Spectral Line Depth Variability in Radial Velocity Spectra
Abstract

Stellar active regions, including spots and faculae, can create radial velocity (RV) signals that interfere with the detection and mass measurements of low-mass exoplanets. In doing so, these active regions affect each spectral line differently, but the origin of these differences is not fully understood. Here we explore how spectral line variability correlated with S-index (Ca H and K emission) is related to the atomic properties of each spectral line. Next, we develop a simple analytic stellar atmosphere model that can account for the largest sources of line variability with S-index. Then, we apply this model to HARPS spectra ofαCen B to explain Feiline depth changes in terms of a disk-averaged temperature difference between active and quiet regions on the visible hemisphere of the star. This work helps establish a physical basis for understanding how stellar activity manifests differently in each spectral line and may help future work mitigating the impact of stellar activity on exoplanet RV surveys.

Authors:
; ; ; ;
Publication Date:
NSF-PAR ID:
10366784
Journal Name:
The Astrophysical Journal
Volume:
930
Issue:
2
Page Range or eLocation-ID:
Article No. 121
ISSN:
0004-637X
Publisher:
DOI PREFIX: 10.3847
National Science Foundation
##### More Like this
1. Abstract

We present the radio properties of 66 spectroscopically confirmed normal star-forming galaxies (SFGs) at 4.4 <z< 5.9 in the COSMOS field that were [Cii]-detected in the Atacama Large Millimeter/submillimeter Array Large Program to INvestigate [Cii] at Early times (ALPINE). We separate these galaxies (“Cii-detected-all”) into lower-redshift (“Cii-detected-lz”; 〈z〉 = 4.5) and higher-redshift (“Cii-detected-hz”; 〈z〉 = 5.6) subsamples, and stack multiwavelength imaging for each subsample from X-ray to radio bands. A radio signal is detected in the stacked 3 GHz images of the Cii-detected-all and lz samples at ≳3σ. We find that the infrared–radio correlation of our sample, quantified byqTIR, is lower than the local relation for normal SFGs at a ∼3σsignificance level, and is instead broadly consistent with that of bright submillimeter galaxies at 2 <z< 5. Neither of these samples show evidence of dominant active galactic nucleus activity in their stacked spectral energy distributions (SEDs), UV spectra, or stacked X-ray images. Although we cannot rule out the possible effects of the assumed spectral index and applied infrared SED templates in causing these differences, at least partially, the lower obscured fraction of star formation than at lower redshift can alleviate the tension between our stackedqTIRs and those of localmore »

2. Abstract

Radial velocity (RV) measurements of transiting multiplanet systems allow us to understand the densities and compositions of planets unlike those in the solar system. Kepler-102, which consists of five tightly packed transiting planets, is a particularly interesting system since it includes a super-Earth (Kepler-102d) and a sub-Neptune-sized planet (Kepler-102e) for which masses can be measured using RVs. Previous work found a high density for Kepler-102d, suggesting a composition similar to that of Mercury, while Kepler-102e was found to have a density typical of sub-Neptune size planets; however, Kepler-102 is an active star, which can interfere with RV mass measurements. To better measure the mass of these two planets, we obtained 111 new RVs using Keck/HIRES and Telescopio Nazionale Galileo/HARPS-N and modeled Kepler-102's activity using quasiperiodic Gaussian process regression. For Kepler-102d, we report a mass upper limitMd< 5.3M(95% confidence), a best-fit massMd= 2.5 ± 1.4M, and a densityρd= 5.6 ± 3.2 g cm−3, which is consistent with a rocky composition similar in density to the Earth. For Kepler-102e we report a massMe= 4.7 ± 1.7Mand a densityρe= 1.8 ± 0.7 g cm−3. These measurements suggest that Kepler-102e has a rocky core with a thick gaseous envelope comprising 2%–4% of themore »

3. Abstract

Precise radial velocity (PRV) surveys are important for the search for Earth analogs around nearby bright stars, which induce a small stellar reflex motion with an RV amplitude of ∼10 cm s−1. Detecting such a small RV signal poses challenges to instrumentation, data analysis, and the precision of astrophysical models to mitigate stellar jitter. In this work, we investigate an important component in the PRV error budget—the spectral contamination from the Earth’s atmosphere (tellurics). We characterize the effects of telluric absorption on the RV precision and quantify its contribution to the RV error budget over time and across a wavelength range of 350 nm–2.5μm. We use simulated solar spectra with telluric contamination injected, and we extract the RVs using two commonly adopted algorithms: dividing out a telluric model before performing cross-correlation or forward modeling the observed spectrum incorporating a telluric model. We assume various degrees of cleanness in removing the tellurics. We conclude that the RV errors caused by telluric absorption can be suppressed to close to or even below 1–10 cm s−1in the blue optical region. At red through near-infrared wavelengths, however, the residuals of tellurics can induce an RV error on the meter-per-second level even under themore »

4. Abstract

We investigate spatially resolved emission-line ratios in a sample of 219 galaxies (0.6 <z< 1.3) detected using the G102 grism on the Hubble Space Telescope Wide Field Camera 3 taken as part of the CANDELS LyαEmission at Reionization survey to measure ionization profiles and search for low-luminosity active galactic nuclei (AGN). We analyze [Oiii] and Hβemission-line maps, enabling us to spatially resolve the [Oiii]/Hβemission-line ratio across the galaxies in the sample. We compare the [Oiii]/Hβratio in galaxy centers and outer annular regions to measure ionization differences and investigate the potential of sources with nuclear ionization to host AGN. We investigate some of the individual galaxies that are candidates to host strong nuclear ionization and find that they often have low stellar mass and are undetected in X-rays, as expected for low-luminosity AGN in low-mass galaxies. We do not find evidence for a significant population of off-nuclear AGN or other clumps of off-nuclear ionization. We model the observed distribution of [Oiii]/Hβspatial profiles and find that most galaxies are consistent with a small or zero difference between their nuclear and off-nuclear line ratios, but 6%–16% of galaxies in the sample are likely to host nuclear [Oiii]/Hβthat is ∼0.5 dex higher thanmore »

5. Abstract

We use Hubble Space Telescope Wide Field Camera 3 G102 and G141 grism spectroscopy to measure rest-frame optical emission-line ratios of 533 galaxies atz∼ 1.5 in the CANDELS LyαEmission at Reionization survey. We compare [Oiii]/Hβversus [Sii]/(Hα+ [Nii]) as an “unVO87” diagram for 461 galaxies and [Oiii]/Hβversus [Neiii]/[Oii] as an “OHNO” diagram for 91 galaxies. The unVO87 diagram does not effectively separate active galactic nuclei (AGN) and [Nev] sources from star-forming galaxies, indicating that the unVO87 properties of star-forming galaxies evolve with redshift and overlap with AGN emission-line signatures atz> 1. The OHNO diagram does effectively separate X-ray AGN and [Nev]-emitting galaxies from the rest of the population. We find that the [Oiii]/Hβline ratios are significantly anticorrelated with stellar mass and significantly correlated with$log(LHβ)$, while [Sii]/(Hα+ [Nii]) is significantly anticorrelated with$log(LHβ)$. Comparison with MAPPINGS V photoionization models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-star formation rate (SFR) galaxies. We do not find evidence for redshift evolution of the emission-line ratios outside of the correlations with mass and SFR. Our results suggest that the OHNO diagram of [Oiii]/Hβversus [Neiii]/[Oii] willmore »