Flaring Stars in the GAIA contest

Flaring Stars in the GAIA
contest
Isabella Pagano
on behalf of
I. Pagano, A.C. Lanzafame, G. Cutispoto,
A.F. Lanza, G. Leto, S. Messina, E. Distefano
INAF- OACT
Flares
•  Blue photographic observations in the
60s-70s in open clusters
•  UBV photoelectric photometry of field
stars in the 70s-80s
•  Multiwavelenght spectroscopic
observations (UV, optical, X-ray,
radio, IR) in the 80s-90s
•  X-ray imaging observations of single
field stars and open clusters in the
90s
•  Galex serendipitous UV observations
•  Flares in SDSS
•  GAIA next
Bologna, 13 Dicembre 2011
NGC 2264 (3-4 Myr)
100 ksec
Flaccomio, Micela &
Sciortino, 2006, A&A
455, 903
Stellar Flares
•  Stellar flares are unpredictable
phenomena due to the sudden
transformation of energy stored in
the magnetic field into thermal
and kinetic energy.
•  Flares are typically multiwaveleght
phenomena: they are detected in
the Optical, UV, X-ray, and radio.
•  Flares involve the whole
atmosphere, from the corona to
the photosphere of solar-type
stars.
•  There are evidence of flares
occurring in the circumstellar
environment in active binaries.
•  Flares are observed also on YSO,
generally originating in the
interaction between coronal
magnetic field and the accreting
disk.
Bologna, 13 Dicembre 2011
Solar Flares
•  Solar Flares occur in active regions
•  The whole elecetromagnetic
spectrum is involved radio, optical,
UV, X, gamma.
•  Energy released by a typical solar
flare 1027 erg/s (range 1024 - 1032
erg).
•  Plasma temperature 10 – 20 MK,
in sporadic cases till 100 MK
•  Particle acceleration (electron,
proton, more heavy nuclei).
•  Flare frequency in phase with the
solar cycle.
Bologna, 13 Dicembre 2011
Flaring Stars
•  Flare stars (UV Cet type)
–  Spectral type: late Ge -> dM9e. Optical spectra show emission lines (H, Ca). There is a
peak at M7 for dMe (Gizis et al 2000)
–  While typical rotation for M star is (vrot ~ 2 km/s), dMe è10 km/s (Marcy, Chen 92)
–  About 30% of dMe are in binary systems (Pettersen 91)
–  Density in the solar neighbourhood: 0.056 stars/pc3 (Shakhovskaya 95)
–  Rotational modulation because of starspots (Rodonò 80)
•  Brown Dwarfs
•  Active binaries (BY Dra, RS Cvn-type, FK Com)
•  T Tauri stars
Bologna, 13 Dicembre 2011
Sp
Shakhovskay
a (1995)
Gershberg et
al (1999)
G0-G9
0.05
0.02
K0-K3
0.11
0.02
K4-K8
0.14
0.06
M0-M3
0.32
0.35
M4-M8
0.38
5
Optical flares in
UV Cet type stars
EV Lac (dM3.5e)
Photoelectric photometry è good
enough resolving power
Dmag (U band) till 8 mag è 3-4
mag in B band (G)
EV Lac
16 Jul 71
UBV 61 cm SLN
Bologna, 13 Dicembre 2011
Optical vs. X-ray flare in UV Cet –type star
Flaring rate in EUV, X-ray
and B-band:
0.16-0.37 flares hr—1
Leto, Pagano et al. (1997)
Osten et al (2005)
:
Bologna, 13 Dicembre 2011
Esempi di flare stellari nei
raggi X soft
EV Lac
Rapid rise and a long double-exponential decay,
Favata et al. 2000, observations with ASCA GIS
Modest, flat-topped flare on AT Mic, Raassen
et al. 2003, observations with XMM-Newton
EPIC).
Sequence of very slowly
decaying flares on the giant
β Cet ( Ayres et al. 2001,
observations with EUVE ).
Bologna, 13 Dicembre 2011
Rapid rise and very slow
decay of a flare on the RS
CVn binary σ Gem (Osten
observations with EUVE ).
Flare characteristic
time scales
Flare stars – optical data
Rise time
YSOs – X-ray COUP
data
Duration
Decay time
Leto et al. (1997)
Few minutes to tipcally less than 1hr
Bologna, 13 Dicembre 2011
Wolk et al. (2005)
1hr to 3 days
Long duration optical flare
YY Men
K2 III
FK Com-type
Flare
duration:
9.8 d
EUBVRI =6.2x1039 erg
LUBV/Lbol=2.7x10-3
Bologna, 13 Dicembre 2011
Cutispoto, Pagano
et al. A&A (1992)
Stellar flare classification
•  Typically X-ray stellar flares with short rise times of order of minutes - and decay times of order of a
few tens of minutes, are considered to be analogs of
solar compact flares, while flares with long decay
times - exceeding one hour - are reminiscent of tworibbon (2-R) flares on the Sun:
•  Compact flares: one or a few individual loops that
brighten up on time scales of minutes. They are of
modest height and show high densities. The most
likely mechanism leading to compact flares is an
interaction between neighboring loops.
•  Two-ribbon flares: longer rise times (e.g., van den
Oord and Mewe 1989). Magnetic field structures are
large (104-105 km) and the densities are low.
Complex loop arcades. Interbinary loops.
Bologna, 13 Dicembre 2011
Superflares (SFs)
•  SFs occur on main-sequence stars in spectral classes
F8-G8 with no unusual properties (specifically rapid
rotation, high chromospheric activity, close binary
companions, or very young age).
•  The observed SF energy ranges from 1033 to 1038 ergs.
•  The typical SF duration is about 1 hr, range from a
fraction of an hour up to days.
•  SFs emit radiation at least from the X-rays to the
optical, with indicated temperatures (Schaefer et al.
2000) from 15,000 K (from the He I emission line) to 10
keV (from X-ray continuum).
Bologna, 13 Dicembre 2011
Superflare origin?
•  Superflares are caused by magnetic
reconnection between fields of the
primary star and a close-in Jovian
planet (Rubenstein et al., 2000)
Bologna, 13 Dicembre 2011
How many stars are
flare stars (UV Cettype)
Sp
Shakhovskaya
(1995)
Gershberg et al
(1999)
G0-G9
0.05
0.02
K0-K3
0.11
0.02
K4-K8
0.14
0.06
M0-M3
0.32
0.35
M4-M8
0.38
0.51
Bologna, 13 Dicembre 2011
How to detect flares in
GAIA
•  Outliers are identified by means of magnitude vs.
colour behaviour (SVD).
•  An outlier is flagged as flare when:
Goutlier- Gquiescent<0
Δ(BP - RP) = (BP - RP) outlier - (BP - RP) quiescent < 0
•  To each star having flares detected we attribute a
Flare Star Membership Probability.
•  When per-CCD photometry is available we can
attempt to measure other parameters (energy,
duration at least as lower limits.
Bologna, 13 Dicembre 2011
What information GAIA
provides on flares
•  Despite Gaia scanning law prevents to study single flares in
detail nor it will be possible to study flaring rates on single
objects, it gives an unprecedented opportunity to assemble
flare data from repeat observations of millions of stars on
timescales of minutes to days.
•  The Gaia flare database will therefore provide unprecedented
flare statistics on several classes of objects:
–  Flare rate occurrence per different class of stars
–  Statistics of long duration flares in the optical should be feasible
for particular regions of the sky depending on the scanning law.
–  Statistics of superflares.
Bologna, 13 Dicembre 2011
SDSS investigation on
flares (GAIA like) Kowalski et al. 2009
•  Flare rate analysis of 50,130 dM light curves in SDSS Stripe 82.
•  271 flares identified.
•  Flaring is found to be strongly correlated with the appearance of
Hα in emission in the quiet spectrum (dMe)
–  Of 99 flare stars that have spectra, 8 as relatively inactive (dMe)
•  The flaring fraction is found to increase strongly in stars with
redder colors during quiescence, è or increasing flare visibility
and/or increasing active fraction for redder stars.
•  The flaring fraction is strongly correlated with |Z| distance such
that most stars that flare are within 300 pc of the Galactic plane.
•  The most luminous flares occur on the earlier type M dwarfs.
•  Lower limit on the flaring rate (averaged over Stripe 82) for flares
with is 1.3 flares hr−1 deg−2 but can vary significantly with the line
of sight.
Bologna, 13 Dicembre 2011
Why statistics on flare
occurence vs spectral matter
•  In assessing planetary atmospheric properties:
–  Flares and UV radiation could erode planet atmospheres
(Lammer et al. 2007).
•  In assessing habitability:
–  Bursts of UV radiation due to stellar flares could be detrimental
for life.
•  The search for habitable SEarths is at moment focused
at M stars:
–  Flare effects can be particularly relevant because the active
phase of M stars can last for billions of years (West et al. 2008).
Bologna, 13 Dicembre 2011