1. Stellar Ashes or Planetary Debris: Studying Light Element Abundances in Red Giant Atmospheres
Joleen Carlberg
July 12, 2017
Abstract:
The light elements Li, Be, and B are generally very depleted in red giant atmospheres, as is expected from the relatively low temperatures at which they are destroyed in stellar interiors and the deep mixing that takes place when stars being their ascent up the red giant branch. Nevertheless, red giants with atmospheres enriched in lithium are well known. Is this Li stellar ash dredged up from even deeper layers in the star, or is it debris from a giant planet that once orbited the star? Beryllium is expected to be enhanced in the latter scenario, but not in the former. In this talk, I will present preliminary results on a project to measure Be in a small sample of Li-rich red giant stars.

2. Outline Background: Beryllium:
Changes in light element abundances (Li, Be, B) during stellar evolution
The Li-rich red giant phenomenon
Beryllium:
Why it’s important
Why it’s hard to measure
Preliminary results

3. Background We tend to think of fusion occurring in stellar cores
Li, Be, and B all efficiently “burn” at relatively cool temperatures  surface abundance changes
Going to talk about LI for a bit, b/c it’s easiest burned, most abundant initially, easiest to observe
What stage of is my plot
Cunha and Smith 1999 has the temperatures
Li burns at 2.5 MK
Be: 3.5 MK
B: 5 MK

4. Background First dredge-up (FDU): dilutes surface Li, Be, B
13C dredge-up  reduced surface 12C/13C
CORE
INTERMEDIATE
CONVECTIVE ENVELOPE
RED GIANT
Li/Be/B pockets – size dependent on M★, rotation, [Fe/H]…
SUN

5. Background Most red giants should be (and are) depleted in Li.
What happened in the ones that are Li-rich?
Sun (present)
Li-rich
Meteorites/young sun
Going to talk about LI for a bit, b/c it’s easiest burned, most abundant initially, easiest to observe
What stage of is my plot
Cunha and Smith 1999 has the temperatures
Li burns at 2.5 MK
Be: 3.5 MK
B: 5 MK
A(Li) = log(NLi/NH) + 12
Data: de Medeiros+ 2000, Carlberg+ 2012

6. Are Li-rich red giants planet eaters?γ ν e-
3He
4He
7Be
7Li or

7. The Beryllium Test
No change
in 12C/13C
12C/13C reduced

8. The Beryllium Test: Selecting Stars
Evidence of deep mixing
Too Li rich(for engulfment)
Clearly Li enriched
Engulfment “sweet spot”
Previous Be non-detections (Melo et al 2005; Takeda & Tajitsu 2017)
What is Li/Si?
Stuff in box can be donw with BDs into a 0.8 M_sun envelope or planets into a 0.1 M_sun envelope
Stuff in circle, hard to explain with engulfment even if BD into a 0.1 M_sun envelope
Melo et al A(Li) 12C/13C # in T17 HD
HD X
HD X
HD X
HD X
HD ?? X HD HD HD
T17 did 18 unique new ones
NAME A(LI) 12C/13C/
HD missing?
HD ??
HD ?? ON PLOT HD
HD ?? ON PLOT
KIC
HD ON PLOT
HD ?? ON PLOT HD
HD ON PLOT HD
PDS
HD ?? HD HD
HD Missing?
Detections Takeda & Tajitsu 2017
New Subaru HDS observations (Li-rich stars selected from Kumar+ 2011)
Data: Carlberg+ 2012, Kumar+ 2011, Gilory 1989

9. The Beryllium Test: Selecting Stars

10. Measuring A(X): Be vs. Li
Vesta/Sun (Subaru spectrum):
various line lists with undepleted Be
Li lines
Be lines
Dmag ~ 2.5
Pollux (STIS spectrum):
various line lists with NO Be
Teff ~ 4600 K, A(Li) < –0.11 dex
Teff ~ 4600 K, A(Li) = dex
Carlberg+ 2017, in prep

11. Measuring A(Be):
“By hand” analysis – adjust parameters interactively in MOOG
Semi-automated analysis:
Amoeba Fitter with different free parameters. All runs fit the overall scale, plus fit abundances of: Be
Be/C/O
Be/Ti
Be/C/O/Ti
Visually inspect to select best fits…

12. Measuring A(Be):

13. Preliminary Results Engulfment prediction FDU prediction A(Be) is…
< FDU predictions for low 12C/13C
for normal 12C/13C,
> FDU predictions
< engulfment predictions
for unknown 12C/13C,
≈ FDU for one
> engulfment for other  post engulfment Li burning?
12C/13C normal
12C/13C low
12C/13C unknown
T17, 12C/13C
◯ unknown, ⬤ low
NAME A(LI) 12C/13C/
HD ?? ON PLOT
HD ON PLOT
HD ?? ON PLOT
HD ON PLOT
HD ?? ON PLOT

14. Comparison to previous results:
Our presumed engulfment A(Be) is within distribution seen in Li-normal stars
Due to line list difference?
Due to sample differences?
Evidence of post-engulfment Li-burning?
Takeda & Tajitsu (2014 – Li normal, 2017 – Li-rich)

15. Summary: Selected sample of Li-rich stars with (mostly) known 12C/13C
Using a newly created spectral line list, measured A(Be) in 8 Li- rich stars
Stars for which we have strongest suspicion of planet engulfment do have largest A(Be)
Observed Be enhancement is generally lower than expected
One Be enhancement is much higher than expected  was star previously more Li-rich?
Future:
Well defined error bars
Tailor engulfment predictions to each star
Investigate biases between this sample and literature sample

16. EXTRA

17. Pollux (STIS spectrum):
various line lists with NO Be

18. Preliminary Results

19. The Beryllium Test Too Li rich(for engulfment) Engulfment “sweet spot”
Evidence of deep mixing
Too Li rich(for engulfment)
Clearly Li enriched
Engulfment “sweet spot”
Previous Be non-detections (Melo et al 2005); Takeda & Tajitsu 2017
What is Li/Si?
Stuff in box can be donw with BDs into a 0.8 M_sun envelope or planets into a 0.1 M_sun envelope
Stuff in circle, hard to explain with engulfment even if BD into a 0.1 M_sun envelope
Melo et al A(Li) 12C/13C # in T17 HD
HD X
HD X
HD X
HD X
HD ?? X HD HD HD
T17 did 18 unique new ones
NAME A(LI) 12C/13C/
HD missing?
HD ??
HD ?? ON PLOT HD
HD ?? ON PLOT
KIC
HD ON PLOT
HD ?? ON PLOT HD
HD ON PLOT HD
PDS
HD ?? HD HD
HD Missing?
New observations of Li-rich stars
(selected from Kumar et al 2011)