Bulk Velocities and Orbits of Open Clusters - SRS!

Bulk Velocities and Orbits of Open Clusters
S. Drew Chojnowski & Peter M. Frinchaboy
Texas Christian University, Department of Physics & Astronomy
Background
Open clusters are temporary associa<ons of a few hundred to a few thousand stars, with the Pleiades being the most notable example visible to the naked eye from northern hemisphere la<tudes. As opposed to globular clusters, which are typically observed in the Galac<c halo and which are permanent associa<ons of tens of thousands of predominantly old stars , open clusters are confined to the Galac<c Disk, host rela<vely young stellar popula<ons, and are subject to disrup<on by gravita<onal encounters and shock waves. Despite their transient nature over long <me scales, open clusters are useful probes of the Milky Way Disc for three reasons: 1.  Open clusters are generally confined to the Milky Way disc. 2.  Cluster member stars traverse the Disc as a group. 3.  Cluster member stars formed from the same material at about the same Ame. Our work takes advantage of proper<es (1) and (2) by using the observed mo<on of open clusters to trace the overall mo<on the Milky Way disc and spiral arms. Specifically we do this by combining catalogued proper mo<ons (veloci<es across line of sight) for cluster members with our radial velocity(veloci<es along line of sight) measurements in order to construct a three-­‐dimensional picture of a given open cluster’s orbit about the Galac<c center. We can poten<ally use property (3) to confirm or reject cluster membership for individual stars, rejec<ng those which deviate from the chemical abundance trend of the cluster overall. Vr radial velocity Figure 1: Velocity components for an object moving in 3D space. We use high-­‐resolu<on spectroscopy to measure the radial component. Star Sun velocity through space proper mo<on μ ht of sig
line 3 2 1 1.  The WIYN telescopes on Kiq Peak. 2.  The WIYN 3.5-­‐m dome. 3.  The Hydra spectrograph (and I). transverse velocity Cluster
Dist. (kpc)
Log (age)
Berkeley 68
1.68
8.39
4
-­‐24.7
-­‐7.3
+3.7
Dolidze 39
1.43
?
6
-­‐18.2
-­‐1.4
+3.0
IC 4996
# Memb
Vr μα (km/s)
(mas/yr)
μδ (mas/yr)
2.40
6.87
13
-­‐12.3
-­‐1.5
-­‐4.3
IC 5146
0.85
6.00
12
-­‐9.4
+2.4
+0.8
NGC 1193
4.57
9.70
10
+6.5
+0.2
-­‐0.5
NGC 1342
0.67
8.66
21
-­‐9.9
+0.2
-­‐4.0
NGC 1912
1.40
8.50
2
-­‐35.8
-­‐0.2
-­‐7.4
NGC 2168
0.91
8.25
4
-­‐8.2
+1.4
-­‐2.7
NGC 6819
2.36
9.17
17
+1.5
-­‐3.5
-­‐3.2
NGC 7082
1.44
8.23
5
-­‐15.2
+0.6
-­‐3.7
Fig.2 -­‐ IC 4996 Abstract
We present bulk 3D dynamics for 10 northern hemisphere open clusters. U<lizing WIYN/Hydra mul<-­‐fiber spectroscopic observa<ons and Tycho and/or UCAC-­‐3 proper mo<ons, we determine cluster membership and bulk veloci<es. Cluster veloci<es are used to derive cluster orbits with comparison to previous studies. Figure 2: RV Kernel Membership. (a)  All stars with RV data (b)  Stars outside the cluster radius (c)  (a) minus (b) (d)  Gaussian fit to (c). Table 1:
Data for the 10 clusters processed thus far. Dist
represents distance from the Sun. ‘# Memb’
indicates the number of stars confirmed as
members by our data. The last 3 columns
respectively list our averaged radial velocity,
catalogued proper motion in RA and catalogued
proper motion in DEC.
Fig.3 -­‐ IC 4996 (b) (a) (d) (c) Figure 3: PM Kernel Membership. (a)  All stars with PM data (b)  Stars with non-­‐member RVs (c)  (a) minus (b) (d)  Gaussian fit to (c). Fig.4 -­‐ Berkeley 68 Fig.6 Data
Data for our 10 new clusters used in this analysis were obtained using the Hydra spectrograph on the WIYN 3.5-­‐m telescope (see Table). Radial veloci<es (RVs) have been determined using the IRAF FXCOR package (Frinchaboy & Majewski 2008). Distances are taken from the Dias et al. (2002) catalog of open clusters. Cluster member stars were determined from our RVs and proper mo<ons (PMs) from the Tycho-­‐2 and UCAC-­‐3 catalogues (Høg et al. 2000; Zacharias et al. 2009) using a non-­‐parametric kernel technique based upon the method from Galadí-­‐Enríquez et al. (1998). The membership result and determined orbit (integrated using model from Johnston et al (1995)). The study yielded an average of 10-­‐50 RV members per cluster resul<ng in 2-­‐10 members per cluster with both RVs and PMs. We combine these data with the 66 open cluster from Frinchaboy & Majewski (2008) for our analysis here. Orbit Analysis
We used the Johnston, Spergel, & Hernquist (1995) orbit integra<on model. We integrated all clusters back according to their age and also back 2 Gyr (if younger than 2 Gyr) to determine their basic orbital parameters (Ra, Rp, e, and zmax), as shown for NGC 1912 in Figure 3. Results
The results of our work thus far indicate that most of our sample of open clusters orbit on mildly ellip<cal orbits (<e> = 0.1) and that truly circular orbits are rare. We find most of our 76 open clusters remain within 500 pc of the Galac<c plane, with the excep<on of Collinder 205, Kharchenko 1, and NGC 1193 which should be inves<gated further. We find that the majority of clusters are closer to Apogalac<con (Ra) and that outer clusters are found at higher z compared to inner clusters. Given the well-­‐behaved orbits of these clusters, we find that most of our sample clusters are well-­‐suited to be used as Galac<c dynamical tracers. In future work, we will add data for 30 more open clusters to our sample, significantly strengthening our inves<ga<on of the Milky Way’s rota<on curve. Figures 4 & 5: Orbit and color-­‐magnitude diagrams for Berkeley 68 and IC 4996. (a) Xgc vs. Zgc. (b) Orbit integrated back in <me (to age of the cluster) vs. Rgc. (c) 2MASS CMD; red dots () PM & RV members, crosses () non-­‐
members, and triangles() RV members w/o PM data. (d) Xgc vs. Ygc. (e) Ygc vs Zgc. (f) Meridonial plot of the orbit. Fig.5 -­‐ IC 4996 Figure 6: DistribuAon of orbital parameters. Black points are from Frinchaboy & Majewski (2008) and red points are from clusters listed in Table 1. (a) Apogalac<con Radius (Ra) vs. current galactocentric radius (Rgc). (b) Maximum height above the Galac<c plane (zmax) vs. Rgc. (c) Perigalac<con radius (Rp) vs. Rgc. (d) Maximum height above the Galac<c plane (zmax) vs. current height |zgc| (a) (b) (d) (c) Fig.7 Figure 7: RelaAons among orbital parameters. (a) Histogram of rela<ve difference between Ra and Rgc. (b) Histogram of cluster eccentrici<es. (c) Histogram of rela<ve difference between Rgc and Rp. (d) Rela<ve difference between |z| and zmax. Note the much larger difference compared with the radial parameters. (a) Mean = 0.068 σ = 0.120 (b) Mean = 0.10 σ = 0.07 (c) Mean = 0.126 σ = 0.104 (d) Mean = 3.9 σ = 7.1