Only g modes calculated i red giant star

[Gyre_Beginner]

Hey everybody,

I have recently started calculating oscillation frequencies of red giant stars with gyre. For this I proceeded as follows: I used a stellar model (which I obtained from Mesa) for a red giant with a mass of 0.6*Msun. For the frequency scan I used freq_min = 3 microhertz and freq_max = 4 microhertz since the frequency of maximum oscillation power v_max = 3.47 microhertz for the star I want to investigate. As a solution I get only gravity modes with thousands of nodes. I would have expected p modes because the convective envelope is large in stars with less than one solar mass. So all in all my question is why I only get g modes through the gyre calculations. Thanks for the help.

kind regards
Gyre_Beginner

[warrick]

For the frequency scan I used freq_min = 3 microhertz and freq_max = 4 microhertz since the frequency of maximum oscillation power v_max = 3.47 microhertz for the star I want to investigate.

How exactly did you estimate νmax?

[Gyre_Beginner]

Hello Warrick,

regarding the v_max I refer to the paper "Models of red giants in the CoRoT asteroseismology fields combining asteroseismic and spectroscopic constraints" (link: https://arxiv.org/pdf/1505.01529.pdf). On page 3 there is a tabular containing information about red giants like v_max, the large frequency spacing, ect. The star I am investigating in this tabular is alpha Boo. So far my challenge was to find a suitable frequency range that I can scan with gyre. In the paper "GYRE: an open-source stellar oscillation code based on a new Magnus Multiple Shooting scheme" (link: http://www.astro.wisc.edu/~townsend/res ... s/gyre.pdf) a star with a mass of 1.5 Msun was modeled by Gyre and therefore the scanning interval of the frequency was chosen so that it loosely correspond to the frequency of maximum power v_max. My approach was the same, so I started to search for publications which contains v_max of red giant stars.

kind regards
Gyre_Beginner

[warrick]

What's the radius of your stellar model compared to the entry for α Boo in that table? How would that affect νmax?

I'm deliberately baiting you into computing νmax of your model for the sake of your own understanding. That is, I don't think it's helpful for me to just tell you what νmax is for your stellar model but I'll give you a clue that it isn't 3.47 μHz. Try using the scaling relations by Kjeldsen & Bedding (1995) to estimate νmax. These are the same relations from which equations 1 and 2 are derived in the first paper you linked.

[Gyre_Beginner]

Hello Warrick,

thank you very much for your answere. I have used the scaling relations from the paper Kjeldsen & Bedding (1995) with the radius and effective temperature of my stellar model which leads to much better results. I could now find both p and g modes, because I now know in which frequency range I have to search (as you said the v_max in my previous calculations was chosen wrong). I have now investigated eigenfunctions of some calculated modes for a red giant (Msun = 0.6) which brings me to my current question: Plotting x=r/R against Re(xi_r) from the file mode.00001 (corresponding to n_pg = -31) leads to the oscillation curve below. Unfortunately this curve dont make any sense for me. Since I am plotting a g mode, I would expect the oscillation to take place much closer to the core of the star. My previous understanding was that g modes do not propagate into the convective envelope which is pretty large in stars with fewer than one solar mass. So as I said I wouldnt expect g mode oscillations take place so close at the surface. Can someone please show me the mistake I am doing. I thank you all for your help.

kind regards
Gyre_Beginner

[warrick]

At this point, your questions aren't really about GYRE but about stellar oscillation theory in general. You may want to consult some course notes or your advisor. I recommend the notes by Jørgen Christensen-Dalsgaard. In Section 5.3.3, he discusses mixed modes in the star η Boo. Fig. 5.19 (eigenfunctions of a mixed mode) is hopefully enlightening.

Anyway, the mode you have plotted is neither purely a g-mode nor purely a p-mode. It is a mixed mode. In the non-convective core, the mode behaves like a g-mode; in the envelope, it behaves like a p-mode. If you look closely at x<0.01, you'll see the g-mode-like pulsations. g-modes are more prominent in horizontal displacement, so try plotting "Re(xi_h)" instead and see what you find.

Note that n_pg is basically n_p - n_g, where n_p is the number of nodes in the p-mode cavity and n_g is the number of nodes in the g-mode cavity. The fact that n_pg is overall negative doesn't mean this is a g-mode. You should find that both n_p and n_g are non-zero (although it isn't recorded in the mode file), indicating that the mode has both g-like and p-like behaviour.

[Gyre_Beginner]

thank you for your help Warrick. That answere helps me a lot

greetings
Gyre_Beginner