The Infrared Ca II lines in Sunspot Umbrae

TL;DR

This paper develops an empirical model for sunspot umbrae that accurately describes observed spectral lines and continuum intensities, highlighting the sensitivity of infrared Ca II lines to temperature gradients and proposing an opacity enhancement to reconcile model discrepancies.

Contribution

It introduces a new empirical model for sunspot umbrae that accounts for line profiles and continuum intensities, incorporating opacity enhancement to resolve previous inconsistencies.

Findings

Infrared Ca II line wings are sensitive to temperature gradients.

K I 7699 line is suitable for umbral spectroscopy due to minimal blending.

Opacity enhancement resolves discrepancies between line profiles and temperature gradients.

Abstract

We present an empirical working model for sunspot umbrae which equally describes observed continuum intensities and line profiles. The wings of the infrared Ca II lines depend sensitively on the temperature gradient at -0.6 < log(tau-0.5) < +0.3 but not essentially on the absolute value of T. These lines are observed to remain almost unchanged from photosphere to umbra and are thus insensitive to parasitic light. It is also shown that the infrared K I 7699 line is suitable for umbral spectroscopy since it is not seriously blended, its continuum is well defined and it is less influenced by parasitic light as compared to lines in the visible spectrum, due to the smaller umbral contrast. Calculations show that the umbral gradient dT/d(tau), required to fit the Ca II triplet lines, strongly conflicts with the observed profiles of K I 7699, NaD2 and Fe I 5434 (g=0), even when assuming vanishing Fe II lines for a maximum correction of parasitic light. It is shown that this discrepancy from the different line profiles may be removed by adopting an opacity enhancement as introduced by Zwaan (1974) from a discussion of continuum contrasts alone. The finally proposed umbral working model is very close to a scaled model of the quiet sun with T(eff)= 4000 K thus resembling a M0 rather than a K5 stellar atmosphere