The Approved AP “Young stars in the field of the Open Cluster Dolidze 25”:observational

requirements, feasibility,expectations

Vincenzo Ripepi INAF-Osservatorio Astronomico di Capodimonte, Naples, Italy

Introduction

Original interest: individuating PMS intermediate mass stars as possible Scuti targets for COROT Interest for young open cluster (age < 10 Myr) in the

vicinity of COROT Primary targets, i.e. observed in any case in the EXO field.

1st COROT Long Run

Dolidze 25: Properties

Dolidze 25 is a young cluster associated with the HII region S284

it is far (heliocentric distance > 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.

PMS stars in the HR Diagram

Palla & Stahler (1993)

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: t=3/7·(GM2)/RL.

Instability Strip

During their evolution toward the MS a number of intermediate mass PMS stars are able to cross the instability strip of more evolved Scuti. Crossing times are of the order of 105 106 years

Why to search for PMS Scuti stars ?

1. Determination of the intrinsic stellar parameters: P=cost. P=P(M,R) but L=L(R,Te) P=P(M,L,Te) if L,Te are known indipendently, the comparison between a measured period and the theoretical value provides the stellar mass.

2. 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.

GOALS OF THE PROJECT

1. 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.

2. 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)

3. Variability of stars in the Association(s) in front of Dolidze 25: study of stellar activity (see Torres talks).

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)

Target stars

Observations in RI with VIMOS@VLT (pre-Imaging data)

Pre-Imaging (+2MASS data+EGBW data) used to select targets for the spectroscopy. High priority to objects showing IR excess.

Spectroscopy performed by using VIMOS@VLT

Identification of stars with H emission and Lithium

Selection of the targets: Imaging and Spectroscopy of DOlidze 25

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)

PMS stars in the HRD

AV > 1.7 mag

Expected observational properties of target stars

1. pulsation ( Scuti regime) very low amplitude (mmag to hundredths of magnitude) with time scales of minutes to a few hours.

2. 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.

3. For Debris Disk stars the expected variation is of a few %.

Feasibility

1. Most of target stars have V<15 color information; Error on single phase point < 1-2 mmag

2. PMS candidates are fainter (V> 15) observations in white light. Difficult to be observed. Which would be the S/N?

Photon noise+noise due to jitter and breathing of the PSF. Dispersion is due to the change in the contamination rate from one star to another at a given magnitude (Guterman, private communication)

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. R~16.9 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 > 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%

Simulations

Breger et al. 2005

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:

1.Apply asteroseismological techniques to PMS Scuti candidates (if they actually pulsate!) in order to constraint their intrinsic parameters and evolutionary status.

2.Study the interaction of PMS stars with circumstellar material

3.Study candidate DD systems

4.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).