Number Counts and Non-Gaussianity

We describe a general procedure for using number counts of any object to constrain the probability distribution of the primordial fluctuations, allowing for generic weak non-Gaussianity. We apply this procedure to use limits on the abundance of primordial black holes and dark matter ultracompact minihalos (UCMHs) to characterize the allowed statistics of primordial fluctuations on very small scales. We present constraints on the power spectrum and the amplitude of the skewness for two different families of non-Gaussian distributions, distinguished by the relative importance of higher moments. Although primordial black holes probe the smallest scales, ultracompact minihalos provide significantly stronger constraints on the power spectrum and so are more likely to eventually provide small-scale constraints on non-Gaussianity.Comment: 19 pages; v2 is published PRD versio

Serendipitous discovery of the faint solar twin Inti 1

Context. Solar twins are increasingly the subject of many studies owing to their wide range of applications from testing stellar evolution models to the calibration of fundamental observables; these stars are also of interest because high precision abundances could be achieved that are key to investigating the chemical anomalies imprinted by planet formation. Furthermore, the advent of photometric surveys with large telescopes motivates the identification of faint solar twins in order to set the zero point of fundamental calibrations. Aims. We intend to perform a detailed line-by-line differential analysis to verify whether 2MASS J23263267-0239363 (designated here as Inti 1) is indeed a solar twin. Methods. We determine the atmospheric parameters and differential abundances using high-resolution (R ≈ 50 000), high signal-to-noise (S/N ≈ 110–240 per pixel) Keck/HIRES spectra for our solar twin candidate, the previously known solar twin HD 45184, and the Sun (using reflected light from the asteroid Vesta). Results. For the bright solar twin HD 45184, we found T_(eff) = 5864 ± 9 K, log g = 4.45 ± 0.03 dex, v_t = 1.11 ± 0.02 km s^(-1), and [Fe/H] = 0.04 ± 0.01 dex, which are in good agreement with previous works. Our abundances are in excellent agreement with a recent high-precision work, with an element-to-element scatter of only 0.01 dex. The star Inti 1 has atmospheric parameters T_(eff) = 5837 ± 11 K, log g = 4.42 ± 0.03 dex, v_t = 1.04 ± 0.02 km s^(-1), and [Fe/H] = 0.07 ± 0.01 dex that are higher than solar. The age and mass of the solar twin HD 45184 (3 Gyr and 1.05 M⊙) and the faint solar twin Inti 1 (4 Gyr and 1.04 M⊙) were estimated using isochrones. The differential analysis shows that HD 45184 presents an abundance pattern that is similar to typical nearby solar twins; this means this star has an enhanced refractory relative to volatile elements, while Inti 1 has an abundance pattern closer to solar, albeit somewhat enhanced in refractories. The abundance pattern of HD 45184 and Inti 1 could be reproduced by adding ≈3.5 M⊕ and ≈1.5 M⊕ of Earth-like material to the convective zone of the Sun. Conclusions. The star Inti 1 is a faint solar twin, therefore, it could be used to calibrate the zero points of different photometric systems. The distant solar twin Inti 1 has an abundance pattern similar to the Sun with only a minor enhancement in the refractory elements. It would be important to analyze other distant solar twins to verify whether they share the Sun’s abundance pattern or if they are enhanced in refractories, as is the case in the majority of nearby solar twins

Constraining the evolution of stellar rotation using solar twins

The stellar Rotation $vs.$ Age relation is commonly considered as a useful tool to derive reliable ages for Sun-like stars. However, in the light of \kepler\ data, the presence of apparently old and fast rotators that do not obey the usual gyrochronology relations led to the hypothesis of weakened magnetic breaking in some stars. In this letter, we constrain the solar rotation evolutionary track using solar twins. Predicted rotational periods as a function of mass, age, [Fe/H] and given critical Rossby number ($Ro_{\rm crit}$) were estimated for the entire rotational sample. Our analysis favors the smooth rotational evolution scenario and suggests that, if the magnetic weakened breaking scenario takes place at all, it should arise after $Ro_{\rm crit}\gtrsim2.29$ or ages $\gtrsim$5.3 Gyr (at 95$\%$ confidence level).Comment: 5 pages, 3 figures, accepted for publication in MNRA

How Magnetic Activity Alters What We Learn from Stellar Spectra

Magnetic fields and stellar spots can alter the equivalent widths of absorption lines in stellar spectra, varying during the activity cycle. This also influences the information that we derive through spectroscopic analysis. In this study, we analyze high-resolution spectra of 211 sunlike stars observed at different phases of their activity cycles, in order to investigate how stellar activity affects the spectroscopic determination of stellar parameters and chemical abundances. We observe that the equivalent widths of lines can increase as a function of the activity index log R'HK during the stellar cycle, which also produces an artificial growth of the stellar microturbulence and a decrease in effective temperature and metallicity. This effect is visible for stars with activity indexes log RHK -5.0 (i.e., younger than 4-5 Gyr), and it is more significant at higher activity levels. These results have fundamental implications on several topics in astrophysics that are discussed in the paper, including stellar nucleosynthesis, chemical tagging, the study of Galactic chemical evolution, chemically anomalous stars, the structure of the Milky Way disk, stellar formation rates, photoevaporation of circumstellar disks, and planet hunting.L.S. and A.I.K. acknowledge financial support from the Australian Research Council (Discovery Project 170100521). A.R.C. acknowledges the support from the Australian Research Council (DECRA 190100656). J.M. thanks support by FAPESP (2018/04055-8) and CNPq (Bolsa de Produtividade). J.Y.G. acknowledges the support from CNPq. This research was supported by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE17010001

The Li-age correlation: The Sun is unusually Li deficient for its age

This work aims to examine in detail the depletion of lithium in solar twins to better constrain stellar evolution models and investigate its possible connection with exoplanets. We employ spectral synthesis in the region of the asymmetric 6707.75 Å Li I line for a sample of 77 stars plus the Sun. As in previous works based on a smaller sample of solar twins, we find a strong correlation between Li depletion and stellar age. In addition, for the first time we show that the Sun has the lowest Li abundance in comparison with solar twins at similar age (4.6 ± 0.5 Gyr). We compare the lithium content with the condensation temperature slope for a subsample of the best solar twins and determine that the most lithium-depleted stars also have fewer refractory elements. We speculate whether the low lithium content in the Sun might be related to the particular configuration of our Solar system.This study was financed in part by the Coordenac¸ao de ˜ Aperfeic¸oamento de Pessoal de N´ıvel Superior – Brasil (CAPES) – Finance Code 001. JM is thankful for the support of Fundac¸ao de ˜ Amparo a Pesquisa do Estado de S ` ao Paulo (FAPESP, 2014/18100- ˜ 4, 2018/04055-8) and Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnologico (CNPq, Bolsa de Produtividade). LAdS ´ acknowledges the financial support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (project FOUR ACES; grant agreement No. 724427)

The Solar Twin Planet Search: V. Close-in, low-mass planet candidates and evidence of planet accretion in the solar twin HIP 68468

Context. More than two thousand exoplanets have been discovered to date. Of these, only a small fraction have been detected around solar twins, which are key stars because we can obtain accurate elemental abundances especially for them, which is crucial for studying the planet-star chemical connection with the highest precision. Aims. We aim to use solar twins to characterise the relationship between planet architecture and stellar chemical composition. Methods. We obtained high-precision (1 m s-1) radial velocities with the HARPS spectrograph on the ESO 3.6 m telescope at La Silla Observatory and determined precise stellar elemental abundances (~0.01 dex) using spectra obtained with the MIKE spectrograph on the Magellan 6.5 m telescope. Results. Our data indicate the presence of a planet with a minimum mass of 26 ± 4 Earth masses around the solar twin HIP 68468. The planet is more massive than Neptune (17 Earth masses), but unlike the distant Neptune in our solar system (30 AU), HIP 68468c is close-in, with a semi-major axis of 0.66 AU, similar to that of Venus. The data also suggest the presence of a super-Earth with a minimum mass of 2.9 ± 0.8 Earth masses at 0.03 AU; if the planet is confirmed, it will be the fifth least massive radial velocity planet candidate discovery to date and the first super-Earth around a solar twin. Both isochrones (5.9 ± 0.4 Gyr) and the abundance ratio [Y/Mg] (6.4 ± 0.8 Gyr) indicate an age of about 6 billion years. The star is enhanced in refractory elements when compared to the Sun, and the refractory enrichment is even stronger after corrections for Galactic chemical evolution. We determined a nonlocal thermodynamic equilibrium Li abundance of 1.52 ± 0.03 dex, which is four times higher than what would be expected for the age of HIP 68468. The older age is also supported by the low log (R'HK) (–5.05) and low jitter (<1 m s-1). Engulfment of a rocky planet of 6 Earth masses can explain the enhancement in both lithium and the refractory elements. Conclusions. The super-Neptune planet candidate is too massive for in situ formation, and therefore its current location is most likely the result of planet migration that could also have driven other planets towards its host star, enhancing thus the abundance of lithium and refractory elements in HIP 68468. The intriguing evidence of planet accretion warrants further observations to verify the existence of the planets that are indicated by our data and to better constrain the nature of the planetary system around this unique starJ.M. acknowledges support from FAPESP (2012/24392-2) and CNPq (Bolsa de Produtividade). M.B. is supported by the National Science Foundation (NSF) Graduate Research Fellowships Program (grant no. DGE1144082). J.B. and M.B. acknowledge support for this work from the NSF (grant no. AST-1313119). J.B. is also supported by the Alfred P. Sloan Foundation and the David and Lucile Packard Foundation. M.A. acknowledges support from the Australian Research Council (grants FL110100012 and DP120100991)

The Solar Twin Planet Search. V. Close-in, low-mass planet candidates and evidence of planet accretion in the solar twin HIP 68468

[Methods]. We obtained high-precision radial velocities with HARPS on the ESO 3.6 m telescope and determined precise stellar elemental abundances (~0.01 dex) using MIKE spectra on the Magellan 6.5m telescope. [Results]. Our data indicate the presence of a planet with a minimum mass of 26 Earth masses around the solar twin HIP 68468. The planet is a super-Neptune, but unlike the distant Neptune in our solar system (30 AU), HIP 68468c is close-in, with a semi-major axis of 0.66 AU, similar to that of Venus. The data also suggest the presence of a super-Earth with a minimum mass of 2.9 Earth masses at 0.03 AU; if the planet is confirmed, it will be the fifth least massive radial velocity planet discovery to date and the first super-Earth around a solar twin. Both isochrones (5.9 Gyr) and the abundance ratio [Y/Mg] (6.4 Gyr) indicate an age of about 6 billion years. The star is enhanced in refractory elements when compared to the Sun, and the refractory enrichment is even stronger after corrections for Galactic chemical evolution. We determined a NLTE Li abundance of 1.52 dex, which is four times higher than what would be expected for the age of HIP 68468. The older age is also supported by the low log(R'HK) (-5.05) and low jitter. Engulfment of a rocky planet of 6 Earth masses can explain the enhancement in both lithium and the refractory elements. [Conclusions]. The super-Neptune planet candidate is too massive for in situ formation, and therefore its current location is most likely the result of planet migration that could also have driven other planets towards its host star, enhancing thus the abundance of lithium and refractory elements in HIP 68468. The intriguing evidence of planet accretion warrants further observations to verify the existence of the planets that are indicated by our data and to better constrain the nature of the planetary system around this unique star.Comment: A&A, in pres