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The Approved AP Young stars in the field of the
Open Cluster Dolidze 25observational
requirements, feasibility,expectations
Vincenzo Ripepi INAF-Osservatorio Astronomico
di Capodimonte, Naples, Italy
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Introduction
Original interest individuating PMS intermediate
mass stars as possible ? Scuti targets for COROT
? Interest for young open cluster (age lt 10 Myr)
in the vicinity of COROT Primary targets, i.e.
observed in any case in the EXO field.
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Dolidze 25 Properties

• Dolidze 25 is a young cluster associated with the
  HII region S284
• it is far (heliocentric distance gt 5.5 Kpc)
• it is metal poor Lennon et al. 1990 found that
  Dolidze 25 is deficient in metals approximately
  by a factor 6 ? somewhat in disagreement with
  respect to the expected abundance gradient of the
  disc.
• Dolidze 25 could be connected with the Canis
  Major Dwarf Galaxy which is supposed to be
  merging with our Galaxy (Martin et al. 2004)
• Dolidze 25 is poorly studies as a SFR. Due to the
  low metallicity, it represents an ideal object to
  study metal deficient PMS objects, which would be
  the counterpart of normal TTauri and
  intermediate mass PMS stars.

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PMS stars in the HR Diagram

• Theory allows to calculate the so-called
  birth-line, i.e. the location of stars of
  different mass when they start their PMS phase.

• PMS stars contract from the birthline to their
  location on the Main-Sequence on the
  Kelvin-Helmoltz time scale t3/7(GM2)/RL.

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Why to search for PMS ? Scuti stars ?

• Determination of the intrinsic stellar
  parameters P??cost. ? PP(M,R) but LL(R,Te) ?
  PP(M,L,Te) ? if L,Te are known indipendently,
  the comparison between a measured period and the
  theoretical value provides the stellar mass.
• Determination of the inner structure Pulsation
  (asteroseismology techniques) also allows us to
  probe the interior of Herbig Ae/Be, whose
  structure is different from that of MS objects.
  Suran et al. (2001)
  showed that some non-radial unstable
  modes are very sensitive to the deep internal
  structure which differs between PMS and post-ZAMS
  ? measurable differences between PMS and post-MS
  in the low frequency regime where observations
  from ground are very difficult even with the best
  multisite campaign.

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GOALS OF THE PROJECT

• Asteroseismology of PMS ? Scuti stars. We
  identified 4 confirmed and 3 probable candidate
  PMS stars which fall in the instability strip. If
  these objects pulsate, they will allow us to use
  asteroseismological techniques to put constraints
  on their evolutionary status and internal
  structure ? constraint on cluster distance ?
  estimate of fundamental parameters of the
  low-mass PMS stars in the cluster ? possibly to
  study the effect of metallicity on the Initial
  Mass Function.
• Variability on longer timescales. In particular
  we plan (i) to study the interaction of the
  stars with the circumstellar structure (ii) to
  verify the variability produced by hotspots
  (iii) to search for eclipses by circumstellar
  material and/or other periodic variations (iv)
  to model the variations in the accretion process
  (v) to study Debris Disk candidates in the
  direction of Dolidze 25 (de la Reza talk)
• Variability of stars in the Association(s) in
  front of Dolidze 25 study of stellar activity
  (see Torres talks).

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Target selection

1. Intermediate mass PMS stars have been selected on
   the basis of both literature data (H? emission
   catalogues, 2MASS, IRAS data) and of a specific
   photometric and spectroscopic survey devoted to
   Dolidze 25
2. In order to study more systematically the
   cluster, additional targets have been selected on
   the basis of membership to the cluster (bright
   stars)
3. Debris Disk candidates were selected on the basis
   of their IR excess in the IRAS bands (see de la
   Reza talk)
4. Stars in nearby associations have been
   individuated on the basis of X-ray data and
   proper motions (see Torres talk)

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Target stars
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Selection of the targets Imaging and
Spectroscopy of DOlidze 25

• Observations in RI with VIMOS_at_VLT (pre-Imaging
  data)
• Pre-Imaging (2MASS dataEGBW data) used to
  select targets for the spectroscopy. High
  priority to objects showing IR excess.
• Spectroscopy performed by using VIMOS_at_VLT
• ?
• Identification of stars with H? emission and
  Lithium

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PMS stars in the HRD
From spectral type ? Teff ?intrinsic colors and
BC Comparing with BVRIJHK photometry for our
stars and assuming the distance (5.5 Kpc) ?
position in the HRD (and Av)
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Expected observational properties of target stars

• pulsation (? Scuti regime) ? very low amplitude
  (mmag to hundredths of magnitude) with time
  scales of minutes to a few hours.
• Interaction with circumstellar material 1)
  variable dust obscuration ? variations up to
  magnitudes time scales of weeks to years. 2)
  clumpy accretion ? lower amplitude variability
  with time scales of the order of days 3)
  rotational modulation due to activity and winds
  ? low amplitude variability (hundredths to tenths
  of magnitude) with time scales of hours to days.
• For Debris Disk stars the expected variation is
  of a few .

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Feasibility

1. Most of target stars have Vlt15 ? color
   information Error on single phase point lt 1-2
   mmag
2. PMS candidates are fainter (Vgt 15) ? observations
   in white light. Difficult to be observed. Which
   would be the S/N?

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Simulations

• Could we obtain asteroseismological measurements
  with this low S/N?
• Frequency spectrum from IP Per (9 frequencies
  with V amplitues from 2.8 to 1.1 mmag, Ripepi et
  al. 2005)
• Used period04 (Lenz Breger 2005) to simulate
  a150 d time series diminishing (regularly) to 94
  the duty cycle (not very realistic)
• Added gaussian noise with ?0.04 mag (i.e.
  R16.9 mag)
• Time series analysed with period04 (Lenz Breger
  2005) uncertainty on the pulsation amplitudes
  estimated on the basis of Monte Carlo
  simulations
• ?
• 8 over 9 frequencies revealed with S/N gt 3.6
  (99 confidence level Kuschnig et al. 1997). In
  general any mode with amplitude larger 1.1-1.2
  mmag can be revealed.
• Amplitudes and Phases uncertain up to 20-30

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Simulations
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Summary

• We identified 40 targets for COROT in the young
  open cluster Dolidze 25.
• The unprecedented precision and long time
  duration of COROT observations will allow us to
• Apply asteroseismological techniques to PMS ?
  Scuti candidates (if they actually pulsate!) in
  order to constraint their intrinsic parameters
  and evolutionary status.
• Study the interaction of PMS stars with
  circumstellar material
• Study candidate DD systems
• Study variability in Association stars
• The PMS candidates are faint and amplitudes
  small simulations show that an
  asteroseismological analysis is possible also for
  faint objects
• No problems in studying interaction with
  circumstellar material and activity (larger
  amplitudes).