Transcript UGA Physics Colloquium - Academics

Dynamic UC HII regions in
Sgr B2: Flickering and
Ionized Flows
Chris De Pree – Agnes Scott College
UGA Colloquium Series
February 7, 2013
Collaborators (Students in ital.)
 D.J. Wilner2 & E.R. Keto2
 R. Gàlvan-Madrid3
 M. Goss4
 R. Klessen5
 M. Mac Low6
 T. Peters7
 R. Banerjee8
 M. Hutcheson1, K. Butler1, A. Monsrud1, J. Heath1, K.
Luong1
1Agnes
Scott College, 2CfA, 3ESO, 4NRAO, 5ITA-University of Heidelberg, 6American
Museum of Natural History, 7ITP-University of Zurich, 8University of Hamburg
Overview
 Background
 A Problem – UC HII Region Lifetimes
 A Solution – Flickering UC HII Regions
 A Test – VLA/EVLA Observations over a 23 year time
baseline
 Current Status – First Complete Continuum Results
(AAS 2013)
 Preliminary Conclusions
Background
 Current models of star formation
 “Classical” versus UC HII Regions
 High resolution imaging and spectroscopy with
the VLA
Current models of star formation
 High Mass Star Formation
1. Collision of multiple lower mass stars in a stellar cluster
(e.g. Bonnell et al. 1998)
2. Infall of material through an accretion disk (McKee & Tan
2003) – similar to low mass stars
 Disks have been observed around high mass protostars (e.g.
Davies et al. 2010)
 Disks can solve some outstanding problems in massive star
formation
 Recent modeling of the accretion process has also given insight
into a number of other outstanding issues in massive star
formation
Classical versus UC HII Regions
 “Classical” HII regions are relatively large (D~100 pc)
 But…high resolution radio interferometers (e.g. VLA) show the
ionized gas surrounding many young massive stars is highly
confined
 Early studies (reviewed in Churchwell 2002) identified subsets
of HII Regions
 Ultracompact (UC) HII regions (D~0.1 pc)
 Hypercompact (HC) HII regions (D~0.01 pc)
 Earliest models to explain UC and HC HII regions assumed that
they were similar to classical HII regions, only smaller
 Steadily expanding into their environments and that the
ionizing star was fully formed (Galván-Madrid et al. 2011)
Background: Previous VLA
Observations
 The Sgr B2 Main 7 mm
continuum image (top) and the 7
mm RRL profiles (bottom) from
De Pree et al. (2011)
 There are several peculiar
sources with multiple-peaked
profiles, and others that show
no clear line emission at all,
possibly due to the limited
bandwidth of the VLA correlator
 These line data have a
bandwidth of 25 MHz and a
velocity coverage of 150 km/s
(old VLA correlator limitations)
Problem: UC HII Region Lifetimes
 The “lifetime problem” (Wood & Churchwell 1989; Kurtz et al. 1994)
 These UC and HC HII regions last longer than they should if they are
simply expanding into their local environments
 Recent observations have led to a revision of these assumptions
 Hot molecular cores are rotating and infalling (e.g. Keto 1990)
 Small-scale ionized gas shows accretion dynamics (e.g. Galván-Madrid et
al 2009)
 Some UC and HC HII regions have rising spectral indices (e.g. De Pree et
al. 2004)
 A sample of UC and HC HII regions have measured flux variations on
timescales of years (e.g. Galván-Madrid et al. 2008)
 These observations strongly suggest that the morphology and
characteristics of UC and HII regions may be related to the accretion
processes that form massive stars
Proposal: Flickering UC HII
Regions
 3-D radiation-hydrodynamic numerical simulations of the
formation of HII regions in accretion flows are now possible
 The dense, rotating, accretion flows required to form massive
stars quickly become gravitationally unstable
 High-resolution simulations (Peters et al. 2010a) show that
when accretion continues in the presence of ionizing
radiation, the UC HII region can be gravitationally trapped
 UC HII regions fluctuate over time between trapped and
extended states as infalling massive filaments of material
interact with the radiation field of the young massive star
 As these “flickering” UC HII regions expand and contract,
they take on the shapes defined by the morphological
classifications of Wood & Churchwell (1989), Kurtz et al
(1994) and De Pree et al. (2005).
Morphology from Models (Peters
et al. 2010a)
Proposal: Flickering UC HII
Regions
 Sources should flicker between HC and UC sizes throughout the
main accretion phase, rather than monotonically expanding
 Peters et al. (2010c) show that this behavior also solves the UC
HII lifetime problem (Wood & Churchwell 1989), since accretion
continues for a period ten times longer than the free expansion
timescale for an HII region. The model predicts that UC HII
regions can experience scale length and flux variations of 5% per
year
 Galván-Madrid et al. (2011) estimate that ~10% of observed UC
and HC HII regions should have significant, detectable flux
variations (of 10%) on timescales of ~10 years
 Such fluctuations have been seen in a few sources with multi-epoch
VLA observations (e.g. Cep A, Hughes 1988; NGC~7358~IRS1, FrancoHernandez & Rodriguez 2004; G24.78+0.08, Galván-Madrid et al.
2008)
Galván-Madrid et al. 2008
 The crosses indicate the
positions of the
components A1 and B from
Gaussian fits to the 1984.36
image
 The negative residuals
observed in the difference
image indicate a decrease
of ~45% in the flux density
of component A1
 Note: measuring flux
variations in an isolated
source can be difficult
Scientific Goals of the Current Work
 The primary scientific goals of this work are
1. To search for size and flux variations in a large sample of
UC and HC HII regions, and
2. To examine the properties of the radio recombination
line (RRL) emission arising from those flickering regions
Test: EVLA Observations (1.3 cm)
Goals
 Determine the frequency and magnitude of UC HII flux and size
fluctuations over a 22 year time baseline (1989 to 2011) in one of the
most source-rich massive star forming regions in the Milky Way
 Constrain the theoretical models described in Peters et al. (2010a,
2010b, 2010c, 2011) and Galván-Madrid et al (2011) with these data
 Observe recombination lines with the improved spectral resolution and
bandwidth of the new EVLA correlator, and characterize line profiles and
velocity gradients
 Examine the dynamics of sources with especially broad or multiply peaked
line profiles discussed in De Pree et al. (2011), and compare the RRL
properties of flickering sources with the predictions of Peters et al
(2011b).
 Look for kinematic signatures of H II region flickering. These signatures
could be shocks of ionized gas, line broadening or asymmetric line
profiles.
Test: EVLA Observations (7 mm) Goals
 Make continuum and RRL observations at 7 mm in only the
BnA configuration to obtain morphological information in the
continuum and RRLs at a short wavelength recombination line
at the highest available angular resolution of 0.06” (650 AU).
These observations will allow us to:
 Provide recombination line data at additional wavelengths
(H52α and H53α) to diagnose the role of pressure broadening
in the most compact sources as described in Keto et al. (2008)
and De Pree et al. (2011)
 Better resolve the dynamics of the ionized gas – which can
result from a combination of rotation of the evaporating
accretion flow and outflow – to test the predictions of Peters
et al. 2011b
Current Status
 CASA versus AIPS
 DnC (1.3 cm) Observations (January 2012)
 CnB (1.3 cm) Observaations (April 2012)
 BnA (1.3 cm, 7 mm) Observations (September-Oct. 2012)
 First image (DnC) – November 2012 - GRAM
 First combined image (DnCnBnA) – January 2013 - AAS
 Line data (not yet reduced) – factor of 3 improvement in both
 420 km/s coverage
 3.25 km/s channels
First EVLA Data
(obs. January 2012)
 GRAM November, 2012
Sgr B2 North
 Unable to compare to
previous observations
(not matched
resolution)
 Indication of data
quality
Sgr B2 Main
EVLA
1.3 cm
DnC (Jan. 2012)
Data Reduction
 Continuum data sets were reduced using AIPS with
Appendix E in the AIPS Cookbook. Appendix E accounts
for specific issues related to the reduction and analysis
of EVLA data since the upgrade.
 Each of the datasets (DnC, CnB and BnA) were
independently flagged, calibrated, imaged and self
calibrated. The calibrated data sets were then
combined and self calibrated again (DnA with CnB, and
then this pair with the remaining BnA data).
 The final cleaned images had an rms noise of 8x10-4
Jy/beam. The new (2012) data were convolved slightly
to the exact beam size of the 1989 observations.
Sgr B2 Main
Sgr B2 Main
Sgr B2 Main (F Sources)
 Highest density region in
Sgr B2 Main
 Sources F1-F4 (shown)
 Three of these sources have
significant flux density
changes
Sgr B2 North
Sgr B2 North
Early Results
 Peak flux density values were examined using the AIPS task IMSTAT
 The source peak flux density values in the 1989 and 2012 images are
remarkably similar for almost all of the 49 sources in Sgr B2 Main, North
and South, with 4 clear exceptions
 Exceptions were noted for sources that had peak flux density values that
changed by more than 10 times the rms noise in the image (~8
mJy/beam):
•
Source Sgr B2 Main F1 (16% increase)
•
Source Sgr B2 Main F2 (20% increase)
•
Source Sgr B2 Main F3 (19% increase)
•
Source Sgr B2 North K3 (16% decrease)
 The frequency and scale of the the changes (10-20% in approximately
10% of the sources in the region) are consistent with the predictions in
Galvàn-Madrid et al. (2011)
New EVLA Data (phase and amp)
 Multiple Sub Bands
 High spectral resolution
 Broad spectral coverage
Future Project Goals (RRLs)
 1.3 cm and 7 mm recombination lines with the improved
spectral resolution and bandwidth of the new EVLA correlator
 Characterize these line profiles and velocity gradients
 Use these recombination line data to determine the relative
contribution of kinematic broadening in the UC and HC
sources in Sgr B2 Main
 Use the new RRL data to explore the dynamics of sources with
especially broad or multiply peaked line profiles discussed in
De Pree et al. (2011) and to use recombination line data at an
additional wavelength (H53α) to diagnose the role of pressure
broadening in the most compact sources in Sgr B2 Main and
North
 Test RRL numerical predictions from the modeling of Peters et
al. (2011b)
Project Goals (Education)
 Involve 2 undergraduate women per year in the reduction, analysis and
presentation of EVLA data (Monsrud & Heath – Summer 2013)
 Students will do most of their work in the summer months (2013-2015),
but will also do some work during the academic year as part of the upper
level course Astronomy 300 (Radiation) which will be taught in the Fall of
2013 and 2015 according to our normal rotation
 Employ a student from the Agnes Scott College graphic design program to
design and maintain a web site that will convey the results of this project
(K. Luong)
 Produce an educational poster about massive star formation that will
feature the new 1.3 cm and 7 mm continuum images of Sgr B2. This
image will be the result of the 3 hybrid configuration imaging carried out
over the period January- September 2012
 To increase the diversity of majors in our Department by actively
recruiting incoming majors with the possibility of summer research
internships at the College
Conclusions

First results in a three year project to investigate the continuum and radio
recombination line emission at K band (1.3 cm) and Q band (7 mm) in the Galactic
massive star forming region, Sgr B2 (Main, North and South). First EVLA continuum
images are comparable in quality to 1989 data

Preliminary analysis indicates that four of the 49 continuum sources in Sgr B2 Main (3)
and North (1) have experienced a significant (> 10s) change in their flux density peak
between matched resolution, full synthesis, hybrid configuration observations made in
1989 and 2012

The sources with flux density changes are F1, F2, F3 and K3. All four sources with
changes in peak flux density are hypercompact HII (HC HII) regions, and all have been
observed in previous observations to have broad radio recombination line (RRL) profiles
(F1, F2, F3 DVFWHM>50 km/s and K3 DVFWHM>35 km/s)

Previous RRL observations at both 1.3 cm and 7 mm were severely bandwidth-limited.
The current RRL observations (taken simultaneously with the continuum observations
presented here) have nearly three times the velocity coverage (32 MHz versus 12.5 MHz)
and nearly three times the spectral resolution (3.5 km/s versus 10 km/s) of any previous
RRL observations in this region.
References
•
De Pree, C. G., Wilner, D. J., Deblasio, J., Mercer, A. J., & Davis, L. E. 2005, ApJ, 101, 104
•
De Pree, C. G., Wilner D.J. & Goss, W.M., 2011, AJ, 142, 177
•
Franco-Hernandez & Rodriguez, L.F., 2004, ApJ, 604, 105
•
Galván-Madrid, R., Rodriguez, L.F., Ho, P., & Keto, E., 2008, ApJ, 674 L33
•
Galván-Madrid, R., Peters, T., Keto, E., Mac Low, M.-M., Banerjee, R., & Klessen, R.S., 2011, MNRAS,
416, 1033
•
Gaume, R. A., Claussen, M. J., De Pree, C. G., Goss, W. M., & Mehringer, D. M. 1995, ApJ, 449, 663
•
Hughes, V.A., 1988, ApJ, 333, 788
•
Mac Low, M.-M. & Klessen, R.S. 2004, Rev. Mod. Phys., 76, 125
•
Peters, T., Banerjee, R., Klessen, R.S., Mac Low, M.-M., Galván -Madrid, R., Keto, E., 2010a, ApJ,
711, 1017
•
Peters, T., Mac Low, M.-M., Banerjee, R., Klessen, R.S., Dullemond, C.P., 2010b, ApJ, 719, 831
•
Peters, Thomas, Klessen, R.S., Mac Low, M.-M., Banerjee, R., 2010c, ApJ, 725, 134
•
Peters, T., Banerjee, R., Klessen, Ralf S., Mac Low, M.-M., 2011, ApJ, 729, 72
•
Peters, T., Longmore, S. N. & Dullemond, C. P., 2012, MNRAS, 425, 2352
•
Wood, D.O.S. & Churchwell, E., 1989, ApJS, 69, 831
This work is supported by the National Science Foundation through grant AST-1211460