AGE/METALLICITY OF OPEN CLUSTERS NGC 581 AND NGC 7160Alexander Owens, 3rd year BSc Physics with Astrophysics, NTU

1. IntroductionAge, metallicity, interstellar reddening and extinction of two young open clusters NGC 581 and NGC 7160 are determined by collecting photometric data on a CCD camera and using aperture photometry to plot Colour-Magnitude Diagrams and fit isochrones.

2.Background/Experimental• NGC 581 and NGC 7160 are observed at NTU observatory with a 14-inch Meade

telescope, using a CCD camera to capture incoming light from stars.• For the photometry, a total of 5 images in B and V filters, were taken of NGC 581 at

an exposure time of 60 seconds.• For NGC 7160, a total of 20 images at an exposure of 3 seconds were taken in the B-

filter and a total of 10 images at an exposure of 1 second were taken in the V-filter.• Data reduction including bias subtraction and flat division is performed on images in

B and V filters to reduce the offset voltage produced from the bias and the dust grains from the flat field.

• Aperture photometry is carried out on 54 stars in NGC 581 and 14 stars in NGC 7160 using MaxIm DL.

• The cluster members are identified using finding charts from SIMBAD astronomical database.

• A calibration with a reference star of known magnitude is used to measure the apparent magnitudes of the cluster members.

• Photometric results are then plotted on a CMD with axis of apparent V-magnitude against B-V colour. The relevant errors calculated from the signal to noise ratio are also applied.

• Isochrones from Bressen et al. (1999) of different metallicities are applied to the CMDs by fitting the main sequence and main sequence turn-off as accurately as possible. From this, an age of the cluster is determined.

• Extinction and interstellar reddening effects are taken in to consideration to provide a more accurate fit to the data.

Fig. 1 NGC 7160 in V-filter with 90s exposure time

Fig. 2 NGC 581 BVR colour combined image

Fig. 3 Diagram of aperture used for photometry on stars

3. Results

Fig. 4 CMD of NGC 581 with isochrones of age, t=21.68Myr at a range of metallicities

Fig. 5 CMD of NGC 7160 with isochrones of age, t=15Myr at a range of metallicities

Object Age(Myr)

Metallicity(Zʘ)

Reddening

(mag)

Distance

(parsecs)

NGC 581 21.7 ± 7 0.025 ± 0.0065 0.43 ± 0.05

2194

NGC 7160

15 ± 4 0.0025 ± 0.00065 0.2 ± 0.05 789Table 1. Results of age determination from the fitting of isochrones

4. Conclusion• The age of two young open

stellar clusters have been determined as shown in the results

• Large uncertainty is due to no visible red giant branch in the data, so it is hard to distinguish reliable age

Acknowledgments• Dr. Daniel Brown, Senior lecturer at

NTU• Bressen et al. (1999) – isochrone

data from PARSEC database• Pierre Vaurs – providing images of

NGC 7160• Wenger et al. (2000) - SIMBAD

astronomical database

• For NGC 581, the main sequence of the isochrone fits nicely to the data, but there is no visible main sequence turn-off in the data, giving large error in age determination.

• For NGC 7160, the images are of the centre of the cluster so there is a lack of data, giving a large uncertainty in the parameters given.

• The age of NGC 581 is in good agreement with the literature, but NGC 7160 needs more research as there is a large uncertainty in the literature and very few studies.