Herschel

Searching for Kuiper Belts around solar-type stars:
DUst around NEarby Stars (DUNES)
Benjamín Montesinos (CAB, CSIC-INTA)
(on behalf of the DUNES consortium)
The DUNES team
Olivier Absil, David Ardila, Jean-Charles Augereau, David Barrado, Amelia
Bayo, Charles Beichman, Geoffrey Bryden, Carlos del Burgo, Carlos Eiroa,
William Danchi, Steve Ertel, Davide Fedele, Malcolm Fridlund, Misato
Fukagawa, Beatriz M. González, Eberhard Grun, Ana M. Heras, Inga
Kamp, Alexander Krivov, Ralf Launhardt, Jeremy Lebreton, Rosario
Lorente, René Liseau, Torsten Löhne, Jesús Maldonado, Jonathan Marshall,
Raquel Martínez, Gwendolyn Meeus, David Montes, Benjamín Montesinos,
Alcione Mora, Alessandro Morbidelli, Harald Mutschke, Sebastian Müller,
Takao Nakagawa, Göran Olofsson, Göran Pilbratt, Ignasi Ribas, Aki
Roberge, Jens Rodmann, Jorge Sanz, Enrique Solano, Karl Stapelfeldt,
Philippe Thebault, Helen Walker, Glenn White, Sebastian Wolf
The Herschel Space Observatory
ESA mission, successor of ISO (Infrared Space Observatory). It operated between
14/05/2009 and 20/04/2013. With a 3.5-m mirror, it has been the largest space
telescope in orbit so far.
• PACS (Photodetector Array Camera and Spectrometer): 55 – 210 microns.
• SPIRE (Spectral and Photometric Imaging REceiver): 194 – 672 microns.
• HIFI (Heterodine Instrument for the Far Infrared): 157 – 625 microns.
Some examples of Herschel’s potential
HST
Calar Alto, 1.7 µm
Herschel
Herschel related the stratospheric water in Jupiter with the SL9 impacts
In July 1994 at least 21 fragments of the comet Shoemaker-Levy 9 impacted on Jupiter.
The water vapour observed in Jupiter by Herschel has been related with those impacts
(Cavalié et al. 2013). The PACS maps provide the covering pattern of water vapour on the
planetary disc, whereas HIFI provides the vertical pressure profile. The water observed is
located in the stratosphere with a clear asymmetry when comparing both hemispheres,
pointing to an isolated event for its origin.
Fomalhaut (α PsA, A3 V, d=7.7 pc, ~440 Ma)
HUBBLE
0.5 micron
The Herschel Space Observatory
ESA mission, successor of ISO (Infrared Space Observatory). It operated between
14/05/2009 and 20/04/2013. With a 3.5-m mirror, it has been the largest space
telescope in orbit so far.
• PACS (Photodetector Array Camera and Spectrometer): 55 – 210 microns.
• SPIRE (Spectral and Photometric Imaging REceiver): 194 – 672 microns.
• HIFI (Heterodine Instrument for the Far Infrared): 157 – 625 microns.
DUNES
DUst around NEarby Stars
“Cold Disks around Nearby Stars. A Search for
Edgeworth-Kuiper Belt Analogues”
• Herschel ‘Open Time Key Programme’ with the aim
of detecting and studying cold dusty discs (analogs
to the Kuiper Belt in the Solar System) around solartype stars in our neighbourhood.
• Tools: PACS photometry at 70, 100 y 160 µm
SPIRE photometry at 250, 350, 500 µm
The Kuiper Belt
The Kuiper Belt (also known as the Edgeworth-Kuiper Belt, EKB), is a region in
the Solar System placed beyond the orbit of Neptune (30 UA) up to ~55 UA. It is
similar to the asteroid belt but 20 times wider and 20-200 times more massive.
β Pic
The Kuiper Belt
The Kuiper Belt contains dust and small bodies,
ices of methane, amonia, water etc. and dwarf
planets like Pluto, Haumea, Makemake…
Although from 1930 (F. Leonard), predictions
were made about its existence (K. Edgeworth,
1943; G. Kuiper, 1951; A.G.W. Cameron, 1962; F.
Whipple, 1964…) its “oficial” discovery from
observations was in 1992 when D. Jewitt and J.
Luu announced the detection of 1992 QB1. In
addition to the dust, models say that there are
more than ~70 000 objets with diameters larger
than ~100 km.
1995 QY9
1999 KR16
http://www2.ess.ucla.edu/~jewitt/kb.html
β Pic
Context
The detection of IR excesses around MS and PMS stars
was one of the main discoveries of the IRAS
observatory (1983).
Vega
• Debris discs: dusty discs continously fed by collisions
between large bodies (planetesimals).
• They are second generation discs: the primordial gas
has disappeared almost entirely.
• Debris discs provide information about the presence
of planetesimales (and planets).
• Relevant contributions from ISO (1995-1998)
and mainly Spitzer (2003-2009) and groundbased telescopes.
β Pic
Fomalhaut
DUNES: objetives
Detect and characterize exo-solar analogs (faint) to
The Edgeworth-Kuiper Belt (EKB)
Herschel has advantages over previous
space observatories:
• Larger mirror, narrower beam width,
better resolution, less confusion.
• Sensitive at λ > 70 µm. PACS 100 µm
is the most appropriate for discs in the
range Tdust ∼ 20 -100 K and optimal for
30 - 40 K.
CEK: Ldust/LSun ~10 - 10
-7
-6
Fluxes at 70-160 µm: ~0.1-0.4 mJy
Detection limits (5σ PACS 100 µm,
~4 mJy) for a G5V star at 20 pc.
Additional goals
• Dependency of the formation of planetesimals with stellar mass.
• Collisional and dynamical evolution of the exo-EKBs.
• Presence of exo-EKBs against presence of planets.
• Properties of the dust and size distributions in the exo-EKBs.
Formation and evolution of planetary systems
Data analysis and interpretation using tools and codes including
radiative, collisional, dynamical... approximations.
Sample and observing strategy
• Sample: 133 FGK stars:
• Distance < 20 pc.
• Stars with known planets (d < 25 pc).
• Debris discs detected by Spitzer (d < 25 pc).
+ 106 stars (81 FGK) stars shared with OTK DEBRIS.
Volume limited sample
• Strategy: integrate as long as necessary to reach the
photospheric flux at 100 µm, with the only limitation of the
background confusion. We need to detect very few mJy above
photospheres with fluxes of the same order and both around
the Herschel detection limits.
• F* (100 µm) ≳ 4 mJy.
• An analog to the EKB at 10 pc, 100 µm: ∼ 7 – 10 mJy.
Spectral energy distributions
WISE
JHK
IRAS, Akari
Herschel
Optical
IRS spectrum
MIPS
Confusion
The sky at 70, 100 y 160 µm is very complex:
• Confusion caused by:
• Extragalactic objects.
• Other stars in the field.
• Extended structures in the ISM,
e.g. cirri.
• The coincidental alignment is a potential
problem for:
• Flux estimations.
• Identification of the target stars.
• Identification of the extended emission.
~10-3 objets/arcsec2 with F160 ~5 mJy.
This implies ~0.1 objets/beam at 160 µm,
i.e. a probability of ~10% of a coincidental
alignment with a background object.
100 µm
160 µm
Summary of results
(DUNES only)
F
G
K
Total
Observed
27
52
54
133
No excess
18
39
44
101
Excess (new)
9 (2)
13 (4)
10 (6)
Dubious objects
3
2
3
8
2
2
4
4 (4)
16 (13)
5 (2,1)
20 (6,2)
Fields “with structure”
Resolved (new)
6 (5)
6 (4)
Planets (excess, new)
5 (2)
10 (2,1)
32 (12)
Summary of results
(DUNES + DUNES-by-DEBRIS)
Resolved discs
HD 207129: G2 V, 16 pc
Most of the material is concentrated near
the external edge of a disc or inclined ring,
placed at ~160 UA from the star.
PACS images: observed (left), star subtracted
(middle) and deconvolved at 70, 100 and
160 µm.
Marshall et al., 2011, A&A, 529, 117
Löhne et al., 2012, A&A, 537, 110
Resolved discs
q1 Eri: F8 V, 17.4 pc
The disc around q1 Eri has been resolved at FIR
wavelengths for the first time. The emission is
thermal and optically thin. From the deconvolved
images, q1 Eri seems to be surrounded by a ring
Of cold dust, with Tdust below 30 K, a width of ~40
UA and an internal radii of ~85 UA.
PACS images: observed (left),
deconvolved (right) at 70, 100 y
160 µm.
Liseau et al., 2010, AA, 518, 132
Discs: large variety of SEDs
Excesses in all λ’s
Discs: large variety of SEDs
SEDs suggesting a ring-like structure for the distribution of dust.
Discs: large variety of SEDs
Small excess at 100 µm (cold disc, Tdust < 40 K)
Cold discs: a result from DUNES
completely new
Six stars show excesses only at 160 µm
This implies cold dusty discs: Tdust ~ 20 -25 K (<30 K) and
faint Ldust/L* ~ 10-6
These discs cannot be explained by any of the standard known scenarios. They
could represent a new physical regime, different from the ones accepted for the
debris discs observed previously.
100 µm
160 µm
(Eiroa et al. 2011, A&A, 536, L4; Krivov et al. 2013, ApJ, 772, 32)
In addition to studies and searches for nearby objects, both in position and in the
background (galaxies) and in the line of sight (objets in the Kuiper belt), statistical
-conservative- arguments tell us that the probability that all six cold discs are
actually background galaxies is ~1.2%.
The data suggest that the dominant grain size is larger than ~100 µm and that the
small grains are strongly underabundant. This clashes with what is observed in
the debris discs studied so far, where the models point to grains in the range of
~µm.
A possible explanation for the scarcity of small grains: population of solids that
grew in the periphery during the protoplanetary phase and have remained unstirred.
To explain the data, those solids must be larger than a few mm but smaller than
a few km.
Radial temperature profiles for three solar chromospheric models :
VALC (solid line, Vernazza et al. 1981), Selhorst et al. (dashed line,
2005), model C7 (dotted line, Avrett & Loeser, 2008). α Cen A presents
a similar chromospheric structure (Judge et al. 2004).
30 -500 micron
Synthetic solar spectrum between 2 GHz and 10 THz as a function of height
over the photosphere. The contours show the efficiency of the total emissivity
εT; blue represents optically thin, red optically thick (De la Luz et al. 2013).
Photosphere in the optical: 5830 K
Chromosphere in the FIR:
Tmin ~ T 160 µm = 3920 K
Summary and conclusions
¡Goals acc
omplished
!...
...but still
w ork to do
• ∼24% - ≲ 30% of debris discs:
- Remarkable increase with respect to previous statistics.
- New discs, mainly around K-type stars.
• Flux levels similar to those of our EKB reached.
• A large number of resolved discs (5 x previous).
• “New class” of debris discs:
- Excesses only at 160 µm: Very cold and faint discs,
Tdust ≲ 30 K, that could be representative of a new physical
regime. More abundant in the latest spectral types.
• Chromospheric minimum detected for the first time directly
in a solar–type star.
ζ2 Ret:another resolved disc
Star: G1V, d = 12.03 pc, 0.97 L, Age ∼ 3 Gyr
Debris discs around ζ2 Ret: excentric structure similar to a ring
with a ~100 UA, e ~0.3, Tdust ~ 40 K, Ldust/L* ≈ 10-5
Asymmetry: Caused by a planet?.
(Eiroa et al., 2010, A&A, 518, L131, Thebault et al., Faramaz et al., 2014, A&A, 563, 72)
SED of the ζ2 Ret complex
ζ 2 Ret: comparison with
similar systems
Fomalhaut
Star
Disc
Planet
ζ2 Ret
Sun
A3 V
2.1 M 16 L
G1 V
1.0 M 1.0 L
G2 V
1.0 M 1.0 L
~0.2 Gyr
∼ 3 Gyr
4.5 Gyr
Ldust/L* ~ 10-4
Ldust/L* ~ 10-5
Tdust ~ 75 K
Tdust ~ 40 K
Ldust/L ~ 10-6 - 10-7
Tdust ~ 40 K
135-160 UA
70-120 UA
40-55 UA
Fomalhaut b (¿?)
e = 0.1
???
e = 0.3 ?
Neptun
e = 0.01
Peculiar SEDs
Spectral index:
For a black body, Δ = 2 (Rayleigh-Jeans)
Peculiar SEDs (“steeped”): Δ > 2
(Ertel et al. 2012, A&A, 541, 148)