Download The science potential of atmospheric Cherenkov arrays used as intensity interferometers

The science potential of atmospheric Cherenkov arrays
used as intensity interferometers
Michael Daniel for
Willem-Jan de Wit
[email protected]
Atmospheric Cherenkov Telescope Arrays
Multiple telescopes image the source of optical Cherenkov light created by charged
secondary particles (“the shower”) from an incoming gamma-ray.
● The faint Cherenkov light requires large light collectors.
● The brief (~nanosecond) Cherenkov flash requires fast photon detectors.
● Shower image reconstruction gives spectral and angular information on the incoming
gamma-ray, hence many telescopes covering long baselines.
●
H.E.S.S.
VERITAS
Similar technical specs of ACTs and I.I.
A proposed CTA lay-out
Fast photon counters/digital
processing
● (very) Large photon collectors
● Large number of telescopes
● Telescope can point independently
● Long baselines
●
red dots : 85m2 dish
blue dots : 600 m2 dish
89 telescopes = 3916 baselines
e.g. Le Bohec & Holder (2006)
ACT projects under study:
● CTA (EU)
● AGIS (US)
Bernloehr et al. (2007)
What do stellar astronomers want?
(apart from some fun)
The accurate determination of the physical properties of
stars,
i.e. mass, radius, luminosity and elemental abundances ...
and everything that either is ex- or accreted from/onto the
star.
Sensitivity of CTA as an I.I.
S/N = 5
● A = 100 -- 600 m2
● quantum efficiency 30%
● n = 5.0 e-5 (1.0e-4)
● degree of coherence = 0.5
● bandwidth 1 GHz
● exposure time 5h
●
2
S / N  RMS= A⋅⋅n ph⋅∣d∣ ⋅  f⋅T /2
(from Hanbury Brown 1973)
Le Bohec & Holder (2006)
limiting magnitude
mv=8.5
[mv= 9.25 using flux zeropoint of
Bessel et al (1990)]
In a simple perspective ...
How many main sequence stars can be imaged (Mv<9.25)?
Three science case examples:
● Young stars
● Rapidly rotating stars
● Cepheids and distance scale
Science case: young stars
the internal stellar structure of pre-main sequence stars
● chromospheric activity (cool spots)
● accretion activity (hot spots)
●
Nearby star formation regions:
Palla & Stahler (2000)
Science case: young stars
In the solar neighbourhood
~50 young stars with mv<8m
In the last decade several young coeval stellar groups have been
discovered in close proximity (~50pc) to the sun. Their closeness
means the members are bright and renders the co-moving group
relatively sparse – making them suitable, unconfused, targets even
with the large optical PSF for an IACT (~few arcminutes).
The spectral type range from A to G
● Sparse, therefore incomplete
membership (N(*) will increase!)
● Ages between 8-50Myr: a
substantial fraction still in the
pre-main sequence contraction
phase
Zuckerman & Song (2004)
●
50pc
oval: 50 pc
small blue dots: Beta Pic Ass.
small red dots: Tucana/Horologium Ass.
grey dot: Pleiades
large blue dot: Beta Pic
The internal structure of young stars
and the calibration of pre-main sequence tracks ...
From PMS tracks masses are inferred. They are therefore
fundamental for a correct understanding of the star formation process.
Interferometry of (non-eclipsing)
spectroscopic binaries deliver dynamical
masses (e.g. Boden et al. 2005), in
combination with spectroscopic data.
● Components V mags: 6.9 and 8.0 at 45pc
● Based on 34 Keck-I interferometry
measurements
In addition:
● Known distances (Hipparcos, GAIA
[launch 2011]) allow direct comparison of
the predicted and I.I. observed sizes of
individual PMS stars.
Boden et al. 2005
The surface structure of young stars
Understanding chromospheric activity and stellar magnetic fields
cool spots (similar to
sunspots) may cover
50% of the stellar
surface and are the
product of the slowly
decaying rapid
rotation of young
stars
hot spots deliver direct
information regarding
the accretion of
material onto the stellar
surface
T Tauri
Magnetically guided accretion process (accretion funnel)
The surface structure of young stars
Understanding chromospheric activity and stellar magnetic fields
Inferred brightness distribution from Doppler imaging
Doppler images of PW And
20Myr, K2V T Tauri star @ 40pc
● Magnitude in V-Band 8.7
● Cool spots are 1200K cooler than
stellar photosphere
●
Cool spots may cover ~50% of surface
● > 65 stars have been doppler imaged
(Strassmeier 2002)
●
Strassmeier & Rice (2006)
The surface structure of young stars
Hot spots and the magnetic accretion phenomenon
2Myr, M0 T Tauri star @ 150pc
● Magnitude in V-Band 14
● Accretion spots are 2000K hotter
than stellar photosphere
●
Doppler imaging of MN Lupi
=>
The surface structure of young stars
Hot spots and the magnetic accretion phenomenon
inferred morphology in
agreement with the
magnetospheric accretion
model
●
the star's fast rotation (a factor
of ~4 from break-up) suggests
that accretion could be
responsible for spin-up, and
hence strong activity
●
magnetic field strength can be
inferred from comparison with
models
(Strassmeier et al. 2005)
●
Science case: distance scale
The zero-point of the Cepheid period-Luminosity relation
Angular size estimate from I.I. and the Cepheid radius estimate from Baade-Wesselink method
Baade-Wesselink method:
1. ∑(vspectro*∆t) = R2-R1
2. L1/L2 = R1/R2
R2
R1
T1
T2
The radius of the cepheid
can be determined from
the observed radial
velocities.
dR
=v t 
dT
high resolution
spectroscopy
Science case: distance scale
The sizes of Cepheids
Angular size measurement (θ)
from intensity interferometry
D
θ
Comparison of angular size and Baade-Wesselink determined size
gives the distance to the Cepheid
Davis et al. 2008 using Sydney University Stellar interferometer (1 Cepheid)
Lane et al. 2000/2002 with the Palomar testbed interferometer (2 Cepheids)
Kervella et al. 2004 with the Very Large Telescope Interferometer (7 Cepheids)
~60 cepheid variables with mv<8m
+
Science case: rapidly rotating stars
Extremely distorted stars near (at?) break-up velocity
Classical Be stars are well-known for the presence of a circumstellar gaseous disk
(the “e” in Be-type stars). The disk is formed in a mass-loss process from the star, and
comes and goes (timescale of months to decades). It is bright in the NIR
(Brehmsstrahlung).
ζ Tauri in Hα at MkIII optical interferometer (2 telescopes)
Quirrenbach et al. (1994)
Science case: rapidly rotating stars
Classical Be disks : how to make them?
Be stars are extremely fast rotators
● How close to break-up?
● Insensitivity of line-width to
rotational velocity
● Pulsations probably also an important
ingredient
●
Townsend et al. 2004
Von Zeipel effect/gravity darkening
0km/s
300km/s
400km/s
487km/s
spectral measurements of rotation can only provide lower limits
owocki
Science case: rapidly rotating stars
The measurement of a deformed star
VLTI-VINCI (K-band!, 2 telescopes)
● alpha Eridani: « Flattest star ever seen »
● v_rot (spectro) ~225 km/s
● v_rot (interfero) ~350 km/s (~break-up)
●
Domiciano de Souza et al. 2003
Science case: rapidly rotating stars
Extremely distorted stars near (at?) break-up velocity
~300 stars with mv<8m corresponding to a distance limit of 700pc
● 50% fraction of B0 stars showing Be phenomenon (i.e. an important
concept within stellar evolutionary theory)
● The disk formation and dissolution are poorly understood
●
Simple examples: baseline coverage
89 telescopes
Nbaseline = Ntel * (Ntel-1)/2. = 3916
(1060 independent baselines)
Simple examples I : perfectly dark spot
3 milli arcsecond
1 single observation with a full 89 telescope CTA array,
using phase information.
Original Image
Power spectrum
Reconstructed image
Simple examples II : Be star
3 milli arcsecond
Original Image
1 single observation with a full 89 telescope CTA array,
using phase information.
Power spectrum
Reconstructed image
Simple examples III : T Tauri
3 milli arcsecond
Original Image
1 single observation with a full 89 telescope CTA array,
using phase information.
Power spectrum
Reconstructed image
Science Summary
Young stars:
- Internal stellar structure by means of dynamical masses of binaries
- PMS stellar radii in combination with known distances (GAIA)
- Stellar rotation, cool spots and dynamo action
- Hot spots and accretion phenomena (less certain)
At least 50 young stars for which CTA-I.I. could provide images
●
●
●
Distance scale:
- Sizes of Cepheid variables to calibrate zeropoint of period-luminosity relation
Rapidly rotating stars:
- Unambiguous determination of the rotational velocities of Be stars
Not or briefly discussed:
X-ray binaries : 15 HMXB and 2 LMXB are brighter than 9.25 in V-band
(from the on-line X-ray binary catalogue (www.xrbc.org)
● Planetary transits (perfectly dark spot)
●
“Since astronomers study objects beyond their control, an
imaging CTA-I.I. will provide a wealth of new discoveries”
Further reading
The potential for intensity interferometry with γ-ray telescope arrays
de Wit et al. arXiv:0710.0190
Towards μ-arcsecond spatial resolution with Air Cherenkov Telescope arrays as optical
intensity interferometers
de Wit et al. arXiv:0811.2377
fin?