Investigating Magnetic Activity in Red Giant Stars

Documenting the development of a funded research project that aims to deepen our understanding of the magnetic fields located within some red giant stars.

[magnetic-red-giants]

Welcome to my blog!

Over the next 2/3 months, I wll pursue research that aims to further our insight into the role of magnetic fields within red giant stars as part of an undergraduate research scheme with my university.

Recent evidence suggests that some red giants, despite being evolved stars, possess magnetic fields. This discovery challenges our prior understanding, as these stars were not expected to harbor efficient dynamos. In my project, I aim to delve deeper into this phenomenon by utilizing archival data from the Kepler and GALEX missions.

My primary goal is to identify key signatures of magnetic activity, particularly flares, within these red giant stars. Of special interest are the stars observed by both Kepler and GALEX. The different wavelengths of these observations, with Kepler capturing white light and GALEX focusing on UV light, offer distinct perspectives on magnetic activity. Even if the data from both instruments is not contemporaneous, comparing the results across wavelengths will be intriguing. It will shed light on the consistency of indicators of increased magnetic activity and their presence across the different wavelengths.

I will also be investigating how measures of activity, such as flaring rate, vary in relation to a range of stellar parameters. Parameters like effective temperature and metallicity will be crucial in understanding the underlying dynamics of these magnetic fields. Additionally, for datasets with sufficient time series I will explore variations in activity over time. These variations may offer insights into the presence of activity cycles, akin to the 11 year solar cycle observed in our Sun.

To conduct this research, I will utilize Python and rely on freely available packages for the Kepler data analysis. Two such tools, lightkurve and altaipony, will play pivotal roles in preparing light curves and searching for flares, respectively.

This blog will function as a place for me to share interesting articles, answer any questions and to provide updates on my research.

I can't wait to begin. Asks are open.

-Morgan

[magnetic-red-giants]

Analysis of GALEX data

Having now calculated Sph for over 300 red giant stars, we will now be moving on to a different measure of magnetic activity for the same set of stars. To do this we move away from data retrieved by the original Kepler mission and on to data retrieved by GALEX. This mission observed stars in the FUV and NUV bands. The python library gPhoton has been vital in downloading and analysing this data, similar to how lightkurve was when it came to Kepseismic data. The following is the abstract from the article:

A correlation between GALEX FUV magnitude and chromospheric activity among red giant stars

It is shown that upon combining GALEX far-ultraviolet and Johnson B magnitudes a resultant FUV–B colour can be obtained that for red giant stars of luminosity classes III and II correlates well with chromospheric emission in the cores of the Mg iih and k lines. Giant stars throughout the colour range 0.8 ≤ B – V ≤ 1.6 exhibit such a phenomenon. The main result of this paper is to show that GALEX far-ultraviolet photometry can provide information about the degree of chromospheric activity among red giant stars, and as such may offer a tool for surveying the evolution of chromospheric activity from the main sequence into the red giant phases of stellar evolution.

[magnetic-red-giants]

Limitations of Sph in assessing stellar magnetic activity

In my previous post, I introduced Sph​ as a metric for gauging stellar magnetic activity. While insightful, Sph​ does have its limitations.

Firstly, Sph is typically determined over segments that span 5 times a star's rotation period. However, there are instances where the rotation period is unknown. In such cases, an alternate method, like determining the median of the light curve's standard deviation over three-day intervals, becomes necessary [1]. This deviation in methodology can introduce disparities in results, making direct comparisons problematic.

Another pivotal factor is stellar inclination, which denotes the angle between a star's rotation axis and our observational line of sight. The visibility and perceived variability of features like starspots are heavily contingent upon this inclination. At a high inclination, nearing 90°, we observe the star edge-on, making most of the starspots' rotational variability discernible as they traverse the star's visible face. Conversely, at low inclinations, approaching 0°, our line of sight hinders the detection of starspots, primarily located near the star's equator. This inclination-induced variability can distort our interpretation of a star's true magnetic activity when using Sph​.

Furthermore, the Signal-to-Noise Ratio (SNR) is an essential aspect to consider. SNR delineates the strength of a desired signal (like stellar variations) relative to the background noise in the data. A low SNR can obscure genuine stellar magnetic activity signals, making them indistinguishable from observational or instrumental noise. Therefore, for Sph​ to be a reliable measure, the data must possess a sufficiently high SNR. Analyzing stars with weak magnetic signals in the presence of significant noise can lead to inaccurate or inconclusive Sph​ values, potentially undermining the metric's efficacy. The KEPSEISMIC data has already undergone a rigorous preprocessing and calibration procedure to ensure the removal of instrumental artifacts and to provide users with the most accurate signals.

[1] Gehan, C. Gualme, P. Yu, J (2022). Surface magnetism of rapidly rotating red giants: Single versus close binary stars.

[s-n-arly]

Skip Google for Research

As Google has worked to overtake the internet, its search algorithm has not just gotten worse.  It has been designed to prioritize advertisers and popular pages often times excluding pages and content that better matches your search terms 

As a writer in need of information for my stories, I find this unacceptable.  As a proponent of availability of information so the populace can actually educate itself, it is unforgivable.

Below is a concise list of useful research sites compiled by Edward Clark over on Facebook. I was familiar with some, but not all of these.

⁂

Google is so powerful that it “hides” other search systems from us. We just don’t know the existence of most of them. Meanwhile, there are still a huge number of excellent searchers in the world who specialize in books, science, other smart information. Keep a list of sites you never heard of.

www.refseek.com - Academic Resource Search. More than a billion sources: encyclopedia, monographies, magazines.

www.worldcat.org - a search for the contents of 20 thousand worldwide libraries. Find out where lies the nearest rare book you need.

https://link.springer.com - access to more than 10 million scientific documents: books, articles, research protocols.

www.bioline.org.br is a library of scientific bioscience journals published in developing countries.

http://repec.org - volunteers from 102 countries have collected almost 4 million publications on economics and related science.

www.science.gov is an American state search engine on 2200+ scientific sites. More than 200 million articles are indexed.

www.pdfdrive.com is the largest website for free download of books in PDF format. Claiming over 225 million names.

www.base-search.net is one of the most powerful researches on academic studies texts. More than 100 million scientific documents, 70% of them are free

[magnetic-red-giants]

Magnetic activity of F stars observed by Kepler

The first measure of magnetic activity I will be looking into is known as Sph. This paper introduces this concept in a very understandable way, however the measure itself is limited in its capabilities due to a number of factors which I will talk about in another post.

Context. The study of stellar activity is important because it can provide new constraints for dynamo models when combined with surface rotation rates and the depth of the convection zone. We know that the dynamo mechanism, which is believed to be the main process that rules the magnetic cycle of solar-like stars, results from the interaction between (differential) rotation, convection, and magnetic field. The Kepler mission has already been collecting data for a large number of stars during four years allowing us to investigate magnetic stellar cycles.

Aims. We investigated the Kepler light curves to look for magnetic activity or even hints of magnetic activity cycles. Based on the photometric data we also looked for new magnetic indexes to characterise the magnetic activity of the stars. Methods. We selected a sample of 22 solar-like F stars that have a rotation period shorter than 12 days. We performed a time-frequency analysis using the Morlet wavelet yielding a magnetic proxy for our sample of stars. We computed the magnetic index Sph as the standard deviation of the whole time series and the index ⟨ Sph ⟩, which is the mean of standard deviations measured in subseries of length five times the rotation period of the star. We defined new indicators, such as the contrast between high and low activity, to take into account the fact that complete magnetic cycles are not observed for all the stars. We also inferred the Rossby number of the stars and studied their stellar background. Results. This analysis shows different types of behaviour in the 22 F stars. Two stars show behaviour very similar to magnetic activity cycles. Five stars show long-lived spots or active regions suggesting the existence of active longitudes. Two stars in our sample seem to have a decreasing or increasing trend in the temporal variation of the magnetic proxies. Finally, the last group of stars shows magnetic activity (with the presence of spots) but no sign of cycle.