Photospheric Emission Research Articles

Photometric activity of the unique X-ray transient CI Camelopardalis (XTE J0421+560)

We present an analysis of the photometric observations of the peculiar X-ray binary and X-ray transient CI Cam (XTE J0421+560), mostly covering the interval following its 1998 outburst. We show that the most prominent variations are observed in the I band, with the amplitude decreasing toward the V (and U) passband. We find that CI Cam displays complicated shifts in the colour diagrams and show that the variations of the continuum play a significant role in the colour changes. We also resolve the signatures of the variations of the very strong Hα emission with respect to the combination of the continuum and other lines in the colours. On the basis of the shifts in the colour diagrams and of the spectral energy distribution determined from the photometry we propose that the division of the dominant contributions of the superimposed spectral components (free–free emission (Clark, J.S., Miroshnichenko, A.S., Larionov, V.M., et al., 2000, A&A, 356, 50) and the (pseudo)photospheric emission) occurs near λ = 550 nm. We make use of this division for an explanation of the decoupling of the variations of U– B and B– V from those in V– R, R– I and V– I. We observed two maxima of brightness (the second one only in R and I) separated by 1350 d in which the (pseudo)photospheric emission, f–f/f–b and Hα emission appear to be involved in a complicated way. We find some indications that the variations of the source of the optical light can be related to those of the X-ray source in quiescence, particularly in an event which we interpret as a density enhancement in the matter exchange. We explain the huge changes of the absorption of the X-ray spectrum, N H, not reflected in the colour variations in the optical region, in terms of the variations of N H confined to the region hotter than the temperature of the condensation of the dust, maybe related to the filling of the disk embedding the compact object. We argue that the dominant part of the X-ray emission comes from the close vicinity of the compact object, not from the donor. Very low-amplitude (∼0.02 mag) intranight optical variations can be present in four nights of our observations in the post-outburst period; they have the form of bumps on the time-scale of about an hour, but without any coherent component. We also point out the similarities in the situation of CI Cam and the microquasar LS 5039/RX J1826–1450, as regards the variations of brightness coming from the vicinity of the compact object.

Spectral and timing properties of a dissipative $\vec\gamma$-ray burst photosphere

We explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy as predicted by the magnetic reconnection model. Previous study of the radiative transfer in the photospheric region has shown that gradual dissipation of energy results in a hot photosphere. There, inverse Compton scattering of the thermal radiation advected with the flow leads to powerful photospheric emission with spectral properties close to those of the observed prompt GRB emission. Here, we build on that study by calculating the spectra for a large range of the characteristics of the flow. An accurate fitting formula is given that provides the photospheric spectral energy distribution in the ∼10 keV to ∼10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of the flow. It facilitates the direct comparison of the model predictions with observations, including the variability properties of the lightcurves. We verify that the model naturally accounts for the observed clustering in peak energies of the E f(E) spectrum. In this model, the Amati relation indicates a tendency for the most luminous bursts to have more energy per baryon. If this tendency also holds for individual GRB pulses, the model predicts the observed narrowing of the width of pulses with increasing photon energy.

Puzzled by GRB 060218

Abstract We study the optical–UV/X-ray spectral energy distribution of GRB 060218 during the prompt and what seems to be the afterglow phases. The results are puzzling, since if the optical–UV and the X-ray emission belong to a single blackbody then its luminosity is too large, and this blackbody cannot be interpreted as the signature of the shock breakout of the supernova. There are also serious problems in associating the emission expected from the supernova shock breakout with either the optical–UV or the X-ray emission. In the former case we derive too small ejecta velocities; in the latter case, in contrast, the required velocity is too large, corresponding to the large radius of a blackbody required to peak close to the UV band. We then present what we think is the most conservative alternative explanation, namely a synchrotron spectrum, self-absorbed in the optical–UV and extending up to the X-ray band, where we observe the emission of the most energetic electrons, which are responsible for the exponential roll-over of the spectrum. The obtained fit can explain the entire spectrum except the blackbody observed in the X-rays, which must be a separate component. The puzzling feature of this interpretation is that the same model is required to explain the spectrum also at later times, up to 105 s, because the optical–UV emission remains constant in shape and also (approximately) in normalization. In this case the observed X-ray flux is produced by self-Compton emission. Thus the prompt emission phase should last for ∼105 s or more. Finally, we show that the blackbody observed in X-rays, up to 7000 s, can be photospheric emission from the cocoon or stellar material, energized by the gamma-ray burst jet at radii comparable to the stellar radius (i.e. 1010–1011 cm), not very far from where this material becomes transparent (e.g. 1012 cm).

Active Galactic Nucleus and Starburst Classification fromSpitzerMid‐Infrared Spectra for High‐Redshift SWIRE Sources

Spectra have been obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope for 20 sources in the Lockman Hole field of the SWIRE survey. The sample is divided between sources with indicators of an obscured AGN, based primarily on X-ray detections of optically faint sources, and sources with indicators of a starburst, based on optical and near-infrared spectral energy distributions (SEDs), which show a luminosity peak from stellar photospheric emission. Ten of the 11 AGN sources have IRS spectra that show silicate absorption or are power laws; only one AGN source shows PAH emission features. All nine of the sources showing starburst SEDs in the near-infrared show PAH emission features in the IRS spectra. Redshifts are determined from the IRS spectra for all nine starbursts (1.0 < z < 1.9) and 8 of the 11 AGNs (0.6 < z < 2.5). Classification as AGN because of an X-ray detection, the classification as AGN or starburst derived from the photometric SED, and the IRS spectroscopic classification as AGN (silicate absorption) or starburst (PAH emission) are all consistent in 18 of 20 sources. The surface density for starbursts that are most luminous in the mid-infrared is less than that for the most luminous AGNs within the redshift interval 1.7 ≾ z ≾ 1.9. This result implies that mid-infrared source counts at high redshift are dominated by AGNs for f_ν(24 μm) ≳ 1.0 mJy.

TheSpitzerView of Low‐Metallicity Star Formation. I. Haro 3

We present Spitzer observations of the blue compact dwarf galaxy Haro 3, with an oxygen abundance of 12 + log(O/H) = 8.32. These data are part of a larger study of star formation and dust in low-metallicity environments. The IRS spectrum of Haro 3 shows strong narrow PAH emission, with high equivalent widths. Gaseous nebular fine-structure lines are also seen. Despite the absence of optical high-excitation lines, a faint high-ionization O IV line at 25.89 μm indicates the presence of radiation as hard as 54.9 eV. A CLOUDY model suggests that the MIR lines originate in two regions: a low-extinction optically emitting region, and an optically invisible one with much higher extinction. The morphology of Haro 3 changes with wavelength. IRAC 4.5 μm traces extended stellar photospheric emission from the body of the galaxy and hot dust continuum coming mainly from star-forming regions, 8 μm probes extended PAH emission coming mainly from the general ISM, MIPS 24 and 70 μm images map compact small-grain warm dust emission associated with active star formation, and 160 μm reflects cooler extended dust associated with older stellar populations. We have derived the optical-to-radio SED of the brightest star-forming region A in Haro 3. The best-fit DUSTY model of the SED gives a total luminosity of 2.8 × 109 L☉ and a mass of 2.8 × 106 M☉ for the ionizing clusters. We infer an extinction AV ≲ 3, intermediate between the optical AV ~ 0.5 and the radio AV ~ 8, consistent with the picture that longer wavelength observations probe more deeply into star-forming regions.

Gamma‐Ray Burst Spectral Correlations: Photospheric and Injection Effects

We present a physical framework that can account for most of the observed spectral properties of the prompt gamma-ray burst emission. This includes the variety of spectral shapes, shape evolutions, and spectral correlations between flux and spectral peak, within bursts described by Borgonovo & Ryde, and among bursts described by Amati/Ghirlanda. In our proposed model the spectral peak is given by the photospheric emission from a relativistic outflow for which the horizon length is much smaller than the radial width. The observed duration of the thermal flash will be given by the radial light-crossing time. This then gives that the typical emission site is at ~10e11 cm from the center, with a Lorentz factor of ~300. This emission is accompanied by non-thermal emission from dissipation locations outside the photosphere. The relative strength of these two components depend on injection effects at the central engine leading to varying relative location of the saturation and photospheric radii. The total emission can then reproduce the observed variety. The spectral correlations are found by assuming that the amount of energy dissipated depends non-linearly on the averaged particle density. Beside the spectral correlations this also gives a description of how the relative strength of the thermal component varies with temperature within a burst.

Spitzer Space TelescopeObservations of Var Her 04: Possible Detection of Dust Formation in a Superoutbursting Tremendous Outburst Amplitude Dwarf Nova

We present four MIPS (24 μm) and two IRAC (3.6, 4.5, 5.8, and 8.0 μm) Spitzer Space Telescope observations of the newly discovered tremendous outburst amplitude dwarf nova (TOAD) Var Her 04 during decline from superoutburst. The four MIPS observations span 271 days, and the two IRAC observations span 211 days. Along the line of sight to Var Her 04, there is a foreground M star within 1 of the variable; as a result, all of the Spitzer photometry presented in this paper is a blend of the foreground M star and Var Her 04. We estimate the quiescent level of the TOAD to be ΔV = 4-5 mag below that of the M star. Based on the spectral energy distribution and the Two Micron All Sky Survey colors, we find the M star to be an M3.5 V dwarf at a distance of 80-130 pc. Based on its outburst amplitude and quiescent apparent magnitude, we estimate the distance to Var Her 04 to be 200-400 pc, suggesting that the line-of-sight foreground star is physically unrelated to the cataclysmic variable. All of the Spitzer photometry is consistent with the photospheric emission of the line-of-sight M3.5 V star, except for one 24 μm observation obtained after the variable rebrightened. This 24 μm flux density is 75 μJy (4 σ) above the preceding and following MIPS observations. We tentatively suggest that the mid-infrared brightening of 75 μJy may be associated with a dust formation event in the superoutburst ejecta. Assuming a dust temperature of 100-400 K, we have estimated the amount of dust required. We find that 10^(-13) to 10^(-11) M_☉ of dust is needed, consistent with the amounts of mass ejection in TOADs expected during superoutburst and possibly making TOADs important contributors to the recycling of the interstellar medium.

Spitzer Far-Infrared Detections of Cold Circumstellar Disks

Observations at 70 microns with the Spitzer Space Telescope have detected several stellar systems within 65 pc of the Sun. Of 18 presumably young systems detected in this study, as many as 15 have 70-micron emission in excess of that expected from their stellar photospheres. Five of the systems with excesses are members of the Tucanae Association. The 70-micron excesses range from a factor of ~2 to nearly 30 times the expected photospheric emission from these stars. In contrast to the 70-micron properties of these systems, there is evidence for an emission excess at 24 microns for only HD 3003, confirming previous results for this star. The lack of a strong 24-micron excess in most of these systems suggests that the circumstellar dust producing the IR excesses is relatively cool (T_dust < 150 K) and that there is little IR-emitting material within the inner few AU of the primary stars. Many of these systems lie close enough to Earth that the distribution of the dust producing the IR excesses might be imaged in scattered light at optical and near-IR wavelengths.

Planetary Nebulae and Their Central Stars in the X-ray and EUV Regions

Einstein, EXOSAT, and ROSAT X-ray observations of planetary nebulae detected soft photospheric X-ray emission from their central stars, but the diffuse X-ray emission from the shocked fast stellar wind in their interiors could not be unambiguously resolved. The new generation of X-ray observatories, Chandra and XMM-Newton, have finally resolved the diffuse X-ray emission from shocked fast winds in planetary nebula interiors. Furthermore, these observatories have detected diffuse X-ray emission from bow-shocks of fast collimated outflows impinging on the nebular envelopes, and unexpected hard X-ray point-sources associated with the central stars of planetary nebulae. Here I review the results of these new X-ray observations of planetary nebulae and discuss the promise of future observations.

Formation and Evolution of Planetary Systems (FEPS): Primordial Warm Dust Evolution from 3 to 30 Myr around Sun‐like Stars

We present data obtained with the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope (Spitzer) for a sample of 74 young (t < 30 Myr old) Sun-like (0.7 < M*/M☉ < 1.5) stars. These are a subset of the observations that comprise the Spitzer Legacy science program entitled the Formation and Evolution of Planetary Systems (FEPS). Using IRAC, we study the fraction of young stars that exhibit 3.6-8.0 μm infrared emission in excess of that expected from the stellar photosphere, as a function of age from 3 to 30 Myr. The most straightforward interpretation of such excess emission is the presence of hot (300-1000 K) dust in the inner regions (<3 AU) of a circumstellar disk. Five out of the 74 young stars show a strong infrared excess, four of which have estimated ages of 3-10 Myr. While we detect excesses from five optically thick disks and photospheric emission from the remainder of our sample, we do not detect any excess emission from optically thin disks at these wavelengths. We compare our results with accretion disk fractions detected in previous studies and use the ensemble results to place additional constraints on the dissipation timescales for optically thick, primordial disks.

Investigating Disk Evolution: A High Spatial Resolution Mid‐Infrared Survey of T Tauri Stars

We present a high spatial resolution, 10-20 μm survey of 65 T Tauri binary stars in Taurus, Ophiuchus, and Corona Australis using the Keck 10 m telescopes. Designed to probe the inner ~1 AU region of the circumstellar disks around the individual stellar components in these binary systems, this study increases the number of binaries with spatially resolved measurements at 10 μm by a factor of ~5. Combined with resolved near-infrared photometry and spectroscopic accretion diagnostics, we find that ~10% of stars with a mid-infrared excess do not appear to be accreting. In contrast to an actively accreting disk system, these passive disks have significantly lower near-infrared colors that are, in most cases, consistent with photospheric emission, suggesting the presence of an inner disk hole. In addition, there appears to be a spectral type/mass dependence associated with the presence of a passive disk, with all passive disks occurring around M-type stars. The presence of a passive disk does not appear to be related to the fact that these objects are in visual binary systems; the passive disk systems span the entire range of binary separations present in the sample, and a similar fraction of passive disks is observed in a sample of single stars. The possibility that the passive disks are caused by the presence of an as yet undetected companion at a small separation (0.3-3 AU) is possible for any individual system; however, it cannot account for the spectral type dependence of the passive disk sample as a whole. We propose that these passive disks represent a subset of T Tauri stars that are undergoing significant disk evolution. The fraction of observed passive disks and the observed spectral type dependence can both be explained by models of disk evolution that include disk photoevaporation from the central star.

On the Possible Connection between Photospheric 5-Min Oscillation and Solar Flare Microwave Emission

Dynamic spectra of low-frequency modulation of microwave emission from solar flares are obtained. Data of 15 bursts observed in 1989-2000 with Metsahovi radio telescope at 37 GHz have been used. During 13 bursts a 5-min modulation of the microwave emission intensity was detected with the frequency of νI = 3.2 ± 0.24 (1σ ) mHz. Five bursts revealed a 5-min wave superimposed on a ∼1 Hz, linear frequency modulated signal generated, presumably, by coronal magnetic loop, this wave frequency is νfm = 3.38 ± 0.37 (1σ ) mHz. Both intensity and frequency modulations detected are in good agreement with the data on 5-min global oscillations of photosphere and with the data on the umbral velocity oscillations observed in the vicinity of sunspots. Possible role of p-mode photospheric oscillations in modulation of microwave burst emission is discussed.

VISIR/VLT observations of brown dwarfs in Upper Scorpius

AbstractWe present mid‐IR observations of a sample of very low‐mass stars and brown dwarfs in the Upper Scorpius association. The main goal of these observations is to derive the disk frequency and the disk properties of the sample in order to shed light on brown dwarf formation theories. The observations have been performed with VISIR at the Very Large Telescope (VLT, Paranal). A total of ten targets were observed and six were detected. In this paper we show preliminary results for two of them: DENIS‐PJ160334 and DENIS‐PJ161939. While the former shows mid‐IR fluxes consistent with pure photospheric emission, the latter displays strong mid‐IR excesses which are probably related with a circumstellar disk. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

VLT K-band spectroscopy of massive stars deeply embedded in IRAS sources with UCHII colours

We have obtained high resolution (R = 10 000) K-band spectra of candidate young massive stars deeply embedded in (ultra-) compact H  regions (UCHIIs). These objects were selected from a near-infrared survey of 44 fields centered on IRAS sources with UCHII colours. Often, the near-infrared counterpart of the IRAS source is a young embedded cluster hosting massive stars. In these clusters, three types of objects are identified. The first type (38 objects) consists of naked OB stars whose K-band spectra are dominated by photospheric emission. We classify the K-band spectra of the OB-type cluster members using near-infrared classification criteria. A few of them have a very early (O3-O4 V) spectral type, consistent with a young age of the embedded clusters. The spectral classification provides an important constraint on the distance to the embedded cluster. The ionising power of the population thus derived is compared to the information obtained from the infrared and radio flux of these sources. In most cases these two different determinations of the ionising flux are consistent, from which we conclude that we have identified the ionising star(s) in about 50% of the embedded clusters. The second type (7 objects) are point sources associated with UCHII radio emission, that exhibit nebular emission lines in the near-infrared. Six of the objects in this group produce He  emission indicative of an embedded O-type star. These objects are more embedded than the OB stars and probably do not dominate the infrared flux as measured by IRAS. They may emit the bulk of their reprocessed UV radiation at mm wavelengths. The third type (20 objects) is characterised by broad (100−200 km s −1 )B rγ emission and no photospheric absorption profiles. Bik et al. (2005, A&A, submitted) show that these objects are massive YSO candidates surrounded by dense circumstellar disks.

Formation and Evolution of Planetary Systems: Cold Outer Disks Associated with Sun‐like Stars

We present the discovery of debris systems around three solar mass stars based upon observations performed with the Spitzer Space Telescope as part of a Legacy Science Program, ``the Formation and Evolution of Planetary Systems'' (FEPS). We also confirm the presence of debris around two other stars. All the stars exhibit infrared emission in excess of the expected photospheres in the 70 micron band, but are consistent with photospheric emission at <= 33 micron. This restricts the maximum temperature of debris in equilibrium with the stellar radiation to T < 70 K. We find that these sources are relatively old in the FEPS sample, in the age range 0.7 - 3 Gyr. Based on models of the spectral energy distributions, we suggest that these debris systems represent materials generated by collisions of planetesimal belts. We speculate on the nature of these systems through comparisons to our own Kuiper Belt, and on the likely planet(s) responsible for stirring the system and ultimately releasing dust through collisions. We further report observations of a nearby star HD 13974 (d =11 pc) that is indistinguishable from a bare photosphere at both 24 micron and 70 micron. The observations place strong upper limits on the presence of any cold dust in this nearby system (L_IR/L_* < 10^{-5.2}).

Deposing the Cool Corona of KPD 0005+5106

The ROSAT PSPC pulse height spectrum of the peculiar He-rich hot white dwarf KPD 0005+5106 provided a great surprise when first analysed by Fleming, Werner & Barstow (1993). It defied the best non-LTE modelling attempts in terms of photospheric emission from He-dominated atmospheres including C, N and O and was instead interpreted as the first evidence for a coronal plasma around a white dwarf. We show here that a recent high resolution Chandra LETGS spectrum has more structure than expected from a thermal bremsstrahlung continuum and lacks the narrow lines of H-like and He-like C expected from a coronal plasma. Moreover, a coronal model requires a total luminosity more than two orders of magnitude larger than that of the star itself. Instead, the observed 20-80 AA flux is consistent with photospheric models containing trace amounts of heavier elements such as Fe. The soft X-ray flux is highly sensitive to the adopted metal abundance and provides a metal abundance diagnostic. The weak X-ray emission at 1 keV announced by O'Dwyer et al (2003) instead cannot arise from the photosphere and requires alternative explanations. We echo earlier speculation that such emission arises in a shocked wind. Despite the presence of UV-optical O VIII lines from transitions between levels n=7-10, no X-ray O VIII Ly alpha flux is detected. We show that O VIII Lyman photons can be trapped by resonant scattering within the emitting plasma and destroyed by photoelectric absorption.

Far‐Ultraviolet Spectroscopy of Magnetic Cataclysmic Variables

We have obtained Hubble Space Telescope (HST) STIS data for a total of 11 polars as part of a program aimed at compiling a homogeneous database of high-quality far-ultraviolet (FUV) spectra for a large number of cataclysmic variables (CVs). Of the 11 polars, eight were found in a state of low accretion activity (V347 Pav, VV Pup, V834 Cen, BL Hyi, MR Ser, ST LMi, RX J1554.2+2721, and V895 Cen) and three in a state of high activity ( CD Ind, AN UMa, and UW Pic). The STIS spectra of the low-state polars unambiguously reveal the photospheric emission of their white dwarf (WD) primaries. We have used pure hydrogen WD models to fit the FUV spectra of the low-state systems (except RX J1554.2+2721, which is a high-field polar) in order to measure the WD effective temperatures. In all cases, the fits could be improved by adding a second component, which is presumably due to residual accretion onto the magnetic pole of the WD. The WD temperatures obtained range from 10,800 to 14,300 K for log g = 8.0. Our analysis more than doubles the number of polars with accurate WD effective temperatures. Comparing the WD temperatures of polars with those of nonmagnetic CVs, we find that at any given orbital period the WDs in polars are colder than those in nonmagnetic CVs. The temperatures of polars below the period gap are consistent with gravitational radiation as the only active angular momentum loss mechanism. The differences in WD effective temperatures between polars and nonmagnetic CVs are significantly larger above the period gap, suggesting that magnetic braking in polars might be reduced by the strong field of the primary. We derive distance estimates to the low-state systems from the flux scaling factors of our WD model fits. Combining these distance measurements with those from the literature, we establish a lower limit on the space density of polars of 1.3 x 10(-6) pc(-3).

Interpretations of gamma-ray burst spectroscopy

We analyze the spectral lags of a sample of bright gamma-ray burst pulses observed by CGRO BATSE and compare these with the results of high-resolution spectroscopical investigations. We find that pulses with hard spectra have the largest lags, and that there is a similar, but weaker correlation between hardness-intensity corre- lation index, �, and lag. We also find that the lags differ considerably between pulses within a burst. Furthermore, the peak energy mainly decreases with increasing lag. Assuming a lag-luminosity relation as suggested by Norris et al., there will thus be a positive luminosity-peak-energy correlation. We also find that the hardness ratio, of the total flux in two channels, only weakly correlates with the spectral evolution parameters. These results are consistent with those found in the analytical and numerical analysis in Paper I. Finally, we find that for these bursts, dominated by a single pulse, there is a correlation between the observed energy-flux, F, and the inverse of the lag, �t: F / �t 1 . We interpret this flux-lag relation found as a consequence of the lag-luminosity relation and that these bursts have to be relatively narrowly distributed in z. However, they still have to, mainly, lie beyond z � 0.01, since they do not coincide with the local super-cluster of galaxies. We discuss the observed correlations within the collapsar model, in which the collimation of the outflow varies. Both the thermal photospheric emission as well as non-thermal, optically-thin synchrotron emission should be important.

Decay of Planetary Debris Disks

We report new Spitzer 24 � m photometry of 76 main-sequence A-type stars. We combine these results with previously reportedSpitzer24 � m data and 24 and 25 � m photometry from theInfrared Space Observatoryand the InfraredAstronomySatellite.Theresultisasampleof266starswithmasscloseto2.5M� ,alldetectedtoatleastthe � 7 � level relative to their photospheric emission. We culled ages for the entire sample from the literature and/or estimated them using the H-R diagram and isochrones; they range from 5 to 850 Myr. We identified excess thermal emission using an internally derived K � 24 (or 25) � m photospheric color and then compared all stars in the sample tothatcolor.Becausewehaveexcludedstarswithstrongemissionlinesorextendedemission(associatedwithnearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emissionmore frequently andwithhigher fractional excess thandothe olderstars. However,asmanyas 50% oftheyoungerstarsdonotshowexcessemission.Thedeclineinthemagnitudeofexcessemission,forthosestarsthat show it, has a roughly t0/time dependence, with t0 � 150 Myr. If anything, stars in binary systems (including Algoltype stars) and k Boo stars show less excess emission than the other members of the sample. Our results indicate that (1) there is substantial variety among debris disks, including that a significant number of stars emerge from the protoplanetary stage of evolution with little remaining disk in the 10‐60 AU region and (2) in addition, it is likely that much of the dust we detect is generated episodically by collisions of large planetesimals during the planet accretion endgame,andthatindividualeventsoftendominatetheradiometricpropertiesofadebrissystem.Thislatterbehavior agrees generally withwhat weknowabouttheevolution of thesolar system, andalsowiththeoretical models ofplanetary system formation. Subject headingg circumstellar matter — infrared: stars — planetary systems: formation Online material: machine-readable table

Starspots: A Key to the Stellar Dynamo

Magnetic activity similar to that of the Sun is observed on a variety of cool stars with external convection envelopes. Stellar rotation coupled with convective motions generate strong magnetic fields in the stellar interior and produce a multitude of magnetic phenomena including starspots in the photosphere, chromospheric plages, coronal loops, UV, X-ray, and radio emission and flares. Here I review the phenomenon of starspots on dierent types of cool stars, observational tools and diagnostic techniques for studying starspots as well as starspot properties including their temperatures, areas, magnetic field strengths, lifetimes, active latitudes and longitudes, etc. Evolution of starspots on various time scales allows us to investigate stellar dierential rotation, activity cycles, and global magnetic fields. Together these constitute the basis for our understanding of stellar and solar dynamos and provide valuable constraints for theoretical models.