Erratum: “A Comprehensive Comparison of the Sun to Other Stars: Searching for Self‐Selection Effects” (ApJ, 684, 691 [2008]) (original) (raw)
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A COMPREHENSIVE COMPARISON OF THE SUN TO OTHER STARS: SEARCHING FOR
If the origin of life and the evolution of observers on a planet is favored by atypical properties of a planet's host star, we would expect our Sun to be atypical with respect to such properties. The Sun has been described by previous studies as both typical and atypical. In an effort to reduce this ambiguity and quantify how typical the Sun is, we identify 11 maximally independent properties that have plausible correlations with habitability and that have been observed by, or can be derived from, sufficiently large, currently available, and representative stellar surveys. By comparing solar values for the 11 properties to the resultant stellar distributions, we make the most comprehensive comparison of the Sun to other stars. The two most atypical properties of the Sun are its mass and orbit. The Sun is more massive than 95% AE 2% of nearby stars, and its orbit around the Galaxy is less eccentric than 93% AE 1% of FGK stars within 40 pc. Despite these apparently atypical properties, a 2 analysis of the Sun's values for 11 properties, taken together, yields a solar 2 ¼ 8:39 AE 0:96. If a star is chosen at random, the probability that it will have a lower value (i.e., be more typical) than the Sun, with respect to the 11 properties analyzed here, is only 29% AE 11%. These values quantify, and are consistent with, the idea that the Sun is a typical star. If we have sampled all reasonable properties associated with habitability, our result suggests that there are no special requirements for a star to host a planet with life.
A Comprehensive Comparison of the Sun to Other Stars: Searching for Self‐Selection Effects
The Astrophysical Journal, 2008
If the origin of life and the evolution of observers on a planet is favored by atypical properties of a planet's host star, we would expect our Sun to be atypical with respect to such properties. The Sun has been described by previous studies as both typical and atypical. In an effort to reduce this ambiguity and quantify how typical the Sun is, we identify 11 maximally independent properties that have plausible correlations with habitability and that have been observed by, or can be derived from, sufficiently large, currently available, and representative stellar surveys. By comparing solar values for the 11 properties to the resultant stellar distributions, we make the most comprehensive comparison of the Sun to other stars. The two most atypical properties of the Sun are its mass and orbit. The Sun is more massive than 95% AE 2% of nearby stars, and its orbit around the Galaxy is less eccentric than 93% AE 1% of FGK stars within 40 pc. Despite these apparently atypical properties, a 2 analysis of the Sun's values for 11 properties, taken together, yields a solar 2 ¼ 8:39 AE 0:96. If a star is chosen at random, the probability that it will have a lower value (i.e., be more typical) than the Sun, with respect to the 11 properties analyzed here, is only 29% AE 11%. These values quantify, and are consistent with, the idea that the Sun is a typical star. If we have sampled all reasonable properties associated with habitability, our result suggests that there are no special requirements for a star to host a planet with life.
Long-term radial-velocity variations of the Sun as a star: The HARPS view
Astronomy & Astrophysics
Context. Stellar radial velocities play a fundamental role in the discovery of extrasolar planets and the measurement of their physical parameters as well as in the study of stellar physical properties. Aims. We investigate the impact of the solar activity on the radial velocity of the Sun using the HARPS spectrograph to obtain measurements that can be directly compared with those acquired in the extrasolar planet search programmes. Methods. We used the Moon, the Galilean satellites, and several asteroids as reflectors to measure the radial velocity of the Sun as a star and correlated this velocity with disc-integrated chromospheric and magnetic indexes of solar activity that are similar to stellar activity indexes. We discuss in detail the systematic effects that affect our measurements and the methods to account for them. Results. We find that the radial velocity of the Sun as a star is positively correlated with the level of its chromospheric activity at ∼95 percent significance level. The amplitude of the long-term variation measured in the 2006−2014 period is 4.98 ± 1.44 m/s, which is in good agreement with model predictions. The standard deviation of the residuals obtained by subtracting a linear best fit is 2.82 m/s and is due to the rotation of the reflecting bodies and the intrinsic variability of the Sun on timescales shorter than the activity cycle. A correlation with a lower significance is detected between the radial velocity and the mean absolute value of the line-of-sight photospheric magnetic field flux density. Conclusions. Our results confirm similar correlations found in other late-type main-sequence stars and provide support to the predictions of radial velocity variations induced by stellar activity based on current models.
The place of the Sun among the Sun-like stars
Astronomy & Astrophysics, 2013
Context. Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship. Aims. We aim to identify possible reasons for the Sun to be currently outside of this relationship. Methods. We employed different scenarios of solar HK emission and irradiance variability and compared them with available time series of Sun-like stars. Results. We show that the position of the Sun on the diagram of photometric variability versus chromospheric activity changes with time. The present solar position is different from its temporal mean position as the satellite era of continuous solar irradiance measurements has accidentally coincided with a period of unusually high and stable solar activity. Our analysis suggests that although present solar variability is significantly smaller than indicated by the stellar data, the temporal mean solar variability might be in agreement with the stellar data. We propose that the continuation of the photometric program and its expansion to a larger stellar sample will ultimately allow us to constrain the historical solar variability.
Accepted for Publication on The Astronomical Journal
1999
The predictions from a Galactic Structure and Kinematic model code are compared to the color counts and absolute proper-motions derived from the Southern Proper-Motion survey covering more than 700 deg 2 toward the South Galactic Pole in the range 9 < BJ ≤ 19. The theoretical assumptions and associated computational procedures, the geometry for the kinematic model, and the adopted parameters are presented in detail and compared to other Galactic Kinematic models of its kind. The data to which the model is compared consists of more than 30,000 randomly selected stars, and it is best fit by models with a solar peculiar motion of +5 km s −1 in the V-component (pointing in the direction of Galactic rotation), a large LSR speed of 270 km s −1, and a (disk) velocity ellipsoid that always points towards the Galactic center. The absolute proper-motions in the U-component indicate a solar peculiar motion of 11.0 ± 1.5 km s −1, with no need for a local expansion or contraction term. The fa...
On the origin of the solar system and the exceptional position of the Sun in the Galaxy
Astrophysics and Space Science, 1983
The solar system's position in the Galaxy is an exclusive one, since the Sun is close to the corotation circle, which is the place where the angular velocity of the galactic differential rotation is equal to that of density waves displaying as spiral arms. Each galaxy contains only one corotation circle; therefore, it is an exceptional place. In the Galaxy, the deviation of the Sun from the corotation is very small -it is equal to AR/R o ~ 0.03, where AR = R c -Ro, R c is the corotation distance from the galactic center and R o is the Sun's distance from the galactic center. The-special conditions of the Sun's position in the Galaxy explain the origin of the fundamental cosmogony timescales T~ ~ 4.6 x 109 yr, /'2 = 108 yr, T 3 = 106 yr detected by the radioactive decay of various nuclides. The timescale T 1 (the solar system's 'lifetime') is the protosolar cloud lifetime in a space between the galactic spiral arms. The timescale T2 is the presolar cloud lifetime in a spiral arm. T3 is a timescale of hydrodynamical processes of a cloud-wave interaction. The possibility of the natural explanation of the cosmogony timescales by the unified process (on condition that the Sun is near the state of corotation) can become an argument in favour of the fact that the nearness to the corotation is necessary for the formation of systems similar to the Solar system. If the special position of the Sun is not incidental, then the corotation circles of our Galaxy, as well as those of other galaxies, are just regions where situations similar to ours are likely t~ be found. i * These are some doubts about the T3-scale reality .
The Activity and Variability of the Sun and Sun-like Stars. I. Synoptic Ca ii H and K Observations
The Astronomical Journal, 2007
Synoptic measurements of activity in Sun-like stars have been performed continuously since 1966, and the largest set comes from the Mount Wilson HK project, in the form of the well-known S index. We have been monitoring the activity and variability of the Sun and a large sample of Sun-like stars, in terms of S and absolute flux, since 1994 with the Solar-Stellar Spectrograph (SSS) at Lowell Observatory. Directly inspired by the similar long-term program at Mount Wilson Observatory, the SSS incorporates both an HK spectrograph and an echelle for visible and far-red observations. This is the first of three papers presenting the results of some 20,000 observations of the Sun and Sunlike stars with the SSS. In this paper we describe our program, review the calibration of solar and stellar fluxes to S and the chromospheric emission fraction R 0 HK , compare our derived stellar activity measures to those from other programs, and discuss the broad characteristics of the activity and variability in our target set, with particular attention to good solar analogs and noncycling stars. In subsequent papers we will discuss the echelle data and present detailed examinations of stars of particular interest.
2016
A new sample of solar analogs and twin candidates have been constructed and studied, with particular attention to their light curves from NASA's Kepler mission. This letter aims to assess the evolutionary status, derive their rotation and ages and identify those solar analogs or solar twin candidates. We separate out the subgiants that compose a large fraction of the asteroseismic sample, and which show an increase in the average rotation period as the stars ascend the subgiant branch. The rotation periods of the dwarfs, ranging from 6 to 30 days, and averaged 19d, allow us to assess their individual evolutionary states on the main sequence, and to derive their ages using gyrochronology. These ages are found to be in agreement with a correlation coefficient of r = 0.79 with the independent asteroseismic ages, where available. As a result of this investigation, we are able to identify 34 stars as solar analogs and 22 of them as solar twin candidates.