[Paper Review] Multi-wavelength variability of the young solar analog iota Hor. X-ray cycle, star spots, flares, and UV emission

J. Sanz‐Forcada, B. Stelzer+3 more|arXiv (Cornell University)|Sep 3, 2019

Stellar, planetary, and galactic studies75 references4 citations

TL;DR

This study confirms a stable 1.6-year X-ray coronal activity cycle in the young solar analog iota Hor, using multi-wavelength XMM-Newton, TESS, and HST observations. It reveals a regular coronal cycle with photospheric-like abundances and a photometric rotation period of 8.19 ± 0.26 days, while UV and X-ray variability are decoupled, likely due to inclination effects and extended coronal emission being less sensitive to hemispheric asymmetries.

ABSTRACT

Chromospheric activity cycles are common in late-type stars; however, only a handful of coronal activity cycles have been discovered. \ihor\ is the most active and youngest star with known coronal cycles. It is also a young solar analog, and we are likely facing the earliest cycles in the evolution of solar-like stars, at an age (~600 Myr) when life appeared on Earth. Our aim is to confirm the ~1.6 yr coronal cycle and characterize its stability over time. We use X-ray observations of iota Hor to study the corona of a star representing the solar past through variability, thermal structure, and coronal abundances. We analyzed multi-wavelength observations of iota Hor using XMM-Newton, TESS, and HST data. We monitored iota Hor throughout almost seven years in X-rays and in two UV bands. The summed RGS and STIS spectra were used for a detailed thermal structure model, and the determination of coronal abundances. We studied rotation and flares in the TESS light curve. We find a stable coronal cycle along four complete periods, more than covered in the Sun. There is no evidence for a second longer X-ray cycle. Coronal abundances are consistent with photospheric values, discarding any effects related to the first ionization potential. From the TESS light curve we derived the first photometric measurement of the rotation period (8.2 d). No flares were detected in the TESS light curve of iota Hor. We estimate the probability of having detected zero flares with TESS to be ~2%. We corroborate the presence of an activity cycle of ~1.6 yr in iota Hor in X-rays, more regular than its Ca II H&K counterpart. A decoupling of the activity between the northern and southern hemispheres of the star might explain the disagreement. The inclination of the system would result in an irregular behavior in the chromospheric indicators. The more extended coronal material would be less sensitive to this effect.

Motivation & Objective

• To confirm the existence and stability of the 1.6-year coronal activity cycle in iota Hor, a young solar analog with known X-ray variability.
• To characterize the thermal structure and coronal abundances of the star using high-resolution X-ray spectroscopy.
• To determine the stellar rotation period from TESS photometry and compare it with spectroscopic measurements.
• To investigate the discrepancy between chromospheric (Ca ii H&K) and coronal (X-ray) activity cycles, particularly the irregularity in chromospheric signals versus the regularity in X-ray modulation.
• To assess the presence of flares and UV emission variability, and evaluate the impact of viewing geometry on observed activity indicators.

Proposed method

• Analysis of XMM-Newton X-ray observations (EPIC and RGS instruments) to derive X-ray light curves, spectral fits, and coronal thermal structure.
• Use of TESS photometry to measure the stellar rotation period via light curve modulation from surface inhomogeneities.
• Spectral analysis of combined RGS and STIS data to determine coronal abundances and assess the first ionization potential (FIP) effect.
• UV flux density measurements from GALEX and HST/OM to quantify chromospheric excess and subtract photospheric contributions.
• Statistical evaluation of flare detection probability in TESS data, using superflare statistics from Kepler data as a reference.
• Inclination estimation via rotational velocity (vsini) and stellar radius, using the rotation period and v sin i.

Experimental results

Research questions

• RQ1 Is the 1.6-year X-ray coronal cycle in iota Hor stable over multiple periods, and does it show evidence of longer-term modulation?
• RQ2 Why is the X-ray coronal cycle more regular than the chromospheric Ca ii H&K cycle in iota Hor?
• RQ3 What is the true rotation period of iota Hor, and how does it compare with previous spectroscopic estimates?
• RQ4 Are there detectable flares in the TESS light curve, and what is the probability of missing such events?
• RQ5 How does the UV emission vary relative to X-ray variability, and what does this imply about the chromospheric and coronal activity connection?

Key findings

• The X-ray coronal activity cycle in iota Hor is stable over four complete periods, with no evidence of a longer cycle, indicating a robust and regular modulation.
• The rotation period of iota Hor is measured photometrically as 8.19 ± 0.26 days, consistent with prior spectroscopic estimates from S-index, radial velocity, and magnetic field measurements.
• No flares were detected in the TESS light curve, with a probability of missing flares estimated at ~2%, suggesting either low flare frequency or high flare energy thresholds.
• Coronal abundances are consistent with photospheric values, with no significant first ionization potential (FIP) effect, contradicting earlier low-resolution X-ray studies and challenging the FIP trend with spectral type.
• The UV emission shows limited variability (~14% amplitude) and does not follow the X-ray cycle, likely due to chromospheric emission being more sensitive to viewing geometry and hemispheric asymmetries.
• The inclination of the system (~56°) is proposed as the cause of the decoupling between chromospheric and coronal variability, as coronal emission is more extended and less affected by viewing-angle effects.