Download Rotational spin-up in the 30-Myr

Application No.
Time Allocation Committee for
MPG time at the ESO 2.2m-telescope
c/o MPI für Astronomie
Königstuhl 17
D-69117 Heidelberg / Germany
Observing period
Received
Apr-Sep 2014
APPLICATION FOR OBSERVING TIME
from
1.
X
MPIA
MPG institute
Telescope:
2.2-m
X
Dr Wolfgang
2.1 Applicant
other
Brandner
MPIA
Name
Institute
Königstuhl 17
69117 Heidelberg
street
ZIP code - city
BRANDNER
[email protected]
ESO User Portal username
e-mail
A. Bayo, J. Bouvier, E. Moraux
MPIA, IPAG, IPAG
name(s)
institute(s)
R. Lachaume,C. Dougados, C. Melo, J. Irwin
CAIUC, UMI LFCA, ESO, CfA
name(s)
institute(s)
2.2 Collaborators
Wolfgang Brandner, Amelia Bayo
2.3 Observers
name
name
By specifying the names under item 2.3 it is obligatory to also send out these observers to
La Silla, if required. Correspondence on the rating of this application will be sent to the
applicant (P.I.) as quoted under 2.1 above.
3.
Category: E
Observing programme:
Title
:
Abstract :
Rotational spin-up in the 30-Myr-old cluster NGC 3766
A young star’s angular momentum is one of the fundamental quantities defining its properties and evolution, from photospheric abundances (mixing processes) to X-ray emission,
spots, and stellar winds (magnetic activity). The evolution of angular momentum provides
insight in the star formation process, accretion/ejection, evolution of stellar activity and its
impact on planets, and physical processes redistributing angular momentum in stellar interiors. Models and observations suggest pre-main-sequence spin-up followed by loss of angular
momentum with age. Yet observations still have to probe ages 20–30 Myr, when spin-up
should result in the shortest rotation periods. We propose to fill this gap by measuring 100s
of rotational periods for low-mass members of the 30 Myr-old cluster NGC 3766.
X
4.
Instrument:
WFI
FEROS
GROND
5.
Brightness range of objects to be observed:
6.
Number of hours:
from
16
applied for
30
no restriction
7.
grey
to
22
I-mag
already awarded
still needed
none
none
dark
Optimum date range for the observations: .................................... 1.4.14 – 31.5.14
Usable range in local sideral time LST: ....................................... 7:00h – 16:00h
8a.
Description of the observing programme
Astrophysical context
was negligeable at earlier PMS phases, becomes dominant, and the strong reduction of the stellar moment
of inertia before the ZAMS leads to rapid acceleration.
The two clusters closest in age are h Per at 13 Myr,
corresponding to the start of the spin-up phase towards the ZAMS, and NGC 2547 at 40 Myr, when
solar-type stars have already settled on the ZAMS. As
shown in Fig.2, the latter cluster lacks rotational period measurements for solar-mass stars (those shown in
Fig.1 are for 0.6-0.9 M stars), and has too few measurements at lower masses down to 0.3 M to allow
a robust derivation of the mass-dependent rotational
distribution at this age (Irwin et al. 2008).
The angular momentum content of a newly born star is
one of the fundamental quantities, like mass and metallicity, defining the star’s properties and evolution. Rotation influences the star’s internal structure and the
mixing processes in the stellar interior that are reflected
in surface elemental abundances. It is also the main
driver for magnetic activity, from X-ray luminosity to
UV flux and surface spots, that is the ultimate source
of stellar winds. Studying the initial angular momentum content of stars and its evolution throughout the
star’s lifetime brings unique clues to the star formation process, to the accretion/ejection phenomenon in
young stellar objects, to the history and future of stellar activity and its impact on surrounding planets, and
to physical processes that redistribute angular momentum in stellar interiors.
Understanding the angular momentum evolution of
low-mass stars is one of the major challenges of modern
stellar physics. Spectacular progress has been achieved
for cool stars and brown dwarfs in the past years.
Thousands of new rotational periods have been derived
for objects over the entire mass range from solar-type
stars down to brown dwarfs at nearly all stages of evolution between birth and maturity (e.g. Irwin & Bouvier 2009). Recent years have also seen a renaissance
in numerical simulations of magnetized winds that are
the prime agent of angular momentum loss (e.g., Matt
et al. 2012), new attempts have been made to understand how young stars exchange angular momentum
with their disks via magnetic interactions (e.g., Zanni
& Ferreira 2013), and new insights have been gained
on the way angular momentum is transported in stellar
interiors (e.g. Charbonnel et al. 2013).
New semi-empirical models reproduce some of the
major trends observed for the rotational evolution of
low-mass stars (e.g. Denissenkov 2010; Irwin et al.
2011; Reiners & Mohanty 2012; Gallet & Bouvier
2013). The models include in a simplified way all the
physical processes at play: star-disk interaction, wind
braking, and core-envelope decoupling. Their predictions are constrained by the observed evolution of the
surface rotation rates of stars from birth ('1 Myr) to
maturity (∼5-10 Gyr), see Fig. 1. The evolution of
surface rotation can be traced observationally from the
Pre-Main Sequence (PMS) through the Zero Age Main
Sequence (ZAMS) to the mid-MS by the period distributions of low mass members of star forming regions
and young open clusters. The changing shape of the
rotational distribution as the stars age is relatively well
accounted for by these models.
Yet, a better sampling of the temporal evolution
of stellar rotation is required. Figs. 1 & 2 reveal
an age gap for observations of the pre-main-sequence
spin-up between about 13 and 40 Myr. This spin-up
phase is critical in several aspects: models predict that
this is the time when core-envelope decoupling is the
strongest (cf. Fig.1), braking by stellar winds, which
Immediate aim
We propose to fill this observational age gap by measuring 100s of rotational periods for low-mass members
(0.1-1.0 solar masses) of the 25–30 Myr-old NGC 3766
cluster, thus providing a statistically robust distributions in each mass range, as for the h Per cluster (cf.
Moraux et al. 2013), whose properties are similar to
those of NGC 3766. This will allow us to fully characterize the pre-ZAMS spin-up phase of solar-type and
low mass stars that is today only scarcely explored.
Previous work
Previous work includes our studies of star cluster in the
age range 1 to 600 Myr (e.g., Kudryavtseva, Brandner
et al. 2012, ApJ 750, L44; Bayo et al. 2012, A&A
547, 80) and on outflows from young stars (e.g., Zhang,
Brandner et al. 2013, A&A 553, 41).
Layout of observations
We aim at detecting rotation periods ranging from a
few hours to 15 days. In order to better discern random
activity (flares) from rotational variation (spots rotating in and out of view), NGC 3766 will be monitored
continuously over two nights followed by 20 nights with
individual 2 hr blocks. The MPIA request is for 15
times 2hr, observed over consecutive nights. 2 nights
and 5 times 2hr block in consecutive nights will be
asked for in Chilean time. Observations will consists
of sets of 300s I-band exposures (plus a single 3600
V-band exposure during Chilean time to characterize
cluster members based on the position in the CMD).
Strategic importance for MPIA
The proposed observations combine our work at MPIA
on young stellar clusters and low mass stars and substellar objects with the studies of our collaborators
on rotational periods and angular momentum evolution. The proposed programme also aims at further
strengthening the collaboration between MPIA, IPAG,
and researchers at universities and observatories in
Chile.
2
8b.
Figures and tables
Figure 1: Comparison between models and observations of the evolution of rotational periods in young
solar-type stars. There is a clear observational gap between 13 and 40 Myr where models predict the largest
rotational velocities (Gallet & Bouvier 2013).
Figure 2: Relation between mass and rotation period
in clusters of different ages (Moraux et al. 2013)
3
9.
Objects to be observed
(Objects to be observed with high priority should be marked in last column)
Designation
NGC 3766
α (2000)
δ (2000)
magnitude in
spectral range
to be observed
priority
11h 36m 00.s 0
−61◦ 360 0000
I=16--22
1
4
10. Justification of the amount of observing time requested:
The 0.1-1.0 M population of the cluster encompasses an Ic -band magnitude range between 16 and 22.
According to WFI ETC, this range can be covered in a 300 sec single exposure. We will adopt the same
observational strategy as we used for the h Per cluster, that allowed us to successfully measure rotational
periods for nearly 600 of its low-mass members (Moraux et al. 2013). Namely, we will monitor the NGC
3766 cluster every night over 2 hour-long blocks of time for 20 nights, and additionally during 2 full nights.
15 of the nights with 2 hr blocks will be requested as MPIA time, thus we ask for a total of 30 hr.
11. Constraints for scheduling observations for this application:
Idealy the observations should be scheduled directly after the Chilean time, and should consist of 2hr
blocks observed over 15 consecutive nights.
12. Observational experience of observer(s) named under 2.3:
(at least one observer must have sufficient experience)
The proposed observers have considerable observing experience.
13. Observing runs at the ESO 2.2m-telscope (preferably during the last 3 years)
and publications resulting from these
Telescope
2.2m
instrument
WFI
date
P91
hours
21
success rate
75%
5
publications
all data pre-red., astrometric analysis on-going
14. References for items 8 and 13:
6
Tolerance limits for planned observations:
maximum seeing:
photometric conditions:
1.800 minimum transparency:
no moon: max. phase /
6
:
80% maximum airmass:
1/30◦ min. / max. lag:
2.0
/ nights