Formation of Rapidly rotating
massive stars as progenitorsof long GRBs	
Sung-Chul Yoon (Seoul National University)	

GRB conference at Yukawa Institute, Kyoto, Japan, Nov. 12, 2013

Many massive stars are born witha large amount of angularmomentum

Necessary condition forGRB

Specific angular momentum ofgalactic main sequence stars.
Wolff et al. 2006

There are also many ways toproduce rapidly rotating massivestars via binary interactions

de Mink et al. 2013

See also Cantiello et al. 2007, Huang et al. 2010

Which evolutionary paths of rapidlyrotating massive stars lead to productionof a long GRB?

Unfortunately not much progress in theory has
been made on this question, during the last
several years.
In this talk, I will explain why.

Long GRBs and Their Progenitors	
§ long GRBs associated with SNe Ic§ Massive helium stars like ordinary SNe Ic progenitors, butwith some special conditions§ long GRBs without SN association§ Mergers of helium star + BH?
§ Massive helium stars?

§ long GRB transient,ultra-long
ThistalkGRBs
willfocus on this. 	

§ Blue supergiant?

§ Supermassive BH?

Q1: According to the collapsar
scenario, long GRBs areassociated with BHs. But,then, which stars make a BH?
It has been long believed that there isa “mass cut” for BH formation. But is itreally so?
Recent studies indicate that thedetails of the core structure aresometimes equally or more important
than the progenitor mass (Ugliano etal 13, Dessart et al. 12, O’Connor &Ott 13). Magnetar-driven explosion
could also prohibit formation of a BH
(Dessart et al. 08).

Ugliano et al. 2013

The stellar evolution at the final stages is highly non-linear withcomplicated convection history, and the final stellar structure isnot easy to predict robustly.

Q2: Why is there no GRB associated with SN Ib?
So far, all supernovae associated with long GRBs
were Type Ic.
Both Type Ib and Ic supernovae originate fromhydrogen-deficient massive stars.

First of all, then, what makes SN Ic progenitors
different from SN Ib progenitors? 	
We still do not know. 	

Evolution of massive stars towards Type Ib/c
supernovae	 Long GRB is a sub-set of SN Ib/c	Single Stars: Conti Scenario
Binary Stars

Winds from RSG	WR star	SN Ibc	What makes SN Ic progenitors different
from SN Ib progenitors? 	
1. Total amount of helium?
(cf. Hachinger et al. 2009) 	

Case BB masstransfer for
relatively lowmass He stars:
This can make“faint SN Ic”, likeSN 2005ek.
Cf. Drout et al.
2013, Tauris etal. 2013. 	

Yoon et al. 2010	

WR winds
formassive
He stars.
Stellarevolutionmodelspredict
more than
0.2 Msunof helium
left until
the end.	

If the total amount of He is the governing factor for
distinguishing SN Ic from SN Ib, SNe Ic should havesystematically larger ejecta mass than SNe Ib.
But, that’s not what we observe (e.g. Drout et al. 2011, ApJ,741, 97). The typical ejecta mass is about 2 Msun for both Iband Ic.	

Yoon et al. 2010	What makes SN Ic progenitors different
from SN Ib progenitors? 	
2. Mixing of 56Ni with He? 	

Mixing seems tobe critical for Helines.
But whichprogenitors are
more prone tomixing?
See also
Hachinger et al.
2009	

Dessart et al. 2012	

Q3: which stars make broad-lined Ic without GRB?

Clueless!!!! (maybe
related to magnetar-driven
explosion?)

Observations seem toindicate that their aresystematically more
massive than ordinary
SN Ic.
Would it be related tomagnetars? Then, why
not broad-lined SN Ib?

Graham &Fruchter 2012

Q4: Why is it so difficult to directly identify SNIbc progenitors?
So far, there is no firm direct identification of SN Ibc progenitors in the
pre-SN images, unlike SN IIP progenitors. If SN Ibc progenitors look likethe observed WR stars, at least a few should have been found in the past
surveys (Smartt et al. 2009, Eldridge et al. 2013; but see ). 	
Smartt 09, Eldridge et al. 13	

The SN Ibc progenitors at the pre-SN stage may look very
different from most of the observed WR stars!
If their final masses are higher than about ~ 8 Msun, they will bevery compact, hot, and optically faint like WO stars.
This would also be the case for GRB progenitors. 	

Yoon et al. 2012	

Q5: How can we explain the metallicity
dependence of the long GRB event rate?
Although it is not conclusive yet, many observations seem to indicatethat low-Z is preferred for the production of long GRBs. (e.g., Modjazet al. 2008, Graham & Fruchter 2012)

Would a low Z be preferred if the amountof angular momentum retained in theprogenitor were the key necessary
condition for long GRB?

NO! if the core braking is
not efficient (nonmagnetic stellar evolution
models; Heger et al.
2000, Hirschi et al.
2004).

Probably yes, if the core braking
is strong (magnetic models),depending on the evolutionary
paths you want to consider.
(Yoon & Langer 05, Woosley &Heger 06)

Both observation and theory indicate that high-Z ispreferred for ordinary SN Ibc
Bossier & Prantzos 2009

If long GRB is a“random” subsetof SN Ibc (asexpected fromnon-magnetic
stellar models)This is whatobservations
seem toindicate.

Then, what’s special with low-metallicity?
High Metallicity

Low Metallicity

Strong winds resulting from Weaker line-driven winds
metal lines
Unstable for many times

Relatively stable? (cf.
Baraffe et al. 2001)

Strong mass loss

Weak mass loss

The evolution of massive
stars is dominated by“mass loss”

The evolution of massive
stars is dominated by“rotation” (i.e., rotational
mixing and centrifugallydriven winds)

CHE: chemically homogeneous evolution via rotationallyinduced chemical mixing

CHE can lead to formation of massive He stars
even at zero-metallicity as GRB progenitors

Yoon et al, 2012

Q6: Can binary systems produce a long GRB?

Binary scenarios might explain long GRBs frommetal-rich host galaxies. Strong preference forlow-metallcity is expected in the CHE scenario.

SummaryThe necessary conditions for long GRB
progenitors, and related questions.
ü Rapid rotation (Q: which mechanism is most importantfor the transport of angular momentum in stars?)ü BH formation (Q: core should be massive enough???)ü No extended hydrogen envelope (Q: OK. But then why
there’s no GRB associated with SN Ib?)
Are there other important necessary conditions that
we are missing? B-fields?