 2009)
Barnes et al.(2005)Kopparapu et al. (2009)
Gravitational microlensing
• Planets can also be detected by gravitational microlensing• This method takes advantage of the fact that, according to general relativity, light rays are bent by a gravitational field -- or, equivalently, space-time is distorted and light travels along straight paths in the distorted reference frame)
http://www.astro.cornell.edu/academics/courses/astro201/microlensing.htm
dL
ds
A microlensing event
http://www.nasa.gov/topics/universe/features/planet20110518-video.html
• When the lensing star passes in front of the source star, the light from the source star is amplified by a factor of as much as 10-20
• The typical duration of a microlensing event is minutes to hours
An event with planets
• If the lensing star has planets, then the light curve can be distorted (i.e., you get spikes)
• The planets must be near the Einstein ring radius Einstein ring radius to be detected– Typically, the ring radius is
outside of the habitable zone, so this technique is not that useful for finding habitable planets
http://exoplanet.eu/catalog-microlensing.php
Planet Mass Distribution (Microlensing)
Cassan et al.(2012)
Sensitivity:0.5 to 10 AU5 Earth to 10 Jup
• The majority of all detected planets havemasses below that ofSaturn, though the surveysensitivity is much lowerfor those planets
• Low-mass planets arethus found to be much morecommon than giant planets.
Planet Mass Distribution (Microlensing)
• 17% of stars host Jupiter mass planets
• 52% of stars host Neptune mass
• 62% of stars host Super-Earths • On average, every star in the
Milkyway has 1.6 planetstwithin 0.5 to 10 AU !!
• Planets around stars in our Galaxy thus seem to be the rule rather than the exception.
Historical astrometry: Barnard’s star
• Second closest star to Earth (6 light yrs), in Ophiucus
• Red dwarf (M3.8)• Largest stellar proper motion
(10.3”/yr)– Moving towards us. Will be
closest star (3.8 l.y.) in about 12,000 yrs
• Discovered by Edward Emerson Barnard (1957-1923)
• Studied hard by Peter van de Kamp from 1938 until his death in 1995. Thought to have a planet, but this hypothesis was later proved to be incorrect
1985
1990
1995
20002005
The sexagesimal system of angular measurement
unit value symbol abbreviations conversion
degree 1/360 circle ° deg 17.45 mrad
arcminute 1/60 degree ′ (prime)arcmin, amin, , MOA
290.89 µrad
arcsecond 1/60 arcminute″ (double prime)
arcsec 4.8481 µrad
milliarcsecond 1/1000 arcsecond
mas 4.848 nrad
http://en.wikipedia.org/wiki/Arcsecond
• Equivalently, there are 1,296,000 arcsec in a circle
Determination of parallax
• A star’s parallax, p, is the angle by which it appears to move as the Earth moves around the Sun
• A star that moves by 1 arcsecond when Earth moves by 1 AU relative to the Sun is defined to be at at distance of 1 parsec
• 1 pc = 1 AU/sin p = 3.0857×1013 km
= 3.262 light years
http://en.wikipedia.org/wiki/Parsec
Astrometric method
• Calculated motion of the Sun from 1960 to 2025, as viewed from a distance of 10 pc, or about 32 light years above the plane of the Solar System, i.e., at i = 0o
• Scale is in arcseconds• You get the actual mass of the
planet because the plane of the planet’s orbit can be determined
• Can do astrometry from the ground, but the best place to do it is in space
http://planetquest.jpl.nasa.gov/Navigator/material/sim_material.cfm
Astrometric missions
• Hipparcos 1989 – 1993 (ESA)
• Precise proper motion & parallax for 118,000 stars (1 milli-arc sec)
Sun-Earth 0.3 micro-arc sec
• Gaia 2013 (ESA)• Parallax for 1 billion stars (20 micro-arc sec)• 3-D map of our Galaxy
SIM – Space Interferometry Mission
http://planetquest.jpl.nasa.gov/SIM/simImageGallery.cfm
• This mission will do extremely accurate astrometry from space