Resolving the Radio Photospheres of Main Sequence Stars
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Stellar Magnetic Activity – Star-Planet Interactions
EPJ Web of Conferences 101, 005 02 (2015) DOI: 10.1051/epjconf/2015101005 02 C Owned by the authors, published by EDP Sciences, 2015 Stellar magnetic activity – Star-Planet Interactions Poppenhaeger, K.1,2,a 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambrigde, MA 02138, USA 2 NASA Sagan Fellow Abstract. Stellar magnetic activity is an important factor in the formation and evolution of exoplanets. Magnetic phenomena like stellar flares, coronal mass ejections, and high- energy emission affect the exoplanetary atmosphere and its mass loss over time. One major question is whether the magnetic evolution of exoplanet host stars is the same as for stars without planets; tidal and magnetic interactions of a star and its close-in planets may play a role in this. Stellar magnetic activity also shapes our ability to detect exoplanets with different methods in the first place, and therefore we need to understand it properly to derive an accurate estimate of the existing exoplanet population. I will review recent theoretical and observational results, as well as outline some avenues for future progress. 1 Introduction Stellar magnetic activity is an ubiquitous phenomenon in cool stars. These stars operate a magnetic dynamo that is fueled by stellar rotation and produces highly structured magnetic fields; in the case of stars with a radiative core and a convective outer envelope (spectral type mid-F to early-M), this is an αΩ dynamo, while fully convective stars (mid-M and later) operate a different kind of dynamo, possibly a turbulent or α2 dynamo. These magnetic fields manifest themselves observationally in a variety of phenomena. -
Arxiv:1110.1805V1 [Astro-Ph.SR] 9 Oct 2011 Htte Olpet Ukaclrt H Lcrn,Rapid Emissions
Noname manuscript No. (will be inserted by the editor) Energy Release and Particle Acceleration in Flares: Summary and Future Prospects R. P. Lin1,2 the date of receipt and acceptance should be inserted later Abstract RHESSI measurements relevant to the fundamental processes of energy release and particle acceleration in flares are summarized. RHESSI's precise measurements of hard X-ray continuum spectra enable model-independent deconvolution to obtain the parent elec- tron spectrum. Taking into account the effects of albedo, these show that the low energy cut- < off to the electron power-law spectrum is typically ∼tens of keV, confirming that the accel- erated electrons contain a large fraction of the energy released in flares. RHESSI has detected a high coronal hard X-ray source that is filled with accelerated electrons whose energy den- sity is comparable to the magnetic-field energy density. This suggests an efficient conversion of energy, previously stored in the magnetic field, into the bulk acceleration of electrons. A new, collisionless (Hall) magnetic reconnection process has been identified through theory and simulations, and directly observed in space and in the laboratory; it should occur in the solar corona as well, with a reconnection rate fast enough for the energy release in flares. The reconnection process could result in the formation of multiple elongated magnetic islands, that then collapse to bulk-accelerate the electrons, rapidly enough to produce the observed hard X-ray emissions. RHESSI's pioneering γ-ray line imaging of energetic ions, revealing footpoints straddling a flare loop arcade, has provided strong evidence that ion acceleration is also related to magnetic reconnection. -
Chapter 11 SOLAR RADIO EMISSION W
Chapter 11 SOLAR RADIO EMISSION W. R. Barron E. W. Cliver J. P. Cronin D. A. Guidice Since the first detection of solar radio noise in 1942, If the frequency f is in cycles per second, the wavelength radio observations of the sun have contributed significantly X in meters, the temperature T in degrees Kelvin, the ve- to our evolving understanding of solar structure and pro- locity of light c in meters per second, and Boltzmann's cesses. The now classic texts of Zheleznyakov [1964] and constant k in joules per degree Kelvin, then Bf is in W Kundu [1965] summarized the first two decades of solar m 2Hz 1sr1. Values of temperatures Tb calculated from radio observations. Recent monographs have been presented Equation (1 1. 1)are referred to as equivalent blackbody tem- by Kruger [1979] and Kundu and Gergely [1980]. perature or as brightness temperature defined as the tem- In Chapter I the basic phenomenological aspects of the perature of a blackbody that would produce the observed sun, its active regions, and solar flares are presented. This radiance at the specified frequency. chapter will focus on the three components of solar radio The radiant power received per unit area in a given emission: the basic (or minimum) component, the slowly frequency band is called the power flux density (irradiance varying component from active regions, and the transient per bandwidth) and is strictly defined as the integral of Bf,d component from flare bursts. between the limits f and f + Af, where Qs is the solid angle Different regions of the sun are observed at different subtended by the source. -
Structure of the Solar Chromosphere
Multi-Wavelength Investigations of Solar Activity Proceedings IAU Symposium No. 223, 2004 c 2004 International Astronomical Union A.V. Stepanov, E.E. Benevolenskaya & A.G. Kosovichev, eds. DOI: 10.1017/S1743921304005587 Structure of the solar chromosphere Sami K. Solanki Max-Planck-Institut f¨ur Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany, email: [email protected] Abstract. The chromosphere is an intriguing part of the Sun that has stubbornly resisted all attempts at a comprehensive description. Thus, observations carried out in different wavelength bands reveal very different, seemingly incompatible properties. Not surprisingly, a debate is raging between supporters of the classical picture of the chromosphere as a nearly plane parallel layer exhibiting a gentle temperature rise from the photosphere to the transition region and proponents of a highly dynamical atmosphere that includes extremely cool gas. New data are required in order settle this issue. Here a brief overview of the structure and dynamics of the solar chromosphere is given, with particular emphasis on the chromospheric structure of the quiet Sun. The structure of the magnetic field is also briefly discussed, although filaments and prominences are not considered. Besides the observations, contrasting models are also critically discussed. 1. Introduction: Chromospheric structure The solar chromosphere is traditionally defined as the layer, lying between the pho- tosphere and the transition region, where the temperature first starts to rise outwards. The lower boundary in this definition is formed by the temperature minimum layer. The upper boundary is less clearly marked, however, with a temperature around 1−2×104 K generally being prescribed. -
The Sun's Dynamic Atmosphere
Lecture 16 The Sun’s Dynamic Atmosphere Jiong Qiu, MSU Physics Department Guiding Questions 1. What is the temperature and density structure of the Sun’s atmosphere? Does the atmosphere cool off farther away from the Sun’s center? 2. What intrinsic properties of the Sun are reflected in the photospheric observations of limb darkening and granulation? 3. What are major observational signatures in the dynamic chromosphere? 4. What might cause the heating of the upper atmosphere? Can Sound waves heat the upper atmosphere of the Sun? 5. Where does the solar wind come from? 15.1 Introduction The Sun’s atmosphere is composed of three major layers, the photosphere, chromosphere, and corona. The different layers have different temperatures, densities, and distinctive features, and are observed at different wavelengths. Structure of the Sun 15.2 Photosphere The photosphere is the thin (~500 km) bottom layer in the Sun’s atmosphere, where the atmosphere is optically thin, so that photons make their way out and travel unimpeded. Ex.1: the mean free path of photons in the photosphere and the radiative zone. The photosphere is seen in visible light continuum (so- called white light). Observable features on the photosphere include: • Limb darkening: from the disk center to the limb, the brightness fades. • Sun spots: dark areas of magnetic field concentration in low-mid latitudes. • Granulation: convection cells appearing as light patches divided by dark boundaries. Q: does the full moon exhibit limb darkening? Limb Darkening: limb darkening phenomenon indicates that temperature decreases with altitude in the photosphere. Modeling the limb darkening profile tells us the structure of the stellar atmosphere. -
Tures in the Chromosphere, Whereas at Meter Wavelengths It Is of the Order of 1.5 X 106 Degrees, the Temperature of the Corona
1260 ASTRONOMY: MAXWELL PROC. N. A. S. obtained directly from the physical process, shows that the functions rt and d are nonnegative for x,y > 0. It follows from standard arguments that as A 0 these functions approach the solutions of (2). 4. Solution of Oriqinal Problem.-The solution of the problem posed in (2.1) can be carried out in two ways, once the functions r(y,x) and t(y,x) have been deter- mined. In the first place, since u(x) = r(y,x), a determination of r(y,x) reduces the problem in (2.1) to an initial value process. Secondly, as indicated in reference 1, the internal fluxes, u(z) and v(z) can be expressed in terms of the reflection and transmission functions. Computational results will be presented subsequently. I Bellman, R., R. Kalaba, and G. M. Wing, "Invariant imbedding and mathematical physics- I: Particle processes," J. Math. Physics, 1, 270-308 (1960). 2 Ibid., "Invariant imbedding and dissipation functions," these PROCEEDINGS, 46, 1145-1147 (1960). 3Courant, R., and P. Lax, "On nonlinear partial dyperential equations with two independent variables," Comm. Pure Appl. Math., 2, 255-273 (1949). RECENT DEVELOPMENTS IN SOLAR RADIO ASTRONOJIY* BY A. MAXWELL HARVARD UNIVERSITYt This paper deals briefly with present radio models of the solar atmosphere, and with three particular types of experimental programs in solar radio astronomy: radio heliographs, spectrum analyzers, and a sweep-frequency interferometer. The radio emissions from the sun, the only star from which radio waves have as yet been detected, consist of (i) a background thermal emission from the solar atmosphere, (ii) a slowly varying component, also believed to be thermal in origin, and (iii) nonthermal transient disturbances, sometimes of great intensity, which originate in localized active areas. -
Stellar Atmospheres I – Overview
Indo-German Winter School on Astrophysics Stellar atmospheres I { overview Hans-G. Ludwig ZAH { Landessternwarte, Heidelberg ZAH { Landessternwarte 0.1 Overview What is the stellar atmosphere? • observational view Where are stellar atmosphere models needed today? • ::: or why do we do this to us? How do we model stellar atmospheres? • admittedly sketchy presentation • which physical processes? which approximations? • shocking? Next step: using model atmospheres as \background" to calculate the formation of spectral lines •! exercises associated with the lecture ! Linux users? Overview . TOC . FIN 1.1 What is the atmosphere? Light emitting surface layers of a star • directly accessible to (remote) observations • photosphere (dominant radiation source) • chromosphere • corona • wind (mass outflow, e.g. solar wind) Transition zone from stellar interior to interstellar medium • connects the star to the 'outside world' All energy generated in a star has to pass through the atmosphere Atmosphere itself usually does not produce additional energy! What? . TOC . FIN 2.1 The photosphere Most light emitted by photosphere • stellar model atmospheres often focus on this layer • also focus of these lectures ! chemical abundances Thickness ∆h, some numbers: • Sun: ∆h ≈ 1000 km ? Sun appears to have a sharp limb ? curvature effects on the photospheric properties small solar surface almost ’flat’ • white dwarf: ∆h ≤ 100 m • red super giant: ∆h=R ≈ 1 Stellar evolution, often: atmosphere = photosphere = R(T = Teff) What? . TOC . FIN 2.2 Solar photosphere: rather homogeneous but ::: What? . TOC . FIN 2.3 Magnetically active region, optical spectral range, T≈ 6000 K c Royal Swedish Academy of Science (1000 km=tick) What? . TOC . FIN 2.4 Corona, ultraviolet spectral range, T≈ 106 K (Fe IX) c Solar and Heliospheric Observatory, ESA & NASA (EIT 171 A˚ ) What? . -
What Do We See on the Face of the Sun? Lecture 3: the Solar Atmosphere the Sun’S Atmosphere
What do we see on the face of the Sun? Lecture 3: The solar atmosphere The Sun’s atmosphere Solar atmosphere is generally subdivided into multiple layers. From bottom to top: photosphere, chromosphere, transition region, corona, heliosphere In its simplest form it is modelled as a single component, plane-parallel atmosphere Density drops exponentially: (for isothermal atmosphere). T=6000K Hρ≈ 100km Density of Sun’s atmosphere is rather low – Mass of the solar atmosphere ≈ mass of the Indian ocean (≈ mass of the photosphere) – Mass of the chromosphere ≈ mass of the Earth’s atmosphere Stratification of average quiet solar atmosphere: 1-D model Typical values of physical parameters Temperature Number Pressure K Density dyne/cm2 cm-3 Photosphere 4000 - 6000 1015 – 1017 103 – 105 Chromosphere 6000 – 50000 1011 – 1015 10-1 – 103 Transition 50000-106 109 – 1011 0.1 region Corona 106 – 5 106 107 – 109 <0.1 How good is the 1-D approximation? 1-D models reproduce extremely well large parts of the spectrum obtained at low spatial resolution However, high resolution images of the Sun at basically all wavelengths show that its atmosphere has a complex structure Therefore: 1-D models may well describe averaged quantities relatively well, although they probably do not describe any particular part of the real Sun The following images illustrate inhomogeneity of the Sun and how the structures change with the wavelength and source of radiation Photosphere Lower chromosphere Upper chromosphere Corona Cartoon of quiet Sun atmosphere Photosphere The photosphere Photosphere extends between solar surface and temperature minimum (400-600 km) It is the source of most of the solar radiation. -
First Firm Spectral Classification of an Early-B Pre-Main-Sequence Star: B275 in M
A&A 536, L1 (2011) Astronomy DOI: 10.1051/0004-6361/201118089 & c ESO 2011 Astrophysics Letter to the Editor First firm spectral classification of an early-B pre-main-sequence star: B275 in M 17 B. B. Ochsendorf1, L. E. Ellerbroek1, R. Chini2,3,O.E.Hartoog1,V.Hoffmeister2,L.B.F.M.Waters4,1, and L. Kaper1 1 Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands e-mail: [email protected]; [email protected] 2 Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany 3 Instituto de Astronomía, Universidad Católica del Norte, Antofagasta, Chile 4 SRON, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands Received 14 September 2011 / Accepted 25 October 2011 ABSTRACT The optical to near-infrared (300−2500 nm) spectrum of the candidate massive young stellar object (YSO) B275, embedded in the star-forming region M 17, has been observed with X-shooter on the ESO Very Large Telescope. The spectrum includes both photospheric absorption lines and emission features (H and Ca ii triplet emission lines, 1st and 2nd overtone CO bandhead emission), as well as an infrared excess indicating the presence of a (flaring) circumstellar disk. The strongest emission lines are double-peaked with a peak separation ranging between 70 and 105 km s−1, and they provide information on the physical structure of the disk. The underlying photospheric spectrum is classified as B6−B7, which is significantly cooler than a previous estimate based on modeling of the spectral energy distribution. This discrepancy is solved by allowing for a larger stellar radius (i.e. -
Triggers of Magnetar Outbursts Are Likely to Affect the Crustal Yielding Threshold, and Cause It to Vary from Place to Place Within a Neutron Star’S Crust
1 Triggers of magnetar outbursts Robert C. Duncan University of Texas at Austin TX USA Abstract. Bright outbursts from Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are believed to be caused by instabilities in ultramagnetized neutron stars, powered by a decaying magnetic field. It was originally thought that these outbursts were due to reconnection instabilities in the magnetosphere, reached via slow evolution of magnetic footpoints anchored in the crust. Later models con- sidered sudden shifts in the crust’s structure. Recent observations of magnetars give evidence that at least some outburst episodes involve rearrangements and/or en- ergy releases within the star. We suggest that bursting episodes in magnetars are episodes of rapid plastic yielding in the crust, which trigger “swarms” of reconnec- tion instabilities in the magnetosphere. Magnetic energy always dominates; elastic energy released within the crust does not generate strong enough Alfv´en waves to power outbursts. We discuss the physics of SGR giant flares, and describe recent observations which give useful constraints and clues. 1.1 Introduction: A neutron star’s crust The crust of a neutron star has several components: (1) a Fermi sea of rela- tivistic electrons, which provides most of the pressure in the outer layers; (2) another Fermi sea of neutrons in a pairing-superfluid state, present only at depths below the 11 −3 “neutron drip” level where the mass-density exceeds ρdrip 4.6 10 gm cm ; arXiv:astro-ph/0401415v1 21 Jan 2004 ≈ × and (3) an array of positively-charged nuclei, arranged in a solid (but probably not regular crystalline) lattice-like structure throughout much of the crust. -
Solar Flare Energy Source Homework W/Solution
ASTRO 101 – M.M. Montgomery - montgomery@physics,ucf.edu Solar Flare Energy Source Homework Problem LWS Heliophysics Summer School 2013 Target Audience: Upper Division Undergraduate Majors or First Year Grad Students Concepts Learned: Four fundamental sources in nature and which is/are the source to CMEs and solar flares Methodology: Calculations Delivery: Handwritten calculations on one side of a standard 8½”x11” white copy paper, folded lengthwise, with printed name on outside jacket Materials: standard 8½”x11” white copy paper, pencil, scientific calculator Assessment: Correct identification from the list of givens, of what is to be found, and of correct assumptions to find the correct answers Texts and chapters used in this homework assignment: 1. Heliophysics I Plasma Physics of Local Cosmos, Chapter 8 2. Heliophysics II Space Storms and Radiation: Causes and Effects, Chapter 5 Usage: To be assigned after the student sees the CME.ppt and prior to the student performing the “Introduction to the “Introduction to the CME Evolution NASA ENLIL Software” Lab. Goal Your goal in this exercise is to determine if the source to the energy is from gravitational, electrical, nuclear, and/or magnetic field sources. Given White light solar flares are observed to have energy E~1032 ergs. These flares occur in localized, transient regions through the chromosphere and corona. For the solar flare, assume the shape is an arch of area A=1018 cm2 height L~109 cm particle mass column density (from the top of the arch to the location in the Sun’s photosphere where the temperature is minimum) ζ=10-2 g/cm2 -6 2 column mass density in the corona ζcor~10 g/cm magnetic field strength B~103 G in the photosphere near the transient region For the Sun, g is acceleration due to gravity 6 Tcor~10 K is the temperature of the corona 4 Tchrom~10 K is the temperature of the chromosphere For constants, -24 mass of hydrogen is mH~10 g k is Boltzmann’s constant in cgs units Find 1. -
Chapter 2: the Structure of The
SOLAR PHYSICS AND TERRESTRIAL EFFECTS 2+ Chapter 2 4= Chapter 2 The Structure of the Sun Astrophysicists classify the Sun as a star of average size, temperature, and brightness—a typical dwarf star just past middle age. It has a power output of about 1026 watts and is expected to continue producing energy at that rate for another 5 billion years. The Sun is said to have a diameter of 1.4 million kilometers, about 109 times the diameter of Earth, but this is a slightly misleading statement because the Sun has no true “surface.” There is nothing hard, or definite, about the solar disk that we see; in fact, the matter that makes up the apparent surface is so rarified that we would consider it to be a vacuum here on Earth. It is more accurate to think of the Sun’s boundary as extending far out into the solar system, well beyond Earth. In studying the structure of the Sun, solar physicists divide it into four domains: the interior, the surface atmospheres, the inner corona, and the outer corona. Section 1.—The Interior The Sun’s interior domain includes the core, the radiative layer, and the convective layer (Figure 2–1). The core is the source of the Sun’s energy, the site of thermonuclear fusion. At a temperature of about 15,000,000 K, matter is in the state known as a plasma: atomic nuclei (principally protons) and electrons moving at very high speeds. Under these conditions two protons can collide, overcome their electrical repulsion, and become cemented together by the strong nuclear force.