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 RegionsHigh resolution imaging and spectroscopy

with the VLA

Current models of star formation

High Mass Star Formation1. 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, Franco-Hernandez & 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

Unable to compare to previous observations (not matched resolution)

Indication of data quality

Sgr B2 MainEVLA1.3 cmDnC (Jan. 2012)

Sgr B2 North

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

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This work is supported by the National Science Foundation through grant AST-1211460